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Hi all, 

A little advice please:  I'm quite pleased with the two images attached except for some of the star shapes which have gone out of round.  Not sure if this is a collimation issue or coma, or something else! I am using canon 1300 D fitted with Baader MPCC at 55mm from the sensor, on my SW 250pds.   Autoguiding with a finder guider, phd2, and the RMS typically is around 0.6"

thanks in advance

Mike

IRIS Nebula NGC7023.jpg

M27 2hr41min ISO800 no Flat or Dark.jpg

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1 hour ago, mikeyj1 said:

Hi all, 

A little advice please:  I'm quite pleased with the two images attached except for some of the star shapes which have gone out of round.  Not sure if this is a collimation issue or coma, or something else! I am using canon 1300 D fitted with Baader MPCC at 55mm from the sensor, on my SW 250pds.   Autoguiding with a finder guider, phd2, and the RMS typically is around 0.6"

thanks in advance

Mike

IRIS Nebula NGC7023.jpg

M27 2hr41min ISO800 no Flat or Dark.jpg

To me it looks like a slight tilt on your imaging train. What’s holding your coma corrector in place? I often find mine has been affected by focuser screws too tight. 

Also your resolution is 0.74 per pixel. Really guiding should be roughly half that. I think :) I would check the screws etc. 

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Thanks, my cc is screwed to the front of the T ring and the camera is secured on the two screws of the 2” holder on the standard focuser. Not sure how I can change the connection? 

How do I know the resolution/pixel on the guidescope?  It’s a QHY5i-ii on a skywatcher 9x50 finder 

Edited by mikeyj1
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Different sites do the same thing. Here is one I use. 

https://astronomy.tools/calculators/field_of_view/

What you can get for the focuser is a compression ring that you swap out of the  focuser. FLO sell them. Maybe 20£ 

Bit of a pain these things sometimes I had problems with mine for ages. Getting your collimation good is important too. 

Now you have said you are using your finder guider it might be worth getting something more suitable for your focal length. I'm sure yours will work but your at quite a long focal length. 

Gerry

 

Edited by Gerry Casa Christiana
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18 hours ago, mikeyj1 said:

Hi all, 

A little advice please:  I'm quite pleased with the two images attached except for some of the star shapes which have gone out of round.  Not sure if this is a collimation issue or coma, or something else! I am using canon 1300 D fitted with Baader MPCC at 55mm from the sensor, on my SW 250pds.   Autoguiding with a finder guider, phd2, and the RMS typically is around 0.6"

thanks in advance

Mike

Hi Mike

If you put your processed images through CCDInspector then it indicates that you have a significant degree of tilt and collimation error on the M27 image. The Iris has a smaller tilt and better collimation. If you look down the right side of the M27 image you can also see that the RGB channels are incorrectly aligned.  The average aspect ratio of the stars is coming out as 24 and 27 for the Iris and M27, respectively - which isn't good.

To find the root cause(s), I'd suggest that:

1. Take some very short subs with the telescope pointing vertically upwards - this will minimise any tilt that may be coming from focuser droop and minimize any detrimental effects of guiding/tracking errors.

2. Repeat the above step but this time with the telescope as horizontal as you can get it whilst still being able to take a picture of the night sky. This will maximise the influence of any focuser droop.

You then need to examine the resultant subframes. If you like to do things quantitatively then I'd suggest you download a free trial of CCDInspector and put the unprocessed but calibrated subframes though it in its averaging mode.  This will enable you to measure the tilt, collimation error and aspect ratio. 

The easiest way to determine your imaging resolution in arc seconds per pixel for either your main system or guide scope is to put a subframe through a plate solving program, alternatively you can work it out by the formula (206.265/Focal Length)*pixel size (here the pixel size is in microns and the focal length in mm). 

Your measured guiding RMS of 0.6 arc seconds also seems high - however, before trying to lower this, I'd suggest you concentrate on lowering any camera tilt and collimation issues.

Alan

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Also, the drawtube on the focuser might be slightly tilted to the imaging plane. I have a similar issue where there is coma more pronounced in the top right of my images. But also my image scale is 0.88 arc seconds per pixel so guiding to that tolerance is difficult as well! Best I've seen on my mount was around 0.65 rms

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On 04/07/2018 at 09:38, Gerry Casa Christiana said:

Now you have said you are using your finder guider it might be worth getting something more suitable for your focal length. I'm sure yours will work but your at quite a long focal length. 

any suggestions?

i have an ST80 that i could mount up piggyback?  but I don't see how I could possibly get it to 1/2 that of the main imaging train?

 

guidescope.JPG

Edited by mikeyj1
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On 04/07/2018 at 10:39, alan4908 said:

If you put your processed images through CCDInspector then it indicates that you have a significant degree of tilt and collimation error on the M27 image.

thanks trying the free version, although not entirely sure what I am doing!  Collimation was out (i didn't check it until the following night!)

Regarding:

On 04/07/2018 at 10:39, alan4908 said:

1. Take some very short subs with the telescope pointing vertically upwards - this will minimise any tilt that may be coming from focuser droop and minimize any detrimental effects of guiding/tracking errors.

2. Repeat the above step but this time with the telescope as horizontal as you can get it whilst still being able to take a picture of the night sky. This will maximise the influence of any focuser droop.

Are you thinking i have a Frac?  only 'vertical' would produce the biggest strain on the focus tilt on a reflector i think?  but i get the idea, I could at least try to orient the camera so its pointing down on the top of the OTA which should help, and i will change to the compression ring also!

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2 hours ago, mikeyj1 said:

Are you thinking i have a Frac?  only 'vertical' would produce the biggest strain on the focus tilt on a reflector i think?  but i get the idea, I could at least try to orient the camera so its pointing down on the top of the OTA which should help, and i will change to the compression ring also!

Yes, for some reason I was.  Anyway, at least you get the idea. Hopefully, you'll soon sort out your issues.

Alan

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Thanks for your responses!  Whilst thinking about the 50mm finder guider I came across some interesting articles on the web:

http://www.wilmslowastro.com/tips/autoguiding.htm

http://lhastro.org/callisto_wp/wp-content/uploads/2016/02/Choosing-a-Guide-Scope.pdf

https://agenaastro.com/selecting-a-guide-scope-and-autoguiding-camera-for-astrophotography.html#aperture

included for reference, but if i get chance i will try to make a direct comparison between the finder guider (50mm dia, 180mm FL) and the ST80 (80mm dia, 400mm FL), as it appears that modern software and camera capabilities should reduce the need of 1/3 focal length (Guide to Main) to something less than 1/5

regards

Mike

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14 hours ago, mikeyj1 said:

any suggestions?

i have an ST80 that i could mount up piggyback?  but I don't see how I could possibly get it to 1/2 that of the main imaging train?

 

guidescope.JPG

I would say that your ST80 would be a good way to test it. It's not 1/2 but 1/3 but as someone has said it seems that is not the current advice but as Mike said it was mentioned 1/5 of the focal length. So 400mm would be exactly 1/3. Is the star travel a F5? 

I do think though your finderscope at 180mm? Is going to struggle  

Let us know what results you get!

Gerry

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