I swapped the 6nm Astronomik OIII filter for the 3nm Antlia Pro. Now I need some clear night skies to try it out!
I've been thinking about portability for my astro setups, and low-power goes along with that. I have a light I use for flat frames, which will cover every scope I have, from the William Optics SpaceCat to the 8-inch Newt. But I also don't mind taking sky flats with a t-shirt or a couple pieces of paper. And these don't use any power.
Here's the scenario: The sun is setting and you've already setup your gear. You're sitting around waiting for the night to arrive. What else are you going to do but spend a little time taking calibration frames, flats when it's still a bit bright out, and dark flats, dark frames, and bias frames as the twilight fades—and if any of those are needed.
That's the idea anyway.
I spent a couple minutes in TinkerCad making a Flat Frame Sleeve for the SpaceCat I can print—inside diameter is 83mm. I ordered some 3 1/2 inch diameter, 1/8in thick "White Acrylic Round" things (Link below). I glued one of the acrylic circles to one side, and placed three pieces of adhesive-backed loop-side velcro strips inside the sleeve to make it fit snuggly on the end of the SpaceCat dew shield. ("loop side" is the soft side). Gluing: I used a Q-tip to put a thin coat of E6000 glue on one side of the printed sleeve and then pressed down the acrylic disc and let it dry overnight.
I've used this sleeve with the Flats Wizard in N.I.N.A. twice and it works well. Give it a try. Let me know what you think!
RedCat 5 Flat Sleeve Model - (Shared/Public) Tinkercad: https://www.tinkercad.com/things/47bakxyG37l
20PCS White Acrylic Round Circle, White Acrylic Rounds: https://www.amazon.com/dp/B081YHZGRL
E6000 Craft Adhesive: https://www.amazon.com/dp/B007TSYNG8
I bought an old Sony A7s because it's a full frame camera with 8.4 micron pixels, and because it's the smallest full frame camera I've ever seen. Also because at the 1350mm focal length of the Astro-Tech AT6RC with the .8x field flattener, I'll end up with 1.6"/pixel resolution, which is damn near perfect for my skies. So, do I care about the "star eater" problem? Not that much. The nebula-imager in me is saying "Thanks, Sony. Can you make it eat all the stars? I hate lots of one-pixel stars more than I hate a full moon". But from the person who actually has to spend $700 USD, I'll see how it goes. For now I'm not going to worry about it—just going to try to take some cool shots. I may have this camera modified to let in more at the red end of the spectrum. Time will tell if my decision to buy a seven year old camera was right, but for now, it's definitely fun to use—and did I mention it's full frame?
Seriously, what about the Sony "Star Eater" issue? As I understand it, the general problem is the Sony line of A7 and A9 mirrorless cameras can confuse hot pixels with undersampled stars, i.e., stars that resolve to 1 pixel or less (and less means they will always resolve to one pixel). I think the reason this hasn't been fully corrected in firmware is because it can't--the hot pixel correction happens at read-out time. It's built into the sensor read functions that also handle read noise, shot noise, fixed pattern noise. That's my guess. Not certain about that. Anyway, with clear skies and great seeing, the typical resolution recommendation is 1 to 2 arcseconds per pixel. If your system (camera + scope) has a resolution higher than 4 or 5 arcseconds/pixel (undersampled) then a lot of the stars in the sky will appear as single white pixels or a small blocky group of pixels—and I'm guessing some of these look like hot pixels to the read function. If you go below 1 arcsecond/pixel in resolution then you're oversampling and stars can appear bloated and soft, covering several pixels. So, it seems that the closer you can get to 1-2 arcseconds/pixel the better your chances of avoiding the "Star Eater" issue. Maybe? For details: https://www.lonelyspeck.com/sony-star-eater-and-how-to-fix-it. Another good article here: http://www.markshelley.co.uk/Astronomy/SonyA7S/sonystareater.html
My William Optics GT81 Apochromatic triplet refractor, the day it arrived (left) and today (right)--that's an FPL-53 front element, 81mm aperture, 478mm focal length, 382mm at f/4.7 with the field flattener. I've been capturing the beautiful night sky with this awesome piece of hardware for nearly seven years. I bought it August 2015 and here we are in May 2022. I have captured hundreds of nebulae, galaxies, supernova remnants, and various phases of our moon. Over the last seven years I've captured the distant light of stars and emission nebulae in thousands of one, two, five, and ten-minute sub-exposures. Here's to seven more. Thank you, William Optics!
The evening weather hasn't been super clear lately, but it has given me a chance to test out the all-sky camera. This takes a pic every 20 seconds through the night and creates a time lapse video, a stacked star-trails image, and a keogram (a keogram is single image visualization that represents an entire night's atmospheric activity). And of course, there's a "Live View" so I can always check on the sky's current conditions.
I'm running Thomas Jacquin's awesome open-source AllSky software for Raspberry Pi:
Today starts my first full test of the setup, with daylight solar charging and continuous operation. Here's the current system, with the dome taped down for now, ZWO ASI120MC color astro camera, 6mm super wide-angle lens, Raspberry Pi 3b, lithium battery, and 10 amp PWM solar controller. I drilled two holes for ventilation, mainly to test the need for separate dew control. My hope is that there's enough heat rising from the Raspberry Pi CPU fans to get warm air circulating under the dome--with enough of a temp boost to prevent condensation from forming.
Examples of the camera's field of view from the back deck, close to the house. I have it in the middle of the backyard now, with a full view of the sky.
A snapshot from the April 3rd night sky:
When you see reviews of Newtonian astrographs, you sometimes hear—if the option is available—that you should get the carbon fiber version—for a variety of good reasons, including reduced weight, less susceptible to temperature fluctuations like steel tubes, and that cool carbon fiber look. Steel OTAs (Optical Tube Assemblies) are cheaper, but they also have an underutilized ferromagnetic advantage—meaning magnets love to stick to them. I bought a ZWO EAF (auto focuser) for my Apertura 8" (203mm) newtonian with an 800mm focal length at f/4, but I ended up removing it and using it with the William Optics SpaceCat 51. I have a couple Deep Sky Dad autofocusers, including a new-ish AF3 I had bought for a scope I sold.
I was tinkering with options for using the AF3 for focusing the Newt. The obvious approach would be to use something like the ZWO EAF bracket and couplers. Or... I could take a completely different path, using strong magnets to hold down the focus motor, with a timing belt to turn the 10:1 reduction focus knob.
This idea wasn't completely out of the blue. I've been using these neodymium bar magnets for a while to hold down Raspberry Pi devices and my Pegasus Pocket Powerbox. I bought a pack of adhesive-backed metal plates (link below) to hold down these devices. But I don't need them for newtonian scope because the whole thing is steel.
First off, these magnets are crazy strong, and you have to be careful with them. Get them around your tools or a box of hex bolts and it's like angry Magneto tussling with the X-Men. But if you glue two of these to a spare guide cam dovetail bracket, you have the perfect base for your autofocus motor on a steel OTA. I always tape over the magnets with gaffers tape (cloth tape) so I don't scratch my scope. I then glued a small 32mm dovetail to the AF3 motor housing and that's it. I let the E6000 glue dry for 24 hours, and tested out the setup up in N.I.N.A. The belt tension was easy to dial in by carefully sliding the dovetail base and testing the belt. I'm happy to report that everything seems to work well. The motor isn't going to budge with two of these magnets holding it down.
E6000 industrial-strength glue
Neodymium Bar Magnets
Adhesive metal plates for magnets
I just ordered the Baader coma corrector for my 800mm f/4 Newt, and spent a few minutes getting the 55mm distance to sensor spacing right for the ZWO ASI071 camera.
It's a gray rainy autumn day, so a good time to do a little upgrade. I ordered an ⅛" (3.2mm) thick aluminum spacer (6"/152mm OD x 3"/76mm ID) for the Orion Atlas pier adapter I use for both the Orion and the Sky-Watcher EQ6-R Pro. The Orion Atlas sits perfectly flat in the machined center and cut out ring, but the EQ6-R Pro sits slightly higher because the depth of the center is probably millimeter too shallow. Although the center bolt held it in place, the EQ6-R Pro always sat at a slight angle. I never liked that. Time to fix it with an aluminum spacer. I drilled a hole for the azimuth post, and we're back in business!