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.


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 two reducers:

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

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

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...

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
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Saltwater Witch copyright © 2017 Chris Howard