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EdgeHD 8" w/Celestron Focal Reducer - Stars Distorted in OAG

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#1 Domer

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Posted 22 September 2022 - 11:18 AM

I recently received my long-awaited EdgeHD 8 and have been running tests before heading out for a full night of imaging.  First, my configuration:

 

Mount: CEM40

Telescope: EdgeHD 8 with and without the Celestron EdgeHD focal reducer

EAF: Rigel Systems nFocus

Imaging camera: ASI2600MC-Pro (APS-C sensor)

Guide camera: ASI290MM-Mini on an OAG w/helical focuser

 

I first wanted to see how it functioned at the native focal length (2032 mm).  I did this a couple weeks ago and didn't notice any issues so I didn't capture any images to use for comparison.  But last light I installed the Celestron 0.7X focal reducer and the results were dismaying.  For starters, I was utterly unable to get anything close to round stars in the guide camera.  The helical focuser allows for very fine adjustments, but the stars vary from upside-down horseshoes to vertical bananas to diagonal bananas.  This is the absolute best I could manage:

 

Guide Star.jpg

 

Needless to say, PHD2 fails to calibrate properly and guiding is deplorable since the center of mass for the guide stars is virtually impossible to find.  When I ran tests a couple weeks ago without the focal reducer, I don't remember having this problem.  If the skies are clear again tonight, I'll try without the FR and see how it looks.

 

But I know a lot of people are using the EdgeHD 8 with the Celestron FR and an OAG, so I'm wondering if there is some trick I'm missing to get this working.  I really don't want to have to use a separate guide scope if I can avoid it.  Any advice would be greatly appreciated.


Edited by Domer, 22 September 2022 - 11:30 AM.


#2 Tapio

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Posted 22 September 2022 - 11:37 AM

When using OAG it's possible to have weird star shapes and PHD can still work.

Did you change the focal length in PHD after you added focal reducer.



#3 Domer

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Posted 22 September 2022 - 12:01 PM

When using OAG it's possible to have weird star shapes and PHD can still work.

Did you change the focal length in PHD after you added focal reducer.

Yes, I set the FL to 1422mm in PHD2.  But the stars are so badly misshapen that calibration failes every time and guiding is very erratic.  



#4 Tapio

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Posted 22 September 2022 - 12:15 PM

What's the error you get when you try to calibrate ?



#5 Domer

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Posted 22 September 2022 - 12:39 PM

Jagged lines, inconsistent orthogonality errors, and widely varying Dec/RA rates:

 

PHD2 Cal1.jpg

PHD2 Cal2.jpg

PHD2 Cal3.jpg

 

Calibration Begins at 2022-09-21 20:13:23
Equipment Profile = CEM40/ASI290MM/1422mm
Dither = both axes, Dither scale = 1.000, Image noise reduction = none, Guide-frame time lapse = 0, Server enabled
Pixel scale = 0.42 arc-sec/px, Binning = 1, Focal length = 1422 mm
Search region = 15 px, Star mass tolerance = 50.0%, Multi-star mode, list size = 12
Camera = ZWO ASI290MM Mini, gain = 65, full size = 1936 x 1096, no dark, defect map in use, pixel size = 2.9 um
Exposure = 3500 ms
Mount = iOptron CEM120/70/40/26, GEM45/28 Mount (ASCOM), Calibration Step = 154 ms, Calibration Distance = 25 px, Assume orthogonal axes = no
RA Guide Speed = 7.5 a-s/s, Dec Guide Speed = 7.5 a-s/s
RA = 19.93 hr, Dec = 0.3 deg, Hour angle = -0.46 hr, Pier side = West, Rotator pos = N/A, Alt = 56.8 deg, Az = 167.4 deg
Lock position = 1250.259, 502.039, Star position = 1250.259, 502.039, HFD = 6.52 px
West calibration complete. Angle = -87.2 deg, Rate = 25.899 px/sec, Parity = Even
North calibration complete. Angle = -175.8 deg, Rate = 17.185 px/sec, Parity = Even

 



#6 Mike7Mak

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Posted 22 September 2022 - 12:51 PM

Do you have room to move the oag prism down closer to the edge of the imaging chip? It may be set way outside the corrected image circle.



#7 Domer

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Posted 22 September 2022 - 01:06 PM

Do you have room to move the oag prism down closer to the edge of the imaging chip? It may be set way outside the corrected image circle.

It's already right at the very edge of the sensor - any more and I start getting shadows.



#8 schellaj

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Posted 22 September 2022 - 01:45 PM

You might want to check that your imager is at the proper backfocus for the EdgeHD reducer.  The EdgeHD 800 with reducer is not as good as the larger scopes and can have elongated stars even when at proper backfocus.

 

The specs from Celestron are 105mm backfocus (from the collar of the reducer to the imaging chip). However, many people have found that it could be anywhere from 105-111 mm.

 

While it is possible to guide on banana stars, it is better when they are round and small. Odd shapes are affected more by seeing, which will impact your guiding.

 

Jason



#9 DirtyRod

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Posted 22 September 2022 - 02:01 PM

I'd suggest looking at the guide camera starting with focus. That HFD at 7 is much higher than the 4ish I usually get. Generally, my FWHM values for my guide star are under 3 using the 290 or a174 in with an OAG and a Reducer with my Edge8.

 

What are the HFD and FWHM values when you are not running the reducer? 

 

Are the stars through your imaging camera similarly distorted?



#10 Domer

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Posted 22 September 2022 - 03:05 PM

I have the back focus set at exactly 105mm:

 

2" SCT-M48 Adapter: 50.0 mm

ZWO OAG:            16.5 mm

ZWO Filter Drawer:  21.0 mm

ASI2600MC-Pro:      17.5 mm

Total:             105.0 mm

 

And for the OAG, it stacks us like this:

 

2" SCT-M48 Adapter: 50.0 mm

ZWO OAG:            40.5 mm

Helical Focuser  :   6.0 mm

ASI90MM-Mini:        8.8 mm

Total:             105.3 mm

 

The OAG has 40.5mm for the OAG instead of 16.5mm because of the prism "corner turn".  I can adjust the back focus for the guide camera up to 6 mm.  But no matter what I do, I cannot get round stars in the guide camera.  



#11 Domer

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Posted 22 September 2022 - 03:13 PM

I'd suggest looking at the guide camera starting with focus. That HFD at 7 is much higher than the 4ish I usually get. Generally, my FWHM values for my guide star are under 3 using the 290 or a174 in with an OAG and a Reducer with my Edge8.

 

What are the HFD and FWHM values when you are not running the reducer? 

 

Are the stars through your imaging camera similarly distorted?

See my previous post regarding guide camera focus.

 

The stars in the imaging camera are nice and tight in the center of the frame, but there is some elongation and chromatic aberration in the corners.  Here is a shot of a random star field and a close-up of the top edge - the guide camera would be seeing stars just above that edge.

 

Star Field (reduced).jpg

Star Field (top edge).jpg



#12 DirtyRod

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Posted 22 September 2022 - 04:51 PM

See my previous post regarding guide camera focus.

 

The stars in the imaging camera are nice and tight in the center of the frame, but there is some elongation and chromatic aberration in the corners.  Here is a shot of a random star field and a close-up of the top edge - the guide camera would be seeing stars just above that edge.

 

attachicon.gifStar Field (reduced).jpg

attachicon.gifStar Field (top edge).jpg

Imperfect stars in the corners is what we all fight so sounds like the imaging camera is straight. If you pull out the guide camera and just try to focus on something down the street do you get a clear image?

 

I assume you've looked at the OAG mirrors and the guide camera glass and its clean. 



#13 Domer

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Posted 22 September 2022 - 05:03 PM

Imperfect stars in the corners is what we all fight so sounds like the imaging camera is straight. If you pull out the guide camera and just try to focus on something down the street do you get a clear image?

 

I assume you've looked at the OAG mirrors and the guide camera glass and its clean. 

I use the same guide camera and OAG on my SkyWatcher Esprit 100 and it gives me crisp, round stars.  I never disassemble it - I just moved it from the ES100 optical train to the EdgeHD.  And hopefully I can confirm tonight, but I'm pretty sure the stars were fairly round when I didn't have the FR installed a couple weeks ago, but I didn't take any snapshots so I cannot be sure.



#14 schellaj

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Posted 22 September 2022 - 05:19 PM

The guide stars are likely round without the FR on because the Edge in corrected out that far (as long as you are at the correct BF (133.3 mm without the FR). When you add in the FR the stars are not corrected quite as well. And many of the Celestron FR suffer from chromatic aberration and are much more sensitive to BF spacing. This is well documented. At a minimum I would try 107mm and see if that changes anything.

#15 Domer

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Posted 22 September 2022 - 05:22 PM

The guide stars are likely round without the FR on because the Edge in corrected out that far (as long as you are at the correct BF (133.3 mm without the FR). When you add in the FR the stars are not corrected quite as well. And many of the Celestron FR suffer from chromatic aberration and are much more sensitive to BF spacing. This is well documented. At a minimum I would try 107mm and see if that changes anything.

Using the helical focuser on the OAG, I can change the back focus by as much as 6 mm.  The screen shot I posted earlier was as good as it got.  Anything less or more and the stars just got worse.



#16 Domer

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Posted 22 September 2022 - 05:40 PM

I just realized something... my SCT T-adapter terminates in M42 threads.  That means there's a 50 mm long pipe that is only 42mm wide from the back or the focal reducer to the front of the OAG.  I wonder if that's clipping the light cone and distoring the star shapes.

 

What does everyone else use to convert the SCT threads to M48?



#17 Domer

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Posted 22 September 2022 - 06:35 PM

I'm currently using the imaging camera to build a new dark library so I can't show that in the photo, but this is what I have:

 

EdgeHD Optical Train.jpg


Edited by Domer, 22 September 2022 - 06:36 PM.


#18 schellaj

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Posted 22 September 2022 - 06:53 PM

This is where to measure your BF from.  Bottom of the threads. 

Attached Thumbnails

  • Screenshot_20220922-204842_Chrome.jpg


#19 schellaj

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Posted 22 September 2022 - 06:57 PM

I might be wrong.  Just looked at the EdgeHD white paper and looks like you measure from the last optical element. 



#20 Domer

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Posted 22 September 2022 - 07:06 PM

My backfocus is correct.  If it wasn't, the main image would be all wonked up and it isn't.



#21 Domer

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Posted 22 September 2022 - 07:12 PM

I decided to purchase the Celestron OAG which is specifically designed to work with their SCTs.  It has a wider inside diameter and a much larger prism so it should yield better results than what I'm getting with the ZWO OAG farther down the optical train and it's tine 8mm prism.

 

Hopefully, that will correct this problem.


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#22 rigelsys

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Posted 22 September 2022 - 08:12 PM

I just realized something... my SCT T-adapter terminates in M42 threads.  That means there's a 50 mm long pipe that is only 42mm wide from the back or the focal reducer to the front of the OAG.  I wonder if that's clipping the light cone and distoring the star shapes.

 

What does everyone else use to convert the SCT threads to M48?

There is this

 

https://agenaastro.c...apter-c-04.html



#23 Phil HD9.25

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Posted 22 September 2022 - 08:30 PM

It's the reducer.  I have the same issue with the Celestron .7x on my Edge 9.25.  Stars look elongated in my guider with the reducer on at 146mm back focus (Celestron OAG, large telescope adapter, + 174 Mini).  I've tried adding and subtracting back focus, no change.  Same type of elongation, although not as bad in the main imager.  Without the reducer, stars are round and crisp on the guider.  I've set aside the reducer, and I've been imaging at f/10, which is not as bad as you would think in terms of exposure time--at least from a dark site.


Edited by Phil HD9.25, 22 September 2022 - 08:32 PM.


#24 dswtan

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Posted 23 September 2022 - 03:37 AM

This is my Edge8 + 0.7x with OAG star shape running right now. It's guiding just fine and I just don't worry about it. PHD is extremely tolerant of star shape.

 

Star

 

In fact, I just noticed my guiding is down to 0.56" total RMS which is as low as I've ever seen!

 

Star2

 



#25 michael8554

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Posted 23 September 2022 - 04:31 AM

Same here. 

 

Poor guidestar shape/good guiding.




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