Looking for a solution for the backfocus issues using a Skywatcher 0.77X reducer with my Esprit 120ED. I am currently on a ZWO 294mm camera so not an APS-C but still see elongated stars at the corners. This is with 55mm backfocus from the back of the FR (I added 0.66mm of shim to cover the filter thickness). Here is how my aberration looks like right now:  I know that Lucomatico on his YT channel said he is using 59.5mm of backfocus. Is that what everyone else is using as well? Thanks! Devesh (AstroDarks)
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It looks like you are still to close. Keep adding shims…
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hey devesh,
I used the 0.77 reducer together with an apsc sensors and only had minimal issues with the corner stars. guess I used something like 56 or 57mm.
cs
anderl
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The stars on the right of the Image don’t look too bad at all, it’s on the left they are elongated, so are you sure this is not a bit of tilt..?
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AstroStew:
The stars on the right of the Image don’t look too bad at all, it’s on the left they are elongated, so are you sure this is not a bit of tilt..? Checked the tilt and it's not too bad.
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Well the stars are not too bad, so how bad is the tilt 🤔
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It looks like you are a little too close still, and there is a bit of tilt. I suggest that you fix the tilt first and then adjust the spacing a little further out if it's still needed. Getting your field perfectly flat can be a bit of a pain.  |
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Update: Tried to fix the tilt and its at 16%. ASTAP says moderate tilt  When I again look at the stars in the corner, they look horrible:  This is currently exactly at 55mm backfocus with 0.6mm shim added for the filter.
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Looking at the latest image, I still think the sensor is too close to the reducer/flattener. Please continue to add shims to at least one corner is round…
CS Tim
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16% is more than enough to give those star shapes, I had stars like that at 5% tilt
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The stars at the bottom left and right look pretty good, but the top right and left look poor, and the goes with the ASTAP tilt analysis above, as your sensor is tilted top to bottom, so more adjustment needed I think. You may consider one on the laser tilt adjusted from www.Astroprecision.no, it will make easy work of adjusting that out
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I got ok results with 58mm Back Focus and APS-C. I used N.I.N.A to do the tests, see my journey here: https://skellner.github.io/projects/Back%20Focus/You will never get it 100% as the focuser will introduce a bit tilt as well. With tilt, I had a similar process (different scope, different camera), see here: https://skellner.github.io/projects/IMX455Tilt/CS Stefan
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Stefan Kellner:
I got ok results with 58mm Back Focus and APS-C. I used N.I.N.A to do the tests, see my journey here: https://skellner.github.io/projects/Back%20Focus/
You will never get it 100% as the focuser will introduce a bit tilt as well.
With tilt, I had a similar process (different scope, different camera), see here: https://skellner.github.io/projects/IMX455Tilt/
CS
Stefan The tool I linked to above makes such easy work out of tilt issues, and no messing in the dark and cold, just a 3 min job indoors in the daytime and warm..👍🏻
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I really dont know where the tilt is coming from. Below is aberration inspector run on M101 ran with the same scope but this time without the reducer. This is flattener only. As you can clearly see, there is no tilt and no issues with backfocusing as well. Stars are nice and round corner to corner.  I am goign to try 60mm backfocus next clear night and will report out the results. Thanks.
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Hey @AstroDarks,
I also have an Esprit 120 and ran into the same issues as you when using the reducer. I had a completely flat field with just the flattener exactly the same as you. You can def try spacing but I really do not think this is a spacing issue. I tried everything from 50-60mm without any improvement. I was equally frustrated so understand how this process is upsetting.
The reducer could be adding tilt or other aberrations in a number of different ways. I also found it increases the effects of any tilt in your imaging train. Even though you are using the specific reducer made by skywatcher for the Esprit 120, it can still not match precisely and cause these issues.
You can use the tilt plate on your camera if there is one, or purchase one to get rid of it. I would also call Skywatcher customer support as I did. They can be very helpful and at least tell you if there is a defect with the reducer. I ended up just using the flattener and returned the reducer.
Feel free to DM me and I can explain everything Skywatcher explained.
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I own both the skywatcher flattener and the reducer for the 120ED and I can confirm that the reducer doesn't deliver as well as the flattener, the overall star quality is definitely a bit lower. This is true for all my scopes, the reduced field is never as good as the flattened field. Also, the reducer makes the field more sensible to tilt overall.
I do not mind that as the overall quality is still outstanding and minor aberrations can easily be fixed in processing. To be honest: I do not find the stars in your above photo so horrible, easy to fix with a little bxt.
Maybe you can exchange the reducer for a different one or try a Riccardi to improve it a bit. Also, you can try find out where the tilt comes from: is it really the sensor (very rare) or something else in the imaging train?
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Hey @AstroDarks,
I also have an Esprit 120 and ran into the same issues as you when using the reducer. I had a completely flat field with just the flattener exactly the same as you. You can def try spacing but I really do not think this is a spacing issue. I tried everything from 50-60mm without any improvement. I was equally frustrated so understand how this process is upsetting.
The reducer could be adding tilt or other aberrations in a number of different ways. I also found it increases the effects of any tilt in your imaging train. Even though you are using the specific reducer made by skywatcher for the Esprit 120, it can still not match precisely and cause these issues.
You can use the tilt plate on your camera if there is one, or purchase one to get rid of it. I would also call Skywatcher customer support as I did. They can be very helpful and at least tell you if there is a defect with the reducer. I ended up just using the flattener and returned the reducer.
Feel free to DM me and I can explain everything Skywatcher explained. Sent you a DM. Thanks.
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Stefan Kellner:
I own both the skywatcher flattener and the reducer for the 120ED and I can confirm that the reducer doesn't deliver as well as the flattener, the overall star quality is definitely a bit lower. This is true for all my scopes, the reduced field is never as good as the flattened field. Also, the reducer makes the field more sensible to tilt overall.
I do not mind that as the overall quality is still outstanding and minor aberrations can easily be fixed in processing. To be honest: I do not find the stars in your above photo so horrible, easy to fix with a little bxt.
Maybe you can exchange the reducer for a different one or try a Riccardi to improve it a bit. Also, you can try find out where the tilt comes from: is it really the sensor (very rare) or something else in the imaging train? The issue is corner stars are trailing. And when I stack the image, you can see a big tail on couple of stars. I guess that's happening because I am dithering each frame. You are right that if stars are a little eggie, BXT will fix it but the tail, I am afraid not (I did blink each frame and I cant see this tail in any of my individual frames). Thanks for your response. Devesh
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Hi, I had the same problem last year with the Skywatcher reducer on my Esprit and eventually switched to the Riccardi reducer. I had problems dialing in backspacing. I eventually noted that my corner stars are distorted only when using L and B filters, R and G filters were well or at least much better corrected. In the images below, backspacing was identical. The filters are Baader CMOS-LRGB filters. With the Riccardi, correction was much better. Best, Dirk Blue:  Red:  |
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With these Esprit scopes the issue is the focus, the Critical focus zone is small, and when outside of this your can get misshapen stars, it’s a very fine line between perfectly in focus with nice round stars, and slightly out and misshapen stars, I think that might be the issue, it certainly happens when using different filters and the focus needs a tweak.
its not a bad thing and these are great scopes I have the 100mm version, but they can take a bit to get dialled in 👍🏻
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UPDATE: Last night, I tested with 59.5 mm backfocus like several others have tried. I got considerably rounder stars at the edges. There is a moderate (17%) tilt in my system due to the reducer (eventhough everything is threaded. My flattener shows no tilt) but is this the best that the reducer could do?  I then ran BXT on this stacked image and it looks good to me. Thoughts?  |
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Those examples look great. You can still play a little with shims on moving the distance. However, it appears you are very close so the juice might not be worth the squeeze on what a small improvement it would make, if any.
I know you mention you have some tilt remaining. Is there improvement with the tilt analysis pre and post BXT?
Thanks Tim
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Tim Ray:
Those examples look great. You can still play a little with shims on moving the distance. However, it appears you are very close so the juice might not be worth the squeeze on what a small improvement it would make, if any.
I know you mention you have some tilt remaining. Is there improvement with the tilt analysis pre and post BXT?
Thanks Tim It did get a little bit better - 12% from 17%
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Thanks for the reply on the tilt analysis. I was curious and impressed that BXT improved tilt that much…
Cheers!
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