29 replies to this topic

[Cey42]

I just upgraded from a borrowed 6” SCT to my own Celestron Advanced VX 8" SCT. My images have a lot more coma (I think that is what it is) than expected.

 

Here is a full image of M51 at 4144x2822 pixels.

 

 

This is a close up of the bottom right corner. I thought coma was supposed to point the other way.

 

 

My setup:
• Celestron Advanced VX 8" SCT
• Meade f/6.3 Focal Reducer & Field Flattener
• Spacers to get to a backfocus of 105mm
• ZWO ASI 294MC camera

 

My questions:
• Is that coma?
• Is the issue that image area is too big and that is why I am seeing so much coma?
• Do I need to change to a smaller ROI?
• Or is there an issue with my setup?

 

 

Thanks,

Cey

[GazingOli]

Is it an Edge HD 8?

 

CS.Oli

[DuncanM]

You might want to check your collimation. There is some coma in the image and the Meade F6.3 reducer has a very small corrected field. The Starizona F6.3 SCT reducer is a far superior unit:

 

https://starizona.co...-coma-corrector

 

or  

 

https://starizona.co...ducer-corrector

[alphatripleplus]

You are also using a large sensor with a FOV that is large compared to the corrected field with that reducer. 

[Cey42]

GazingOli: It is not an Edge HD.

 

DuncanM: I did check collimation and it looked good to me. Thank you for pointing out the Starizona SCT Corrector IV. Need to figure out if it will cover the 294. And I thought I was done buying equipment. : )

 

alphatripleplus: That is what I was figuring that my FOV was too big. Will probably reduce image size down to 2816x2822.

 

 

Do others with this setup see this much coma?

 

 

Thanks,
Cey

[GazingOli]

---> I thought I was done buying equipment. : ) <---

 

You will never be

 

CS.Oli

 

Edited by GazingOli, 07 April 2021 - 11:35 PM.

[KTAZ]

First off, don't jump to the Starizona offering just yet for $400. Besides, folks have been waiting for that to come back in stock for 6 months and it just keeps pushing.

 

From all I've read, the Meade .63 is not a good match to the Celestron products. Spend $100 on a used Celestron .63 reducer and you should see improvement. The second thing to check is your back focus. If this is off by just a few MM it will result in elongated stars.

 

You will never get a perfectly flat field with a standard SCT, but what you have is much worse than what I have seen using the Celestron .63 reducer.

[SanjeevJoshi]

+1 to what KTAZ says above.   Agree on Celestron F6.3 and testing backfocus.  I am guessing you need to increase the back focus a bit.

[Noah4x4]

The OP is already achieving the ideal back focus of 105mm for a Celestron x0.63 reducer. Other reducers might differ. However, I have now used my SCT with x 6.3 FR at 105mm in a straight optics light path, 137mm using a 2" diagonal and less in a 1.25" diagonal. Apart from the 1.25" diagonal clipping the larger 4/3" sensor I have not noticed any difference as the mirror is moved to reach focus and that adjusts f/ratio. I favour the 2" diagonal as that resolves rear end clearance problems on my Alt-az.

[garyhawkins]

I use the Celestron F6.3 focal reducer with my Celestron 8 inch SCT and it works fine. I also use a 533MC, and I can vary the back focus distance from 85mm to 105mm to customize FOV without any coma issues developing.  This focal reducer seems a good match to the scope - I don't think you need to spend $400.

 

+1 to what KTAZ says above.   Agree on Celestron F6.3 and testing backfocus.  I am guessing you need to increase the back focus a bit.

[Feccy]

I think you may have the wrong back focus dimension - see link below, post#2 suggests it should be 95mm, not 105mm. I have an Antares FR and both 105 & 95mm do not work

 

https://www.cloudyni...pacing-for-sct/

 

I have been having the same issues with too big a back focus and am awaiting a clear night to to see if my solution works. CCD inspector can help to analyse your image - free for 30 days - see below for the 3d map of my sensor FOV - slight hump!! This was at 105mm back focus from memory

 

[KTAZ]

The OP is already achieving the ideal back focus of 105mm for a Celestron x0.63 reducer.

What the OP says and what is reality can be 2 different things!

 

It took me several attempts while I failed to properly measure things before I finally got it to 105mm. Depending on the reducers and how they connect together, sometimes you can be several MM off just due to the way they assemble.

[donstim]

This is a bit confusing. The OP states that the setup uses a Meade 0.63x reducer, but the signature references a Celestron reducer. Whatever is being used indicates either the back focus spacing is way off, or the image labeled as full scale isn't actually full scale.  I ran the image through Astrometry.net, and it came up with an image scale of 1.97 arc-sec/pixel. For the ASI294 camera's pixel size, this works out to a focal length of 485 mm, which is more like 0.24x, not 0.63x, for a Celestron 8" f/10 SCT.

Edited by donstim, 08 April 2021 - 01:33 PM.

[Cey42]

First thanks to everyone jumping in and helping on this.

 

 

It seems that I should try a Celestron Focal Reducer instead of the Meade one that I am using. Would you agree? It is a cheaper alternative than the Starizona and I can get it now. I am not expecting perfection but as I mentioned in the OP, the coma is worse than expected.

 

 

As to backfocus, I could be off by 1-2mm. I have run my images through astrometry.net, then using the results to calculate my focal reduction and I am getting .63.

 

 

 

 

Thanks to call,

Cey

[KTAZ]

First thanks to everyone jumping in and helping on this.

 

 

It seems that I should try a Celestron Focal Reducer instead of the Meade one that I am using. Would you agree? It is a cheaper alternative than the Starizona and I can get it now. I am not expecting perfection but as I mentioned in the OP, the coma is worse than expected.

 

 

As to backfocus, I could be off by 1-2mm. I have run my images through astrometry.net, then using the results to calculate my focal reduction and I am getting .63.

 

 

 

 

Thanks to call,

Cey

Yeah...no.

 

Don't do that. I can tell you from experience (and Celestron will also tell you) that the reducer specifications are approximate. As an example, at precisely 105mm my C9.25 gives 1350mm and f5.7 (and good stars). That is nowhere close to what you might calculate (1480mm and f6.3).

 

105mm is the starting point, then you do trial and error until you get the best stars. Painful? Yes. Worth it? Yes.

 

EDIT: By all means, DO buy the Celestron .63 reducer.

Edited by KTAZ, 08 April 2021 - 03:49 PM.

[KTAZ]

And here is a frame of M51 that I shot last night in the configuration that I described above (105mm BF, 1350mm and f5.7):

 

 

And after running Aberration Inspector:

 

Perfect stars in the corners? Nope.

 

But much better than you are seeing. The improvement potential is there.

Edited by KTAZ, 08 April 2021 - 05:36 PM.

[garyhawkins]

Yes, try the Celestron F6.3 focal reducer.

 

No, I have found back focus is not critical.  In fact, you can vary it to tune the FOV.

 

First thanks to everyone jumping in and helping on this.

 

 

It seems that I should try a Celestron Focal Reducer instead of the Meade one that I am using. Would you agree? It is a cheaper alternative than the Starizona and I can get it now. I am not expecting perfection but as I mentioned in the OP, the coma is worse than expected.

 

 

As to backfocus, I could be off by 1-2mm. I have run my images through astrometry.net, then using the results to calculate my focal reduction and I am getting .63.

 

 

 

 

Thanks to call,

Cey

[Cey42]

I have ordered a Celestron F6.3 focal reducer which is set to arrive on Monday. Monday also happens to be the start of a predicted four night stretch of clear nights so that will give me time to experiment.

 

 

I will report back what I find.

 

 

Thanks,

Cey

[alphatripleplus]

Just curious if the Meade reducer you were using was purchased brand new, or used. If it is an older reducer was it made in Japan, or China? 

[Cey42]

alphatripleplus, It was purchased 6 months ago on Amazon and made in China. The "A too-short Meade f/6.3 focal reducer" post made me wonder if it was one of these. Today the sun came out and I tried "“firestarter” method of focal length measurement" mentioned in that post. I measured a focal length of about 230mm which is the expected value for the 6.3 reducer.

 

 

 

Cey

[alphatripleplus]

Yeah, that matches the focal length of my made in Japan Meade reducers. So no worries on the focal length at least. The too-short Meade reducers were a batch made years ago, soon after production first moved from Japan to China. 

[DSO_Viewer]

I use the Celestron F6.3 focal reducer with my Celestron 8 inch SCT and it works fine. I also use a 533MC, and I can vary the back focus distance from 85mm to 105mm to customize FOV without any coma issues developing.  This focal reducer seems a good match to the scope - I don't think you need to spend $400.

He has a 294 not the 533, so yes your 533 will have less chance for coma because the sensor is much smaller.

 

Steve

[garyhawkins]

Fair point, however, it is important not to get too fixated on exactly 105mm.  Further, as the 294 sensor is bigger than the 533, the OP can always set a ROI to select the best 'coma free' viewing area. Yes, the FOV will be smaller, but the resolution will still be better than can be displayed on a monitor.

 

He has a 294 not the 533, so yes your 533 will have less chance for coma because the sensor is much smaller.

 

Steve

[Cey42]

Quick report. More details tomorrow.

 

 

Focal reducer came in today. Clear skies. Viewing M51 now. A night and day difference. The Celestron reducer removed almost all of the coma.

 

 

THANK YOU ALL!!!

 

Cey

[KTAZ]

Glad to be of help!

 

Clear Skies!