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New FR for Edge 9.25": Critical back focus distance

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#1 schmeah

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Posted 21 July 2018 - 11:16 AM

I received the long awaited 0.7x focal reducer for the 9.25" Edge. It is a heavy beast. I have it paired with a QSI 640 wsg and have an Optec TCF-SI in the imaging train. With the FR I now have an image scale of 0.93"/pixel. Previously, at F/10 the Edge has always given me round stars to the corners, despite the fact that I suspected that I might have been slightly long on the recommended optimal backfocus distance (146mm). The relatively small imaging circle (22' vs 31.7' with the FR) was fairly forgiving I suspect. I was happy that per Celestron the backfocus distance is unaffected by the new FR. My first test images with this setup, however, were suboptimal. After critical focusing with the Optec at mid focus position, there was severe corner star abberation. I assumed it was tilt because it affected primarily the right upper corner:

 

EdgeFRbadRU.jpg

 

However, after moving the Optec focuser in by 2500 steps (about 5.2 mm) and refocusing initially with the primary focuser then relocking the primary, then fine focusing again with the Optec,  test exposures now showed nice stars to the edges. I'm still curious why incorrect backfocus distance would produce such aberration in one corner only. Any thoughts? This is a single two minute Lum exposure (after adjusting the backfocus by 5 mm), with minimal processing (levels).

Full resolution of edge/corner crops:   

http://www.pbase.com...image/167842242

 

Derek

 

EdgeFRTestFinal.jpg


Edited by schmeah, 21 July 2018 - 11:19 AM.


#2 jfrech14

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Posted 21 July 2018 - 11:58 AM

I am looking forward to testing mine out with the FR and STT8300. I know I tried getting my sensor to 146mm by pulling the nosepiece out a bit and it introduced tilt that was only seen on the left side. So, even around 12mm short of 146mm I was seeing perfectly round stars from edge to edge. Until I get the Optec Leo, I will still be around 12mm short when I have the FR on. How is your collimation? Also, did you focus at a low altitude? I have noticed I don't get very good focus across the sensor when I lock my mirrors at a very low altitude. I haven't investigated why, but I just make sure to focus above 45 deg so I can hopefully avoid needing to refocus as it goes across the sky (I don't have a rear focuser as of now). I accidentally locked and focused my 11 edge once and it caused some issues at the edges but that was years ago and I don't remember the details of it.

 

Also, celestron says it is 42mm imaging circle with and without the FR. What imaging circles were you referring to above? They have spent a ton of time on the 9.25 reducer because apparently, they put some of the most effort into the 9.25 scope out of all 4 ( I heard someone say that somewhere). So I would expect their claim of 146mm to be right. Also, celestron recommends being short rather than long if you can manage it.


Edited by jfrech14, 21 July 2018 - 12:01 PM.


#3 schmeah

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Posted 21 July 2018 - 01:24 PM

I am looking forward to testing mine out with the FR and STT8300. I know I tried getting my sensor to 146mm by pulling the nosepiece out a bit and it introduced tilt that was only seen on the left side. So, even around 12mm short of 146mm I was seeing perfectly round stars from edge to edge. Until I get the Optec Leo, I will still be around 12mm short when I have the FR on. How is your collimation? Also, did you focus at a low altitude? I have noticed I don't get very good focus across the sensor when I lock my mirrors at a very low altitude. I haven't investigated why, but I just make sure to focus above 45 deg so I can hopefully avoid needing to refocus as it goes across the sky (I don't have a rear focuser as of now). I accidentally locked and focused my 11 edge once and it caused some issues at the edges but that was years ago and I don't remember the details of it.

 

Also, celestron says it is 42mm imaging circle with and without the FR. What imaging circles were you referring to above? They have spent a ton of time on the 9.25 reducer because apparently, they put some of the most effort into the 9.25 scope out of all 4 ( I heard someone say that somewhere). So I would expect their claim of 146mm to be right. Also, celestron recommends being short rather than long if you can manage it.

Hi Jfrech. Those are good questions. My collimation was good, but I haven’t checked it in over a year. Until then it had held collimation very well. I focused on Vega which was fairly high in the sky. Regarding the “imaging circle”, ignore that. I was referring to my FOV in arc-min, and relates to my complete ignorance of the terms and concepts. My assumption was that because of the relatively small size of my chip (15x15mm), the aberrations present at the edges of the imaging circle without the FR were well outside my FOV, but with the FR I’m seeing closer to the edge of the imaging circle.

 

Derek



#4 roofkid

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Posted 21 July 2018 - 01:29 PM

Hi Derek,

 

interesting to know that they finally came through with it. How much is it?

 

I can think of a few scenarios where it could affect only one corner:

- tilt (most likely, had that a few times when I do not lock the 360 rotator tightly on my Esprit

- If you do not image the center of the projected image perfectly but rather offset to it you could see aberations only in one corner

 

On the Esprit (f/5.5) I need to be within ~0.2mm to get a "perfect" image, so it's nice to have the capability to put the reducer into the focuser (or at least that is how I understood what you have described).


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#5 jfrech14

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Posted 21 July 2018 - 01:47 PM

The reducer is in the 300s. I don't remember exactly but the quantities are ridiculously small it seems. I ordered mine months ago and was backordered until August  thankfully they came through sooner.

 

 

Normally I would expect a reducer to do as you guessed but this reducer is pretty intense and they claim that the reducer also keeps a 42mm imaging circle so you should be good edge to edge in either setup. As said above, tilt definitely seems to be a possibility  but if nothing changed except your position on the rear focuser, that doesn't seem likely. If you moved the primary and rear focuser, then it's possible that something occurred to throw the combo off or that collimation is slightly off to cause being too far from the 146mm to show aberrations and being out of collimation would throw the optical axis off to the side a bit. I would just check collimation, get as close to the 146mm as possible with the primary and then experiment by moving the rear focuser in and out to see how it affects it. With a 20-25mm sensor, we should see nearly no difference. Especially with your larger pixels. It might have been one of those perfect storms of changing too many things at once and one or all of them contributed to the aberrations  


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#6 schmeah

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Posted 21 July 2018 - 08:25 PM

Hi Derek,

 

interesting to know that they finally came through with it. How much is it?

 

I can think of a few scenarios where it could affect only one corner:

- tilt (most likely, had that a few times when I do not lock the 360 rotator tightly on my Esprit

- If you do not image the center of the projected image perfectly but rather offset to it you could see aberations only in one corner

 

On the Esprit (f/5.5) I need to be within ~0.2mm to get a "perfect" image, so it's nice to have the capability to put the reducer into the focuser (or at least that is how I understood what you have described).

Hi Sven. Don’t think it was tilt because it resolved just by decreasing the backfocus by bringing in the draw tube of my rear focuser by 5mm. The Optec does not have the capability to fit a FR within it like the moonlite. And this FR is too big to fit into anything. 

 

Derek



#7 schmeah

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Posted 22 July 2018 - 07:16 PM

A theoretical concern. When using a rear cell focuser like the Optec, every adjustment moves the backfocus distance to the sensor. The optimum distance is 146mm, and the tolerance spec by Celestron is +/- 0.5mm. Clearly from my example above 5mm has a large effect on peripheral star quality. The measured temperature coefficient of my Edge is approximately 100 microns / degC. So say the temperature dropped 15C (extreme example) overnight, moving the sensor in 1.5mm, and say I was using a CCD with a large chip that utilized the full imaging circle of the OTA, couldn’t just the movement of the focuser cause enough astigmatism to effect the star quality in the corners?

 

Derek



#8 jfrech14

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Posted 22 July 2018 - 07:38 PM

If you have something like a 16803 then you might very well see some issues introduced by temperature compensation which is exactly why optec offers secondary focusers for the larger scopes. For a small sensor, the central part of the imaging circle should stay flat enough over the course of just correcting temperature assuming you have the zero point at 146mm and a well collimated scope with an imaging train that is properly adjusted to the optical axis. Like I said, I am almost 12mm away from 146 last time I calculated it and I have pinpoint stars edge to edge on my 8300. But I can try to get a better number on the 12mm for you.

Edit: I was pretty wrong haha. I'm around 6mm short of 146mm. Not sure how I got 12mm before haha

Edited by jfrech14, 22 July 2018 - 07:46 PM.



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