October 26, 2018
According to several weather sources it's not going to be clear after 8pm tonight, but I set up the William Optics GT81 anyway. Two reasons: I need to test out an adjustment I made in the spacing from the WO FLAT 6AII, to see if it corrects some star elongation at the corners--I added a 3mm M48 spacer just between the flattener/reducer and the off-axis guider. Second, I'm testing out some plans I have with Stellarmate (INDI, Ekos, KStars) on a Rasp Pi 3B+ (faster, with AC wifi). This one will be dedicated to this color imaging rig, with an auto-starting Ekos equipment profile.

I put together a list of modifications and upgrades I have added or built for my IOptron CEM25P mount, and included them in one handy image--with arrows and descriptions (see below). Over the last year this has become my primary telescope mount (over my Orion Atlas EQ-G), partly because I've been doing most of my astro-imaging work with 81 and 61mm aperture refractors (that's a William Optics GT81 in the upper right). The other advantage of the CEM25P is quick setup time. I can have this bolted to the pier plate, polar aligned, and ready to image in about five minutes.


October 21, 2018
Here's my working color imaging train with the William Optics GT81 APO. The leftmost component is the William Optics FLAT 6AII field flattener/reducer which brings the focal length of the GT81 to 382mm at f/4.7. Pretty fast. The problem, of course, with reducers and flatteners is nailing that 55mm T2 distance to the camera sensor. That means the off-axis guider has to be thin (Orion Thin OAG) and the inline filter drawer does as well. The last piece of the puzzle, or bit of trouble you have to deal with is focusing the off-axis guide camera, matching the guide camera sensor distance to the primary camera sensor distance. And I finally had a bit of clear sky at night--bright moon however--to dial this in.

I added the filter drawer to be able to support UHC and other filters with the ZWO ASI071MC color camera. I don't plan on running with a filter most of the time, but some targets call for a bit more contrast.


August 25, 2018
Small Refractor Mods #1

There were several high-quality 60mm apochromatic refractors that entered the market last year. They were pitched as portable wide-field scopes, and also marketed here in the US for the solar eclipse last summer. Starting around $450 USD, these little refractors, like the William Optics ZenithStar 61 sold out quickly. I didn’t get a chance to purchase one until May of this year.

The ZS61 has a 360mm focal length at f/5.9, synthetic fluorite objective lens--FPL-53, which has some amazing optical properties. It’s a great scope, with a solid focuser. But there’s an easy modification that will make it even better. I found one thing when I added the imaging train--here’s my narrowband setup, with an Atik414EX monochrome CCD, a ZWO filter wheel with 5 filters, hydrogen-alpha, oxygen3, sulfur2, clear, and a near IR 685nm longpass. With the field flattener this ends up around 3.2 pounds or 1.45kg. These scopes--I keep saying these scopes because there are several varieties of the same basic components, a few of them with the same focuser, focal length, and aperture, differing--as far as I can tell--only with the hardware, knobs, lens caps.

Anyway, the first thing you probably want to do with these is strengthen the scope’s connection to the dovetail bar. The stock version comes with this clamshell ring and shoe, and if you’re going to do anything other than some light visual astronomy, you will want to backup the stock ring with another. I found that when I added the camera, filter wheel, and guider, the whole system had a slight flex to it if I lifted or pushed down with the camera. At first I thought it was the focuser and was a bit bummed about that, but then I noticed it was the whole scope moving, and it all relied on this rather slender ring and shoe. The focuser itself is very smooth and very solid. It’s a dual-speed rack-and-pinion type, and so you may want to adjust some of the tension screws depending on the load you’re planning to add--a DSLR or more, but out of the box, this focuser along with FPL-53 glass makes this scope worth considering for your wider-field work.

To remove that flexure in the system, I bought a ZWO 78mm Holder Ring for ASI Cooled Cameras to see if it would work. The tube’s diameter is around 76mm, and with a delrin shim or something similar, the 78mm inside diameter of the ZWO ring worked almost perfectly. The one gap--literally--was with the two shoes of each ring. The stock William Optics one is ¼” (6.35mm) taller than the ZWO ring. Easy solution: I went to my favorite aluminum supplier (you have one, right? See the links below) and bought a set of stock aluminum pieces, 2” x 3” x ¼”, then drilled, and stacked it with the dual ring setup. Now the whole system is perfectly rigid with two strong foundations.

The other advantage of going with the ZWO holder ring are the risers with the threaded holes on the top and bottom. I added one of these SmallRig cheese grater mounting plates on the top--you should always have one or two of these on hand for bolting things together. They’re tough, anodized aluminum, and full of threaded holes of varying sizes. I use these on the ZS61 and my William Optics GT81 to connect the control hardware and power--usually a Raspberry Pi3b and 12v battery pack. What’s nice is I can use a couple hexcap screws to quickly add or remove all devices from one scope to the other.

So, there you have it. An easy way to build more rigidity into a nearly perfect portable wide-field setup. Let me know if you have questions, or a better way to accomplish this. I added some links below for the components I used.
For aluminum: Stoners Tools and Raw Materials
Ebay listing for the 2” x 3” x ¼” aluminum bar stock:


June 25, 2018
The Astro-Tech AT6RC f/9 Ritchey-Chrétien on the iOptron CEM25P EQ mount.

Good counterweight info, with a bit of physics of balancing and moment of inertia:

Regarding more counterweights closer in, versus less counterweights farther out, it does make a big difference - but not for any of the reasons yet mentioned ("antique clock pendulum effect", whatever that is, or vibrations).

In terms of balance, the equation is just mass times distance from the fulcrum. So 100 lb counterweights at 10" is exactly the same as 50 lb counterweights at 20". They will balance the same, and this might seem like a good thing, since the mount is carrying less weight.


The moment of inertia is the mass times the distance from the fulcrum squared. In this case, 100 lb counterweights at 10" is half the moment of inertia as 50 lb at 20". Moment of inertia affects momentum of the axis. So any time that your start or stop or change the motion of the axis (ie. guide inputs), this is the number that's important. In practice, it is far more important than total weight. The bearings supporting the axis can carry far more weight than the mount is rated for (orders of magnitude more weight - probably in the thousands of pounds for a mount like the CEM120).


June 10, 2018
The pier-baseplate-on-the-tripod project is complete, and tonight's slogan is "Summer: portable, Winter: pier" (a slogan typically means "a short and striking or memorable phrase used in advertising", but you can probably tell I'm using it in the historic "Scottish war cry" sense here). The skies look clear this evening, and I'll still be using the William Optics ZS61 on the "pier", which stands just off the back deck. I set this up last fall so that I could do my astro stuff through the winter, and not have to lug a lot of heavy, expensive equipment through snow and ice, and all winter long I've been carrying my EQ mount, scope, and counterweights five steps out the back door to set it down and bolt it to the aluminum baseplate. It was easy and I could be ready to go in a few minutes. Well, it's almost time to move the whole operation further from the house. See the shed in the background there? This time of year, and through the autumn, I usually work off the tripod out there, about 120 feet/36m from the house. This gives me a lot more sky west of the meridian--a lot more than the view from the deck, which has a tree obstructing things to about 45° to the northeast, and I can only reach 10-15° past the meridian to the south. So, it's almost time. I'm ready with my portable tripod (with the pier plate). I'm ready to shout that war cry to the summer night sky: "SUMMER PORTABLE!"


June 8, 2018
I spent a few hours last night dialing in the Orion "TOAG" or Thin Off-Axis Guider, which I bought a couple years ago, but have never been able to get working properly. I've tried seven or eight times, added it to my imaging train a couple times a year, attempting to get things working without success. Well, I went at it again last night, and you know what? It came together. I still have some weirdness to tinker with--to work out, but for the first time I wasn't guiding with a separate scope. I was guiding at the same focal length as the William Optics ZS61 using a pick-off prism that directs a portion of the field of view up into the guide camera. Here's the setup I used last night to dial-in the distance between the primary camera and guide camera, and then bring everything into focus. I ended up with some pretty cool shots, but my main purpose was to get Off-Axis Guiding (OAG) adjusted and working--and that was with me slewing around the sky to clear areas between banks of clouds to find some halfway interesting targets. In this setup I'm using my trusty ZWO Astronomy Cameras ASI120MM-S for guiding, and the ZWO ASI071MC cooled color camera for the primary. The goal here is to be able to guide (track the motion of the earth against the star field to a very fine degree, and make small incremental adjustments to the EQ mount) so that I can take long exposures without worrying about the external guidescope issues I know all of you care deeply about, like field rotation and differential motion between a guide scope and imaging telescope. I have been able to take 20 minute exposures with a guidescope and camera, but the stars are not as sharp as I would like--think pressing down the button of your camera and holding the shutter open for 20 or 30 minutes and have everything in the field of view remain in sharp focus. That's essentially what the guiding system accomplishes, taking continuous images of the stars and feeding them to some pretty sophisticated software that controls the motion of the equatorial mount (that's the white z-shaped device with the black boxes on which the telescope is fastened and balanced).


May 24, 2018
Here's my setup for tonight's astro session: William Optics ZS61 and the ZWO ASI071MC cooled color camera. I'll be running this at f/4.7 with a field of view of 4.7° × 3.1°. The moon's already up--you can see it in the background of the right and bottom frames, so I'm hoping for pretty clear and dark skies around 1 - 2am.


May 12, 2018
Well, look what arrived in the mail! Yes, that's a William Optics ZenithStar 61, a very portable apochromatic refractor from my favorite telescope maker. This cool little scope is like a baby version of my GT81, with a synthetic fluorite objective lens (which has some of the best lens properties, including ultra-low dispersion of light, etc.) I will write up a review after a few nights out with the ZS61. I can't wait for clear skies!

A few shots of the astronomy workbench, where I'm testing out the ZS61 with a guidescope, focuser, and the ZWO ASI071C camera.


May 5, 2018
I ordered the William Optics Zenithstar 61 APO today. This is a very small f/5.9 360mm focal length refractor, and I'm going to be using it as an ultra-portable scope, mostly for wide field targets like NGC7000 North America Nebula, M42, M31, Cygnus Loop, and other massive deep sky objects. I didn't order the Flat-61 field flattener because I want to see how well the WO Flat6A 0.8x Field Flattener/Reducer works with the ZS61. The Flat6A will bring this scope to f/4.7 288 focal length, which is great--I normally work at this f ratio with the GT81.

William Optics ZS61 APO - Field of View Info

No Reducer
Atik414EXCCD - f/5.9 Res: 3.69"x3.69" FoV: 1.43° x 1.07° (85.8 x 64.2)
ZWO ASI071MC - f/5.9 Res: 2.74"x2.74" FoV: 3.76° x 2.5° (225.6 x 150)
QHY 5iii178C - f/5.9 Res: 1.37"x1.37" FoV: 1.18° x 0.79° (70.8 x 47.4)

WO 0.8x Reducer/Flattener
Atik414EXCCD - f/4.72 Res: 4.61"x4.61" FoV: 1.78° x 1.33° (106.8 x 79.8)
ZWO ASI071MC - f/4.72 Res: 3.42"x3.42" FoV: 4.7° x 3.12° (282 x 187.2)
QHY 5iii178C - f/4.72 Res: 1.72"x1.72" FoV: 1.48° x 0.99° (88.8 x 59.4)


March 29, 2018 - ZWO ASI071MC-Color Camera Review
The ZWO ASI071MC-C camera is a new purchase for me. I'm coming from a couple years of narrowband imaging with an Atik 414EX mono CCD, and I was specifically looking for a cooled color camera. There are a lot of great OSC cameras out there, and I put some time into researching Atik, QHY, and ZWO. First off, if the ASI071 is in your potential buying list for gear, you probably know ZWO has improved the design around dew control with the next-generation ASI071 Pro, along with a couple other nice-to-haves like 256MB DDR3 buffering. So far, with a few astro sessions using the ASI071 I haven't experienced anything that makes me regret buying the original (lower-cost) version.

A couple things to be aware of: 1) I went out of my way to look for the issue (reported on Cloudy Nights and elsewhere) with dew freezing on the AR window, and if you’re just starting out for the night and push the temp below -10 or -20C you may see this, but as soon as you start taking subs that evaporates--literally. Simply put: there’s moisture in the air and you’re chilling this device down well below zero, under the right conditions you will see condensation freezing on the glass, with that distinctive pattern showing up in an exposure or two. ZWO has corrected this in the Pro by preheating the window, but if you come across it, you can accomplish the same thing by cooling down slowly--going from ambient to 5 to 0 to -5 to -10, etc. You can also try taking a few 30 second test exposures. Basically, once you start using the camera this issue goes away. 2) The Pro version uses faster memory for more efficient transfer speeds and buffering. That’s great--faster is pretty much always better, but even so, I am impressed with the download speeds from the ASI071MC-C, given that a single 16-bit FITS file from this camera is 32MB. With my last couple astro imaging sessions I’ve used an old Intel Celeron-based laptop running Ubuntu MATE and Ekos/KStars/INDI. Not the fastest setup around but more than adequate for the file sizes and transfer speeds. I haven’t tried this camera out with Ekos/KStars/INDI running on one of my Raspberry Pi boxes, or with SGP on Windows 10, but soon!

For my criteria, budget, and style of astrophotography the ASI071MC-C seems to have it all. The main features that tipped the balance are: very competitive price, 2-stage TEC, low read noise, 14-bit ADC, APS-C sized 16MP sensor, 4.8μ pixels, and full well capacity of 46ke (You can really play with gain and offset, and still have enough capacity for capturing those soft nebulous gradients). Of course I was also impressed by some of the shots others have taken with the camera--take a look on Astrobin. One final thing: the built-in USB hub. I thought this was an interesting idea, but didn’t know how useful it would be until I started using the camera. Now, it bothers me that other cameras (I have Atik, QHY, and other ZWO cameras) don’t have a couple extra USB ports. These really came in handy for my guide cam and focuser.


March 11, 2018
Setting up the William Optics GT81 tonight with my new ZWO ASI071MC camera. I'm also trying out--again--the 30mm apt/135mm fl guide scope with the ZWO ASI120MM-S, this time with the camera threaded directly to the scope, reducing the backfocus a few more millimeters. This will be my first time out with the new main camera, and I've been anxious to see what it can do for almost a week. The ASI071MC is a color CMOS camera with an impressive dynamic range, TEC cooling, 16MP APS-C sized Sony sensor with 4.8μ pixels. The idea is to have the new color camera compliment the Atik 414EX monochrome CCD I normally use. The Atik with the Nikon 180mm f/2.8 lens has almost the same field of view as the ASI071MC with the William Optics GT81. (The Atik + Pentax Takumar 200mm f/4 is even better). You can't tell with the way I've configured the top dovetail rail and guidescope, but the plan is to be able to run both cameras simultaneously with similar FOV, one color, the other with narrowband filters, in some ways doubling my productivity.

Resolution & FOV
William Optics GT81 f/5.9 + ZWO ASI071MC-C
Res: 2.06"x2.06"/pixel FOV: 2.83° x 1.88° (169.8' x 112.8') Dawes Limit: 1.43 arc/secs

William Optics GT81 f/4.7 + 0.8x Reducer/Flattener + ZWO ASI071MC-C
Res: 2.58"x2.58"/pixel FOV: 3.54° x 2.35° (212.4' x 141.0') Dawes Limit: 1.43 arc/secs

Gain & Offset
• Max Dynamic Range: 0 Gain, 8 Offset
• Unity Gain: 90 Gain, 20 Offset
• Min Read Noise: 240 Gain, 65 Offset


February 27, 2018
Setting up the William Optics GT81 tonight, with the Atik414Ex Mono CCD. There's a big bright moon out there tonight, so I'm not sure what I'm going to shoot. I'm mostly testing out a few things. First, a 2" UHC/LPR filter I've had for a few years--and I don't think I've used it for almost that long. The other is my smaller, short focal length guide scope--135mm FL (that black tube on the other side of the main refractor).


January 10, 2018
This is my "astro cart", which allows me to get everything connected, bolted on, balanced, and ready to go while still in the house--before I go out in the freezing temps. I can roll it right up to the back door that leads out to the deck. One of the real advantages of the iOptron CEM25P is its light weight to carrying capacity specs. The lower total weight allows me to pick up this whole setup (OTA, camera, counter-weights and all) and carry it to the pier. (That's my William Optics GT-81 APO next to the AstroTech, right side of the cart).

Getting ready for some potential clear skies tomorrow or the next day, and I'm doing the prep setup with the Atik414EX on the AstroTech RC scope--f/7.2 with the focal reducer/field flattener. I have some smaller targets picked out, and might even go back for more of the Horsehead nebula (with the RC's longer focal length), as well as other interesting stuff in Orion.

Note on the iOptron CEM25P: The mount only weighs 10.4 lbs (4.7 kg), but has a payload capacity of 27 lbs (12.3 kg) without the counterweights. Total weight of the William Optics refractor, imaging train (Atik CCD, filter wheel, filters, and extensions), guide scope and camera, and various other gear ends up around 15 lbs (6.8 kg), and with the AstroTech it's another 5 lbs, so still well under the 27. That's where you're going to get the best guiding and alignment results--not pushing that capacity boundary. (Just for comparison, my Orion Atlas EQ-G mount weighs 54 lbs (24.5 kg) just by itself, and has a payload capacity of 40 lbs(18 kg)--so yeah, it can carry more, but there's no way I can handle setting up except when everything is in pieces).


My Refractor Setup as of October 5, 2017

  • William Optics GT-81 (81mm, 487mm focal length)
  • William Optics 50mm (200mm FL) guide scope
  • ZWO ASI120MM-S (mono) guide camera
  • Orion Atlas EQ-G or iOptron CEM25P equatorial mount
  • Atik 414EX Monochrome CCD camera
  • Atik EFW2 Filter Wheel (filters: L R G B Ha OIII SII Clear DarkFrame)
  • Raspberry Pi 3 64-bit computer Ubuntu Mate distro with INDI, Ekos, KStars remote observatory control software for all the hardware
  • Shoestring Astronomy FCUSB Focuser Controller
  • USB 3 hub (powered)
  • GL-AR150 Wifi Router (Dedicated network for the setup)
  • Thousand Oaks Optical dew heater controller
  • Dew control heater strips


December 25, 2017
My Winter Astro Setup, essentially the same William Optics GT-81 - iOptron CEM25 combo, with the addition of the iOptron Tri-Pier Adapter, some 6x8 aluminum plates off eBay, and one treated 4x4 post from Home Depot.

The good side of winter and astrophotography is it’s usually dry on clear nights--and “clear” really means clear in terms of astronomical seeing, atmospheric turbulence and all that. Downside is that it’s freakin’ cold. Tonight it’s supposed to get down to about -8°C (about 20°F), pretty cold to be out for a long time, but not painfully cold.

Here are a couple shots of my setup for tonight--and possibly the rest of the winter. (William Optics GT-81, CEM25P EQ mount, Atik414Ex mono CCD, WO 50mm guidescope with ZWO ASI120S-MM guide cam, INDI/KStars/Ekos observatory control). What’s cool is that I can unbolt the mount with the pier adapter and aluminum base plate (they’re all bolted together) from the 4x4 post and top plate, and carry in the entire setup--mount, scope, cameras, etc. What I especially like about this is the ease with which I can setup and tear down each night. The whole thing remains balanced and ready to go, with polar alignment reduced to very fine adjustment to zero in on the NCP.

Note on my other mount: I probably won’t use my Orion Atlas EQ-G mount until spring when things start to warm up, and that’s based on the weight of the Atlas and the low temperatures--with ice adding some difficulty to the setup process. (Yeah, I don’t want to lug around this monster with any probability of slipping, landing on my back, and having to catch fifty or sixty pounds of metal out of the air before it kills someone). The average winter low in New Hampshire is around -12°C, and the average winter high temp is still below 0°C (around 30°F). It’s not unusual for things to get down to -20 to -30°C (-10 to -20°F). We’re still in December, so early in the season, but we’re already getting repeated snow storms interspersed with temps above 0°C (32°F). We typically get a few days of warmer weather here and there, snow and ice melting weather, but there haven’t been enough of them to make a dent in the accumulating snow and ice we have in the yard, driveway, or back deck. We just ended yesterday (the 25th) with another 6 inches or so. When there’s ice, lighter is better.

Best combinations for resolution:
Astro-Tech AT6RC + Atik414EX w/0.8x Reducer = 1.23"/pixel
William Optics GT-81 + QHY5III178 = 1.03"/pixel
William Optics GT-81 + ZWO ASI120MM = 1.62"/pixel

Atik EFW2 Filter Wheel config

Filter Wheel (5 filter Solomark manual)
I mainly use this for moon shots, with the #58 Green and #80a Blue filters, but I also have the UHC and OIII filters in this wheel
5 Green - #58 Green (24% Transmission)
4 Blue - #80A Medium Blue (28% Transmission)

NOTES on blue and green filters:
#58 Green (24% Transmission)
Lunar surface detail
Contrast of blue and red structures on Jupiter, including The Great Red Spot
Melt lines around Martian polar ice caps
Contrast of Saturn's subtle cloud belts

#80A Medium Blue (28% Transmission)
Details of Jupiters cloud belts and Great Red Spot
High clouds and polar caps of Mars
Lunar surface details
Contrast of some comet tails

Field Flattener / Reducer Notes:

I have three reducers:

William Optics 0.8x FLAT-6A II
William Optics APO 0.8x Flattener for GT81 (F6-A)
Lens-to-chip suggested distance: 64.38 mm
Reduces f/6 to f/4.7; f/9 to f/7

For the ZWO ASI071MC (17.5mm distance to sensor)
WO Flat 6AII needs 37.5mm spacing. Go with 38mm.

Here's my ZWO ASI071MC with the correct spacing
between the WO FLAT 6AII and the sensor:

2" 0.5x Reducer sold by several companies, GSO/Highpoint
Recommended working distance is 53mm

Camera info:
Atik 414EX Backfocus: 13.5mm

Atik 414EX with Barlow

Atik 414EX with WO 0.8x Flattener/Reducer

ZWO ASI 120MM Backfocus: 12.5mm

ZWO ASI071MC-Cool Backfocus: 17.5mm

Backfocus measurements for ZWO ASI071MC
Nikon Lenses 46.5mm distance to sensor
Geoptik = 21mm width
Filter Drawer = 16mm width
Orion OAG = 10.5 width

ZWO ASI071MC = 17.5mm distance to sensor
William Optics APO 0.8x Flattener for GT81 (F6-A) = 64.38mm distance

ASI071MC + Filter Drawer = 33.5 (13)
ASI071MC + Geoptik Adapt = 38.5 (8)

WO 0.8x Reducer/Flattener distance = 64.38

ASI071MC + Filter Drawer = 33.5 (30.88)
needs 30.88mm from WO 0.8x reducer/flattener

ASI071MC + OAG = 28 (36.38)
ASI071MC + Filter Drawer + OAG = 44 (20.38)

Gain and Offset settings:
• Maximum Dynamic Range: 0 Gain, 8 Offset
• Unity Gain: 90 Gain, 20 Offset
• Minimum Read Noise: 240 Gain, 65 Read Noise

ZWO Article on Gain & Offset settings for the ASI1600 and ASI071

William Optics GT81
Focal Reducer/flattener 0.8x

Focal Ratio: 4.72
Resolution: 2.58"x2.58" per pixel
Field of View: 3.54° x 2.35° / 212.4' x 141'
Dawes Limit: 1.43 arc/secs

Nikon D750 Backfocus: 46.5mm (Lens flange to sensor, lens flange to film)

QHY5III178 Backfocus: 10mm - 12mm

PENTAX Takumar 200mm f/4 M42 Lens
Flange to sensor distance: 45.46 mm

PENTAX Takumar 135mm f/2.5 M42 Lens
Flange to sensor distance: 45.5 mm

Nikon Backfocus: 46.5mm

Spacing from the center of the pickoff mirror
to the base of threads for the guide cam: 31mm
ZWO ASI071MC distance is 17.5, so we need a 13.5mm space between OAG and camera

Filter Wheel Info:
Atik EFW2 width: 22mm

Spacing notes:
Nikon D750 + T-Adapter + spacer + WO 0.8x reducer...Scope
Nikon D750 + T-Adapter + M42/M48 Adapter + 0.5x Reducer...Scope

           13.5mm +  22mm +                       10mm +    20mm
Atik 414EX + Atik EFW2 Filter Wheel + spacer + M48-to-M42 Adapter + WO 0.8x reducer...

Astro-Tech AT6RC Diagram with backfocus:

Backfocus is 150mm from 2” accessory holder, without 1.25” adapter

Edmund Optics spacers
Astrophotography Tutorials on Reducers and spacing: Astrophotography Field Flattener & Focal Reducer Spacing

ATIK 414EX Monochrome CCD Camera Notes: Atik Cameras, in their selecting a camera video, suggests that I should be aiming between 1 and 2 for the following image scale formula. (The Atik 414EX CCD sensor has a pixel size of 6.45µm). Calculating this out with the focal length of my two scopes will give me the number of arc-seconds / pixel. Under 1.0 means I'm over-sampling the image, and I will get large and bloated stars. Undersampling will give me blocky stars. The first time I tried out the 414EX I connected it and the Atik filter wheel right up to the AstroTech, and sure enough, I was looking at frames that looked exactly like the oversampled example below.

    Pixel size (µm)
--------------------------- x 206 = arc-seconds/pixel
Focal Length (mm)

Astro-Tech AT6RC Ritchey-Chrétien Reflector Specifications
Aperture 152mm (6”)
Focal Length 1370mm
Focal Ratio f/9

Atik 414Ex - Monochrome Camera
Pixel Size 6.45 µm
Pixel Array 1392 x 1040

Deep Space Astro camera scope combinations

WO GT81 + Atik414EX (2.78"x2.78" per pixel)
WO GT81 + Atik414EX + 2xBarlow (1.39"x1.39" per pixel)
WO GT81 + Atik414EX + 0.8x Reducer (3.47"x3.47" per pixel) Plate Solving: Focal Length: 382.00 - Aperture 64.83
WO GT81 + Nikon D750 + 0.8x Reducer (2.57"x2.58" per pixel)
WO GT81 + QHY5III178 (1.03"x1.03" per pixel)
WO GT81 + QHY5III178 + 0.8x Reducer (1.29"x1.29" per pixel)

AT6RC + Atik414EX (0.98"x0.98" per pixel)
AT6RC + Atik414EX + 0.8x Reducer (1.23"x1.23" per pixel)
AT6RC + Nikon D750 + 0.8x Reducer (1.14"x1.14" per pixel)

Nikon NIKKOR 180mm f/2.8 + QHY5III178 (2.75"x2.75" per pixel)
Nikon NIKKOR 180mm f/2.8 + Atik414EX (7.38"x7.38" per pixel)
Nikon NIKKOR 180mm f/2.8 + Nikon D750 (6.83"x6.84" per pixel)

Daytime shots of my two setups Left: William Optics GT-81 (81mm, 478mm FL) with Atik 414EX monochrome CCD, Atik Filter Wheel (EFW2 with LRGB Ha O3 S2 Clear DarkFrame filters), William Optics 50mm guide scope with ZWO AS120 guide camera, all on an Orion Atlas EQ-G mount. Right: Astro-Tech AT6RC Ritchey-Chrétien telescope, Nikon D750 fullframe DSLR, on an iOptron CEM25P mount.

AstroTech RC6 with Atik 414EX
6.45 / 1370 x 206 = .9698  (Oversampled image)

AstroTech RC6 + WO 0.8 Field Flattener/Reducer with Atik 414EX
6.45 / 1096 x 206 = 1.2123  (Correctly sampled image)

AstroTech RC6 + WO 0.5 Field Flattener/Reducer with Atik 414EX
6.45 / 685 x 206 = 2.448  (Slightly undersampled image)

William Optics GT-81 (with the 2" extension to handle backfocus")
6.45 / 478mm x 206 = 2.7797  (Slightly undersampled image)

William Optics GT-81 + WO 0.8 Field Flattener/Reducer with Atik 414EX
6.45 / 382.4 x 206 = 3.474  (Undersampled image)

William Optics GT-81 + WO 0.5 Field Flattener/Reducer with Atik 414EX
6.45 / 239 x 206 = 5.559  (Undersampled image)

William Optics GT-81 + 2x Barlow with Atik 414EX
6.45 / 956 x 206 = 1.3898  (Correctly sampled image)

Nikon D750
AstroTech RC6 + 0.5x Field Flattener/Reducer with Nikon D750
5.9 / EFL 685 x 206 = 1.7743  (Correctly sampled image)

AstroTech RC6 + 0.8x Field Flattener/Reducer with Nikon D750
5.9 / EFL 1096 x 206 = 1.108  (Correctly sampled image)

Nikon 300mm Lens with Nikon D750
5.9 / 300 x 206 = 4.051  (Undersampled image)

ZWO ASI120-MM - Monochrome Camera
Pixel Size 3.75 µm
Pixel Array 1280 x 960
Minimum Exposure Time 64µs (0.000064s)
Maximum Exposure Time 1000s

AstroTech RC6 with ZWO ASI120-MM
3.75 / 1370 x 206 = .5238  (Oversampled image)

AstroTech RC6 + WO 0.8 Field Flattener/Reducer with ZWO ASI120-MM
3.75 / 1096 x 206 = .7048  (Oversampled image)

AstroTech RC6 + WO 0.5 Field Flattener/Reducer with ZWO ASI120-MM
3.75 / 685 x 206 = 1.127  (Correctly sampled image)

William Optics GT-81 with ZWO ASI120-MM
3.75 / 478mm x 206 = 1.616  (Correctly sampled image)

William Optics GT-81 + WO 0.8 Field Flattener/Reducer with ZWO ASI120MM
3.75 / 382.4 x 206 = 2.02  (Correctly sampled image)

QHYCCD - QHY5III CMOS USB 3.0 Color Camera
Pixel Size 2.4 µm
Pixel Array 3072 x 2048
50FPS, Planetary/Lunar Camera

AstroTech RC6 with QHY5III-178
2.4 / 1370 x 206 = .3607  (Oversampled for stars and DSOs)

AstroTech RC6 + WO 0.8 Field Flattener/Reducer with QHY5III
2.4 / 1096 x 206 = .4510  (Oversampled for stars and DSOs)

AstroTech RC6 + WO 0.5 Field Flattener/Reducer with QHY5III
2.4 / 685 x 206 = .7217  (Oversampled image)

William Optics GT-81 with QHY5III
2.4 / 478mm x 206 = 1.034  (Correctly sampled image)

William Optics GT-81 + WO 0.8 Field Flattener/Reducer with QHY5III
2.4 / 382.4 x 206 = 1.292  (Correctly sampled image)

Nikon 70-300mm f/4.5-5.6G ED IF AF-S VR Nikkor
Nikon AF-S FX NIKKOR 24-85mm f/3.5-4.5G ED

24 / 3.5 = 6.85mm aperture
85 / 4.5 = 18.8mm aperture
300 / 5.6 = 53.57mm aperture
70 / 4.5 = 15.5mm aperture

Imaging train:
With the Barlow I need the extension in almost every case--the barlow increases focal length, and so the camera sensor needs to be moved back in the train.
2x Barlow --> 2" Extension --> Filter Wheel --> Camera
2x Barlow --> 2" Extension --> WO 0.8 Field Flattener/Reducer --> Filter Wheel --> Camera

Setup Weight

William Optics GT81 - 81 mm f/5.9 FPL53 Apo Refractor, Gold Edition
tube weight 8 lbs. (3.7 kg), tube length 440 mm

Atik 414EX
Weight approx. 400 gr (0.4 kg)

Atik EFW2
Weight: approx. 500 gr (.5 kg, ~.85kg with filters)

William Optics 50mm CCD Guide Scope # M-G50
Tube weight (without eyepiece) = 15.0oz (0.425 kg)

ZWO ASI120MM Monochrome
Camera Weight (lb.)‎: ‎0.22, 100gr

Spacers, Extension tubes
Weight: approx. 1 kg

Total 6.475 kg, 14.27 lbs.

Beyond the meridian test
I love the iOptron CEM25P mount. It's center-balanced, lightweight, very accurate if you don't push the 27lb limit, and easy to work with under almost any app that uses ASCOM or INDI. I regularly shoot 5 - 10 minute guided subs without issue, even 20 minute subs, depending on the target. Besides capacity, the one difference that stands out from my Orion Atlas EQ-G is how far the CEM25 can take things beyond the meridian. This is a quick shot--yes, I know it's not directly centered, but it does give a rough post-meridian estimate of 20° or so. That's enough when you're chasing M42 with those last ten minutes of exposures. I do think the Atlas has more room here though, but I'll have to test that. (This is my wide-field setup--Nikon lens, QHY primary cam, ZWO guide cam)

# This is a config file for the 'astrometry-engine'
# program - it contains information about where indices are stored,
# and "site policy" items.

# Check the indices in parallel?
# -if the indices you are using take less than 2 GB of space, and you have at least
#  as much physical memory as indices, then you want this enabled.
# -if you are using a 64-bit machine and you have enough physical memory to contain
#  the indices you are using, then you want this enabled.
# -otherwise, leave it commented-out.


# If no scale estimate is given, use these limits on field width.
# minwidth 0.1
# maxwidth 180

# If no depths are given, use these:
#depths 10 20 30 40 50 60 70 80 90 100

# Maximum CPU time to spend on a field, in seconds:
# default is 600 (ten minutes), which is probably way overkill.
cpulimit 300

# In which directories should we search for indices?
add_path /usr/share/astrometry

# Load any indices found in the directories listed above.

## Or... explicitly list the indices to load.
#index index-219
#index index-218
#index index-217
#index index-216
#index index-215
#index index-214
#index index-213
#index index-212
#index index-211
#index index-210
#index index-209
#index index-208
#index index-207
#index index-206
#index index-205
#index index-204-00
#index index-204-01
#index index-204-02
#index index-204-03
#index index-204-04
#index index-204-05
#index index-204-06
#index index-204-07
#index index-204-08
#index index-204-09
#index index-204-10
#index index-204-11
#index index-203-00
#index index-203-01
#index index-203-02
#index index-203-03
#index index-203-04
#index index-203-05
#index index-203-06
#index index-203-07
#index index-203-08
#index index-203-09
#index index-203-10
#index index-203-11
#index index-202-00
#index index-202-01
#index index-202-02
#index index-202-03
#index index-202-04
#index index-202-05
#index index-202-06
#index index-202-07
#index index-202-08
#index index-202-09
#index index-202-10
#index index-202-11
#index index-201-00
#index index-201-01
#index index-201-02
#index index-201-03
#index index-201-04
#index index-201-05
#index index-201-06
#index index-201-07
#index index-201-08
#index index-201-09
#index index-201-10
#index index-201-11
#index index-200-00
#index index-200-01
#index index-200-02
#index index-200-03
#index index-200-04
#index index-200-05
#index index-200-06
#index index-200-07
#index index-200-08
#index index-200-09
#index index-200-10
#index index-200-11


Follow, friend, communicate

Saltwater Witch copyright © 2017 Chris Howard