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Backfocus or tilt?


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I've just started imaging with my recently purchased ASI2600MC and I'm noticing what I first thought might be a backfocus issue but I'm starting to think might be tilt.  Up till now I've used an ASI533Mc camera and haven't seen this before. I'm pretty certain the backfocus is right - I'm using the WO flat73R which has a backfocus of 56.8mm so the 17.5mm from the sensor to the camera front plate plus the 21mm and 16.5mm extensions give me 55mm to which I then add 1.8mm adjustment on the adjustable reducer.  This measurement has always worked well with the ASI533 but I'm getting eggy stars with the ASi2600.  The thing is, these eggs look much worse on the top left and bottom right corners although the right hand side also shows some distortion.  

I've attached two subs below, the first - of the western veil - looks pretty bad in all 4 corners.  I adjusted the reducer out a millimeter before I took the second image (andromeda) and some of the distortion has been reduced but it's still there AND it still looks worse down the left hand side (at least to my eyes).    

So, what do you think?  do I simply need to wind the reducer out a bit further or do I have a tilt issue?  

Graeme

westveil.jpg

andromeda.jpg

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Looking at your second image Andromeda I'd say it's still backfocus and needs to come out a bit more as they are still pointing outwards in all 4 corners but to a lesser degree 

Cheers 

Lee 

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I would agree. Adjust your back focus until at least one corner is right. After that it is probably tilt. Looking at the image, I think if you get it close enough you probably won't notice any tilt unless you pixel peep.

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JackO, I took the liberty of downloading your images, and since you specify what camera you used- I had my plate solver measure the images and give the measured focal length for the images. Assuming the ASI2600 was used for both images: The measured focal length for the Andromeda image is 346.2mm, and for the westveil it is 347.6mm. That is a 1mm shift in focal length between the two, one possibility is one includes a filter in the optical path that the other does not. But the focal lengths should be almost identical when the images are in focus.

So what SHOULD the focal length be with the flatner and without it? You did not specify what optics train was involved in each case. I thought the results worth sharing as they may help suggest adding or removing spacers until the design focal length matches the measured value.

Note- the plate solver does not know what sensor is being used, so several common cameras are given as possibilities. It assumes the correct value can be chosen from the table. Also the output is in HTML format, which is a container for a copy of the image, annotations, etc.

I will attach the plate solver output for each image. Hopefully this helps with deciding what correction may be most productive.

andromeda.jpg.962de187d741884cc019fd3cdcf384b7_data8.html westveil.jpg.0473a1dc1ef3505f30e1aa15b1151e34_data8.html

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8 hours ago, SoftwareDeveloper said:

JackO, I took the liberty of downloading your images, and since you specify what camera you used- I had my plate solver measure the images and give the measured focal length for the images. Assuming the ASI2600 was used for both images: The measured focal length for the Andromeda image is 346.2mm, and for the westveil it is 347.6mm. That is a 1mm shift in focal length between the two, one possibility is one includes a filter in the optical path that the other does not. But the focal lengths should be almost identical when the images are in focus.

So what SHOULD the focal length be with the flatner and without it? You did not specify what optics train was involved in each case. I thought the results worth sharing as they may help suggest adding or removing spacers until the design focal length matches the measured value.

 

Thanks for this. Sorry, but I have no idea what your plate solver is telling me.  The Western veil was indeed taken with an l-enhance filter. I'd removed the filter for the M31 image but I also adjusted the reducer out another mm.  I should have stated that I was using a William Optics ZS73 with the Flat73R 0.8 reducer giving a focal length of 344mm.  For last night's attempt on the north american nebula I set the reducer adjustment to 3mm instead of the 1.8mm advised and overall the blobbiness has gone down apart from down the left hand side. (there's still a tiny bit in the right hand corners but nowhere near as prominent as earlier.   Next clear night I think I'll swap the reducer for the flattener and see if the problem is restricted to the reducer. 

Graeme

ngc7000.jpg

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1 minute ago, jacko61 said:

Thanks for this. Sorry, but I have no idea what your plate solver is telling me.  The Western veil was indeed taken with an l-enhance filter. I'd removed the filter for the M31 image but I also adjusted the reducer out another mm.  I should have stated that I was using a William Optics ZS73 with the Flat73R 0.8 reducer giving a focal length of 344mm.  For last night's attempt on the north american nebula I set the reducer adjustment to 3mm instead of the 1.8mm advised and overall the blobbiness has gone down apart from down the left hand side. (there's still a tiny bit in the right hand corners but nowhere near as prominent as earlier.   Next clear night I think I'll swap the reducer for the flattener and see if the problem is restricted to the reducer. 

Graeme

ngc7000.jpg

I'll be interested to see what happens when you use the non-reducing flattener. 

I've got the WO FLT120 and 0.8 reducer and in a similar position to you (moving from ASI533 to QHY268M, which is same sensor as ASI2600). I had a bit of backfocul eggy stars even though I dialled the back focus in exactly as the flattener requires, with the correct adjustable distance for the WO flattener. 

Before messing with tilt, check that your focus tube is not sagging and if it is, see if you can tighten up some of the screws to make it solid. Then check that the adjustable ring on the WO flattener is snug, and not causing tilt. What I did was loosen the locking ring, pull back on the camera so the extension ring is flush against the flattener threads, then tighten the locking ring. This should keep the imaging train flat to the flattener. 

Attached are two images. The first (Cygnus wall) is with camera too far away (like 10mm too far...). The second image (Sh2-171) is with back focus really close to what it should be, but out just a little bit. Ignore the processing of the images (just learning how to process narrowband data!), just look at the star shapes in the corners. Unless I pixel peep, I cannot tell the star shapes look bad so I might settle for now. 

156340675_NGC7000SHOpartial.thumb.jpg.bea1f5c04f68cce0b22d4fa4056de5d1.jpg

1853678215_Sh2171SHO90minstotal.thumb.jpg.d7654cefa618755fec90caeba0b77d29.jpg

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Posted (edited)

Those are interesting results @Richard_  Everything I've read tells us that the sensor is too close if the edges look star-treky but your image at 10mm TOO FAR shows just that. Very odd.   I did notice some play in the focus tube so I've tightened the 2 screws to take that out although they only address the vertical movement - it would be better if they had adjustments at 120 degrees around the tube.  This last image is the first I took with the new camera - 20th July.  Same scope but it has the Flat73A - i.e. flattener, not reducer.  In this one I paid very close attention to getting the backfocus exactly on spec i.e. 11.4mm (ish) on the adjuster and the edge stars don't look too bad at all. I think I'll have a play with the focuser and make sure the reducer is set to exactly 1.8mm next session. 

Graeme

eastveil.jpg

Edited by jacko61
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5 hours ago, jacko61 said:

Those are interesting results @Richard_  Everything I've read tells us that the sensor is too close if the edges look star-treky but your image at 10mm TOO FAR shows just that. Very odd.   I did notice some play in the focus tube so I've tightened the 2 screws to take that out although they only address the vertical movement - it would be better if they had adjustments at 120 degrees around the tube.  This last image is the first I took with the new camera - 20th July.  Same scope but it has the Flat73A - i.e. flattener, not reducer.  In this one I paid very close attention to getting the backfocus exactly on spec i.e. 11.4mm (ish) on the adjuster and the edge stars don't look too bad at all. I think I'll have a play with the focuser and make sure the reducer is set to exactly 1.8mm next session. 

Graeme

eastveil.jpg

That's what threw me. In my "wisdom", I had accidentally put a 20mm spacer into my imaging train thinking it was 10mm, hence the +10mm distance I was out by 😊 Yet, the stars were saying that the camera was too close so perhaps the star shapes are only a useful indicator when you're within a few millimetre to focus? 

The stars look lovely and flat in your image! Perhaps the back focus distance is a bit more critical for flattener with reducing optics than flattener which do not have reducing optics? 

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