I too have the AT72EDII and the AT flattener/0.8 reducer. One thing I noticed watching your video, you are running the field flattener in to the 2” compression ring accessory holder (the black ring with the 3 thumb screws you mention). I removed the 2” compression ring accessory holder and screwed the flattener straight in to the rotator (red ridged ring) My back focus is working well at 55mm. I wonder if that would change the back spacing needed for you, as the flattener is picking up the image cone in a different place?
It's been awhile, but I think I did try that same approach with a different flattener (maybe the Williams Optics?) and still had problems. I ended up selling the telescope because I just wasn't getting much use out of it. I hope to try a RASA 8 when I get back into the hobby. Glad to hear it's working out for you, though!
I am having a similar issue. My next test is to se what I get without the field flattened. If the stars look all symmetric then I would guess it’s the flattener. If it’s to one side then it could be flexure or collimation. Have you rotated the camera to see which way the comma moves if at all?
Hi good work on this, any update? I just ordered this scope with the 0.8 RF/FF and plan to test the 0.6 RF/FF on it making it an F/3.6 fast scope at 254mm.
I was really hoping to get this scope sorted out, but just couldn't get the stars to be acceptable. I ended up selling it. I might try a RASA 8 in the future.
@@deepskyutah5257 yeah I now expect to be returning it! On SVBONY site they say 1/3 to 1/2 inch sensors as it has too much softness in the outside from the center!
Great video. I thought I would chime in with my recent experience. I have a William Optics Z73 with a William Optics Flat 73r flattener/0.8 reducer and struggled like you have. A friend suggested N.I.N.A. Hocus Focus Aberration Inspector. Great tool. It tells you if you need to increase or decrease your back focus and measures tilt. I dialed in my flattener in one evening. I ended have to adjust my flattener 1.7mm beyond the initial recommended. The initial recommended is for Crop Sensors and mine is a 4/3. I have no idea why that would make a difference. I ordered some Astrodymium tilt rings and adjusted my tilt. Problems solved quick and efficient.
Thanks for the reply! It's so frustrating trying to adjust backspacing and tilt in the dark when you're basically just guessing. I will definitely look into N.I.N.A.
Good video showing the examples of the different back focus lengths. I too have been playing around with the same issue using the Orion Flattener for short focal length refractors. (Mine is the Orion ED80T). Are you sure your image train is perfectly aligned? That would cause the same effect of oblong stars in the corners. One way to maybe check that out is the set the backspacing to the recommendations ... take a test image, then rotate the optical train, say 180° and take another test image. If the deformed stars are in the opposite quadrant, then I would think it's a backspacing issue. If the deformed stars are in the same quadrant, then it might be an error in optical train alignment. Just a thought. (Patrick Prokop, Savannah, GA)
Hi Patrick. Thanks for the comment and suggestion! I did try rotating the optical train, and the stretched stars did seem to rotate along. I picked up the Orion flattener for short focal lengths, and I did get better results with it over both the Hotech and Astro-tech flatteners....but still not perfect. I have the William Optics Flat 73a arriving tomorrow to try out. I think my Astro-tech telescope might be the same design as the William Optics 73II, and someone on Cloudy Nights claimed to get perfect stars with the WO Flat 73a + AT72EDII telescope. It's such a pain doing all the back and forth testing when the temperature is about 10 degrees out, haha.
Have the same issue with my AT80EDT and their field flattener. One solution I came across was to use a 'tilt corrector'. Way too much $$ snd more weight on the mount. In the end, my opinion is that it's a flaw in the flattener that the manufacturer should fix.
Thank you for the useful information! Have you later tried to figure the best back focus for the Hotech FF? I am having exactly the same issue as yours. My images taken with the Hotech CSA FF attached to my ES 80ED f6 show stretched stars on one side of the image while the other sidr is almost good. I have also doubted tilt, but really am not sure.
I have been tempted to look at getting one of those variable adapters when messing around with back focal lengths to speed up the process of determining optimum length, so can then get the correct fixed rings required. The Baader VariLock is an example of one of the ones I have seen. They have 2 versions, 20-29 mm and 29 to 46 mm.
Thanks, Dave. I have a cheap Svbony variable extension, but it's a little difficult to adjust without taking the imaging train off. I hear good things about the VariLock. I'll see if I can find a used one.
@@deepskyutah5257 then there is something very wrong with documentation. According to what you showed in the beginning for the flattener, the backfocus should be 57mm from the flattener, which is 17.5mm camera +40mm equipment with roughly 2mm filter, or + 39.5mm without filter.
@@bokvaj that was the whole point of making the video. The measurements given in the instructions did not produce good results. I am not the only person having this issue with this telescope. If you do some research on Cloudy Nights, you will see there are others with a similar experience. Contacting someone at Astrotech was no help. They are mainly just salesmen with limited experience or desire to help. There is probably an issue with the telescope, since several flatteners did not seem to work properly. In any event, I no longer have the telescope. I'll be getting a RASA8 instead. Best of luck to you.
that fieldflattner is not the best quality,i had the same one..also bad stars. you need a flatner special for short scopes. like these. www.teleskop-express.de/shop/index.php/cat/c138_Reducers-Converter-Correctors.html there are multiple versions.
I will do some research on flatteners for shorter scopes. It looks like Orion sells when here in the US. And now it makes sense why the image improved when I added backspacing. One telescop-express, one of the flatteners says the shorter the focal length, the longer the working distance must be. Thanks for your help. If I don't figure it out after this winter, I will just trade for an 8" RASA, haha.
I too have the AT72EDII and the AT flattener/0.8 reducer. One thing I noticed watching your video, you are running the field flattener in to the 2” compression ring accessory holder (the black ring with the 3 thumb screws you mention). I removed the 2” compression ring accessory holder and screwed the flattener straight in to the rotator (red ridged ring) My back focus is working well at 55mm. I wonder if that would change the back spacing needed for you, as the flattener is picking up the image cone in a different place?
It's been awhile, but I think I did try that same approach with a different flattener (maybe the Williams Optics?) and still had problems. I ended up selling the telescope because I just wasn't getting much use out of it. I hope to try a RASA 8 when I get back into the hobby. Glad to hear it's working out for you, though!
I am having a similar issue. My next test is to se what I get without the field flattened. If the stars look all symmetric then I would guess it’s the flattener. If it’s to one side then it could be flexure or collimation. Have you rotated the camera to see which way the comma moves if at all?
Hi good work on this, any update? I just ordered this scope with the 0.8 RF/FF and plan to test the 0.6 RF/FF on it making it an F/3.6 fast scope at 254mm.
I was really hoping to get this scope sorted out, but just couldn't get the stars to be acceptable. I ended up selling it. I might try a RASA 8 in the future.
@@deepskyutah5257 yeah I now expect to be returning it! On SVBONY site they say 1/3 to 1/2 inch sensors as it has too much softness in the outside from the center!
Great video. I thought I would chime in with my recent experience. I have a William Optics Z73 with a William Optics Flat 73r flattener/0.8 reducer and struggled like you have. A friend suggested N.I.N.A. Hocus Focus Aberration Inspector. Great tool. It tells you if you need to increase or decrease your back focus and measures tilt. I dialed in my flattener in one evening. I ended have to adjust my flattener 1.7mm beyond the initial recommended. The initial recommended is for Crop Sensors and mine is a 4/3. I have no idea why that would make a difference. I ordered some Astrodymium tilt rings and adjusted my tilt. Problems solved quick and efficient.
Thanks for the reply! It's so frustrating trying to adjust backspacing and tilt in the dark when you're basically just guessing. I will definitely look into N.I.N.A.
Good video showing the examples of the different back focus lengths. I too have been playing around with the same issue using the Orion Flattener for short focal length refractors. (Mine is the Orion ED80T). Are you sure your image train is perfectly aligned? That would cause the same effect of oblong stars in the corners. One way to maybe check that out is the set the backspacing to the recommendations ... take a test image, then rotate the optical train, say 180° and take another test image. If the deformed stars are in the opposite quadrant, then I would think it's a backspacing issue. If the deformed stars are in the same quadrant, then it might be an error in optical train alignment. Just a thought.
(Patrick Prokop, Savannah, GA)
Hi Patrick. Thanks for the comment and suggestion! I did try rotating the optical train, and the stretched stars did seem to rotate along. I picked up the Orion flattener for short focal lengths, and I did get better results with it over both the Hotech and Astro-tech flatteners....but still not perfect. I have the William Optics Flat 73a arriving tomorrow to try out. I think my Astro-tech telescope might be the same design as the William Optics 73II, and someone on Cloudy Nights claimed to get perfect stars with the WO Flat 73a + AT72EDII telescope. It's such a pain doing all the back and forth testing when the temperature is about 10 degrees out, haha.
Have the same issue with my AT80EDT and their field flattener. One solution I came across was to use a 'tilt corrector'. Way too much $$ snd more weight on the mount. In the end, my opinion is that it's a flaw in the flattener that the manufacturer should fix.
Thank you for the useful information! Have you later tried to figure the best back focus for the Hotech FF? I am having exactly the same issue as yours. My images taken with the Hotech CSA FF attached to my ES 80ED f6 show stretched stars on one side of the image while the other sidr is almost good. I have also doubted tilt, but really am not sure.
I have been tempted to look at getting one of those variable adapters when messing around with back focal lengths to speed up the process of determining optimum length, so can then get the correct fixed rings required. The Baader VariLock is an example of one of the ones I have seen. They have 2 versions, 20-29 mm and 29 to 46 mm.
Thanks, Dave. I have a cheap Svbony variable extension, but it's a little difficult to adjust without taking the imaging train off. I hear good things about the VariLock. I'll see if I can find a used one.
Hi man, i have a ZS81 and Flat 6AIII, and a qhy 165c, could you tell me what accesories i need to buy to have correct backfocus?, wich spacer? thanks
your camera will cut 17.5mm from that 57mm range.
so from the BASE of the camera you have to have ~40mm max to the flattener. which in my opinion you really dont have :) or at least ti looks like
Sounds like you didn't want the video. I tried different spacings from 55mm up to 80mm. The best result was around 73mm.
@@deepskyutah5257 then there is something very wrong with documentation. According to what you showed in the beginning for the flattener, the backfocus should be 57mm from the flattener, which is 17.5mm camera +40mm equipment with roughly 2mm filter, or + 39.5mm without filter.
@@bokvaj that was the whole point of making the video. The measurements given in the instructions did not produce good results. I am not the only person having this issue with this telescope. If you do some research on Cloudy Nights, you will see there are others with a similar experience. Contacting someone at Astrotech was no help. They are mainly just salesmen with limited experience or desire to help. There is probably an issue with the telescope, since several flatteners did not seem to work properly. In any event, I no longer have the telescope. I'll be getting a RASA8 instead. Best of luck to you.
that fieldflattner is not the best quality,i had the same one..also bad stars. you need a flatner special for short scopes. like these. www.teleskop-express.de/shop/index.php/cat/c138_Reducers-Converter-Correctors.html there are multiple versions.
I will do some research on flatteners for shorter scopes. It looks like Orion sells when here in the US. And now it makes sense why the image improved when I added backspacing. One telescop-express, one of the flatteners says the shorter the focal length, the longer the working distance must be. Thanks for your help. If I don't figure it out after this winter, I will just trade for an 8" RASA, haha.