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Celestron C9.25 SCT + Celestron OAG puzzle

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#1 Sean1980

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Posted 25 November 2021 - 06:45 AM

Think this type of question has probably been asked hundreds of times but need a little reassurance this is the right way to go. Will be getting a  used C9.25 SCT in December which I will use for planetary but will also want to use for DSO with the 0.63 reducer.

Have in mind the following imaging train: (before anyone asks am using the ZWO ASI183MM pro because there is no budget yet for a different camera+filters+filterwheel)

post-339093-0-68365500-1637830564.png

I can remove the 0.5mm shim as its purely decorative really to align the camera with filterwheel. This would mean the missing piece of the puzzle would be the connection from reducer to OAG would probably be an 11mm SCT to M48 adapter putting me 0.2mm over optimal backfocus.

 

Am planning on using the ASI174MM mini as a guidecamera, this however brings me to the next challenge which extension pieces are needed to get it parfocal with the main camera. I estimate approx 76mm from prism to main camera so the same for the guidecam.

Which gives me 76mm - 8.5mm (guidecam sensor depth) - 10mm Prism stalk (very unsure about this) so a 57.5 mm extension needed. Also about 8mm to work with on the adjustable focuser on OAG. but a 50mm or 60mm extension feels long to me.

 

Any help/insights or experiences are welcome

 

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  • C9.25 OAG image train.png


#2 DesertCrawler

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Posted 25 November 2021 - 08:51 AM

Your math looks about right.

 

I've done this, minus the Pegasus rotator, on my C9.25XLT and EdgeHD925 with a few different OAGs including the latest Celestron 93648. I also used a spreadsheet to do the calcs complete with lookup in a device database. Makes mixing and matching configurations so much easier.

 

You've probably watched James Lamb's excellent Youtube videos on the topic. If not, it may be worth the views.

 

If you wanted to save ~18mm +/- adapters, you could put the OAG between the FW and rotator. You could orient it opposite the FW for balance. Guide area will always be the same relative to imaging sensor in this configuration and guide stars in the same place relative to the image sensor should you choose to rotate for framing. This may be easier to achieve the parfocal distance required, and makes it a little easier for planning if you're using the OAG option in Stellarium's image reticle. Either way should work, but you're right, the guide sensor will need a lot of extension.

 

You'll be able to add the ASI1600mm without making any adjustments by the looks of it should you go that route. You'd need to find 12.5-17.5mm reduction should you opt for the ASI2600MM or similar. 

 

The COAG should allow you to rotate the guider separately on the dovetail should the need arise assuming COAG #93648 version.

 

I think you'll find the ASI174 mini works really well with the COAG and SCT. I see nightly averages no worse than 0.60" RMS with extended periods as low as 0.25" on the CEM70G depending conditions and where it is pointing.

 

I am in the process of switching over to the ZWO OAG-L so I can bolt everything together at ~55mm. This will allow the mono imaging train to be easily moved to my refractors which all seem to want the DSLR throwback 55mm back spacing. Hopefully this means everything is fairly predictable should I choose to do that. Currently it requires complete disassembly and too much fiddling. I hesitate to touch what I have since it is working really well. Price of change I suppose.

 

Hope that helps, and good luck with the quest. Should work well once dialed.

 

Quick edit: IIRC, I did not use the COAG helical focuser. I found one that sat a little higher up and allowed more flexibility. I am not sure if it is the ZWO or some generic one found on Amazon, plus a 1.25" extension. It's been a while since I set it up, I'll try to remember to take a look. 


Edited by DesertCrawler, 25 November 2021 - 08:58 AM.


#3 Sean1980

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Posted 25 November 2021 - 10:42 AM

Your math looks about right.

I've done this, minus the Pegasus rotator, on my C9.25XLT and EdgeHD925 with a few different OAGs including the latest Celestron 93648. I also used a spreadsheet to do the calcs complete with lookup in a device database. Makes mixing and matching configurations so much easier.

You've probably watched James Lamb's excellent Youtube videos on the topic. If not, it may be worth the views.

If you wanted to save ~18mm +/- adapters, you could put the OAG between the FW and rotator. You could orient it opposite the FW for balance. Guide area will always be the same relative to imaging sensor in this configuration and guide stars in the same place relative to the image sensor should you choose to rotate for framing. This may be easier to achieve the parfocal distance required, and makes it a little easier for planning if you're using the OAG option in Stellarium's image reticle. Either way should work, but you're right, the guide sensor will need a lot of extension.

You'll be able to add the ASI1600mm without making any adjustments by the looks of it should you go that route. You'd need to find 12.5-17.5mm reduction should you opt for the ASI2600MM or similar.

The COAG should allow you to rotate the guider separately on the dovetail should the need arise assuming COAG #93648 version.

I think you'll find the ASI174 mini works really well with the COAG and SCT. I see nightly averages no worse than 0.60" RMS with extended periods as low as 0.25" on the CEM70G depending conditions and where it is pointing.

I am in the process of switching over to the ZWO OAG-L so I can bolt everything together at ~55mm. This will allow the mono imaging train to be easily moved to my refractors which all seem to want the DSLR throwback 55mm back spacing. Hopefully this means everything is fairly predictable should I choose to do that. Currently it requires complete disassembly and too much fiddling. I hesitate to touch what I have since it is working really well. Price of change I suppose.

Hope that helps, and good luck with the quest. Should work well once dialed.

Quick edit: IIRC, I did not use the COAG helical focuser. I found one that sat a little higher up and allowed more flexibility. I am not sure if it is the ZWO or some generic one found on Amazon, plus a 1.25" extension. It's been a while since I set it up, I'll try to remember to take a look.


Interesting idea on placing the oag between FW and rotator. Would mean recalibration on target switch but prism orientation vs main camera would become fixed

Edited by Sean1980, 25 November 2021 - 10:44 AM.


#4 DesertCrawler

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Posted 25 November 2021 - 12:39 PM

Yes, good point, PHD2 calibration is one thing I didn't mention. I do it fairly frequently with setting up and tearing down every night, so skipped over it.




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