Edge HD 9.25 and Esatto 2" focuser

Celestron EdgeHD 9.25" Equipment Focuser
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Jerry Gerber avatar
Jerry Gerber avatar
I am installing an Edge HD 9.25 with the Esatto 2" focuser at a dark sky site. 

I understand there's 146.05mm of backfocus. i will be imaging at native focal length (2350mm) and will not be using a reducer.

Image train will be something like this:
Scope - 2mm adapter - focuser (67mm) - OAG  (?) - filter wheel (20mm) - ASI 2600MC camera (17.5mm). 

The problem seems to be the OAG.  Although it's 29mm in thickness, it needs an SCT adapter (or does it?) on the scope side and 12.5mm adapter on the other side.  This doesn't seem like it's going to work. 

Anybody using the Edge 9.25 with the Primaluce Esatto 2" focuser and the Celestron OAG?

Thanks!!
Jerry
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Scott Badger avatar
Scott Badger avatar
If you’re not using a reducer,  don’t you just need to make sure you’re optical train length puts you in the middle of the focus range vs a specific BF length? Put another way, aren’t you changing the “backfocus” length whenever you adjust focus? I thought BF is a set distance between a reducer/flattener/corrector (if you’re using one) and the sensor. All that said, some diagrams of the 9.25 edge show a corrector built into the primary baffle….

Cheers,
Scott
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andrea tasselli avatar
andrea tasselli avatar
Jerry Gerber:
I understand there's 146.05mm of backfocus. i will be imaging at native focal length (2350mm) and will not be using a reducer.


That is the optimal backfocus but not necessarily the one right for you and in fact small variations (of few mm up or down) won't make a lot of difference in the end results. In addition you can remove the tilt plate and fasten the FW (assuming it's ASI compatible) directly to the back-face of the camera, as shown in the ASI website, thus gaining an extra 5mm. Also use a ASI OAG-L will fit the bill.
John Hayes avatar
John Hayes avatar
andrea tasselli:
Jerry Gerber:
I understand there's 146.05mm of backfocus. i will be imaging at native focal length (2350mm) and will not be using a reducer.


That is the optimal backfocus but not necessarily the one right for you and in fact small variations (of few mm up or down) won't make a lot of difference in the end results. In addition you can remove the tilt plate and fasten the FW (assuming it's ASI compatible) directly to the back-face of the camera, as shown in the ASI website, thus gaining an extra 5mm. Also use a ASI OAG-L will fit the bill.

The tolerance on the back working distance varies with the size of the sensor.  Since Jerry is planning on using a “full-frame” sensor, he’ll need good image quality out to a 42 mm image circle.  Getting the spacing right is critical to minimizing astigmatism in the field.  I know that on the Edge14, the tolerance at an image height of 24 mm is roughly +/- 50 microns.  I’m not familiar with the Edge 9.25 but I’m sure that the spacing tolerance will be much tighter than “a few mm”.   Using an external focuser to make very small adjustments to within ~ +/- 3*DOF will work but it wouldn’t be my first choice for holding focus on a SCT.  I personally think that the SMFS system from Optec is a better way to go.

John
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andrea tasselli avatar
andrea tasselli avatar
The OP's intended sensor is an IMX571 so 28 mm image circle diameter and I'll reiterate my original statement, few mm difference won't show up in a meaningful way in the image circle. I also know as I tested it on a friend's scope a while ago.
Jerry Gerber avatar
Jerry Gerber avatar
Yes, I've thought about a full-frame sensor, but will most likely go with the ASI2600MC, not quite full frame, but not small either.  I'm already dealing with 50MB subs and my SSD drives are filling up fast.  The file size of full-frame are over 100MB per frame, so I think I'll stick with the ASI2600, which gives excellent results.
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John Hayes avatar
John Hayes avatar
andrea tasselli:
The OP's intended sensor is an IMX571 so 28 mm image circle diameter and I'll reiterate my original statement, few mm difference won't show up in a meaningful way in the image circle. I also know as I tested it on a friend's scope a while ago.

Right.  I some how flipped 2600 to read 6200.   If you've tested it on the same scope with the same sized camera and found that that +/- 1-2 mm doesn't make a difference--and you carefully checked the astigmatism, then I'll buy it.  It is a LOT more restrictive on a Edge 14 with a larger sensor.

John
Jerry Gerber avatar
Jerry Gerber avatar
I am assuming that the image train for the Edge 9,.25 and the Esatto 2" would be as follows:


SCOPE - 3.3" adapter - Focuser - Celestron OAG - ZWO filter wheel - Camera

Does this seem right?   Since I will not be using a reducer I am not sure I need to worry about backfocus as the Edge 9.25 has 146mm of backfocus, which should be enough.

Anyone disagree?

Thank you!
Jerry
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Charles Tremblay-Darveau avatar
Charles Tremblay-Darveau avatar
Can someone confirm that this focuser works for full frame with and without reducer on the edge 9.25? Any vignette or reflections/scatter?
ScottF avatar
ScottF avatar
Charles Tremblay-Darveau:
Can someone confirm that this focuser works for full frame with and without reducer on the edge 9.25? Any vignette or reflections/scatter?

I asked that question to someone I know that uses it with full frame on an edge 14" and he says he has no vignetting issues.
ScottF avatar
ScottF avatar
Jerry Gerber:
I am assuming that the image train for the Edge 9,.25 and the Esatto 2" would be as follows:


SCOPE - 3.3" adapter - Focuser - Celestron OAG - ZWO filter wheel - Camera

Does this seem right?   Since I will not be using a reducer I am not sure I need to worry about backfocus as the Edge 9.25 has 146mm of backfocus, which should be enough.

Anyone disagree?

Thank you!
Jerry
Website
Astro Music Videos

I have almost the same setup. I racked the focuser out to the halfway point and then used adapters/spacers to get my 146mm BF distance; sequence is sct adapter, 2" esatto, adapter to two OAG-L, 2" FW, and 2600MC. Due to other ongoing projects, I haven't managed a night with it yet, so I can't say how well it performs. However, I am almost done with my present object list and will be mounting it shortly.
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Kostas Papageorgiou avatar
Kostas Papageorgiou avatar
With this setup you reach 135.5mm of backfocus...so you are off 10.55mm.. i think you can focus with main focuser on telescope then extent the essato focuser to reach optimal back focus.. i was looking too for this. Maybe this will help you: 
https://eu.primalucelab.com/aplanatic-schmidt-cassegrain/celestron-edgehd-925-with-losmandy-bar-and-esatto-2.html

Kostas.
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