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Looking for guidance on my M78 project.

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#1 DarkMain

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Posted 14 January 2021 - 03:46 AM

I started this project last night to take part in the monthly imaging challenge.
I'm still really new to DSO (This will be my 4th target).

This is the result of about an hours worth of integration (Seeing was pretty bad last night, and at the end of the session the clouds rolled over so I ended up not using more than 1/2 the subs I took).

Telescope is an 8" RC with a 0.75x FR and an IR/UV cut filter.
Mount is an iOptron CEM25p
Camera is a QHY 168c.

-10 degrees

gain 1

offset 10

Sky Magnitude is about 20.18 (bortle 4.7)

 

  • 19x 180 sec subs
  • 50x darks
  • 150x bias
  • 15 flats
  • 15 dark flats

 

I'm hoping to get 5+ hours on this if I can, but I wanted to get some feedback and see if there's anything I can do to improve as I gather more data.

Iy4JAr3.png


Edited by DarkMain, 14 January 2021 - 03:54 AM.

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#2 Tapio

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Posted 14 January 2021 - 04:18 AM

Amazing start for 4th image.

M78 is no easy target but you have done well.



#3 Rasfahan

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Posted 14 January 2021 - 04:46 AM

Yes, very good start. No problems really „pop out“ to me. If you change nothing, this will become a great image. For perfection, I can see some tiny things that maybe could be improved (but make sure you make nothing worse in the process):

* There is a little tilt in your imaging train

* Your stars are slightly oblong. Without some subs I cannot say if this is a tracking error or miscollimation. The slightly thickened diffraction spikes point to the latter. Beware, it is very easy to mess up the RC collimation. If I was you, I would be happy with this result and not touch it. If it is the former: How do you guide?

* You have a dark „ring“ around your target, this means either your flats do not work right (Maybe a reflection: Look up the tube without camera), or a scaling problem (do not use the bias, only flat darks and darks), or it is because of high clouds (check the calibrated subs if you see it there) or it can be a postprocessing artifact (did you use any masks?). 


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#4 DarkMain

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Posted 14 January 2021 - 05:45 AM

Yes, very good start. No problems really „pop out“ to me. If you change nothing, this will become a great image. For perfection, I can see some tiny things that maybe could be improved (but make sure you make nothing worse in the process):

* There is a little tilt in your imaging train

* Your stars are slightly oblong. Without some subs I cannot say if this is a tracking error or miscollimation. The slightly thickened diffraction spikes point to the latter. Beware, it is very easy to mess up the RC collimation. If I was you, I would be happy with this result and not touch it. If it is the former: How do you guide?

* You have a dark „ring“ around your target, this means either your flats do not work right (Maybe a reflection: Look up the tube without camera), or a scaling problem (do not use the bias, only flat darks and darks), or it is because of high clouds (check the calibrated subs if you see it there) or it can be a postprocessing artifact (did you use any masks?). 

Cheers for that.

1 - Where is the indication that there it tilt in my image train? I have a tilt plate I can install to try and fix this but I'm not sure what I'm looking for to see the tilt in my image.

2 - Its probably a combination of both t.b.h. I'm right on the weight limit for my CEM25p. Max payload is 12.3kg and I'm rocking about 10.3kg. I know its a lot but the CEM mounts seem to be able to take a bit more than the GEM ones. (I'm planning on upgrading to a CEM40 which has a max payload of 18kg soon. Just need to sell the cem25p).
I'm guiding with a QHY5L-II and a 60mm guide scope. I'll see if I can dig up the guiding log from PHD2.
I'm pretty sure the collimation is a bit out as well. The scope took a big knock just after I got it, and although I have recollimated it with a Takahashi collimating scope I have always felt it was never quite right. But with RCs being rather difficult I have been hesitant to tweak it too much as I don't want to make things worse. I'll figure it out one day though.

3 - I am using a 0.75 focal reducer that has been known to cause reflections with bright stars as can be seen here (top left corner there is a reflection of the star in the bottom right. This is just a trash sub taken while testing).
In saying that, this image was taken without the focal reducer and if you really stretch it, it also has a 'dark ring' in the center.

The M78 image was stacked and processed in DSS, but I'll try using PI to calibrate and process it to see if its present in the subs (still learning PI so tend to gravitate to DSS for stacking for its ease of use).

And just out of curiosity, why should I not be using the bias frames?



#5 Rasfahan

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Posted 14 January 2021 - 06:26 AM

Cheers for that.

1 - Where is the indication that there it tilt in my image train? I have a tilt plate I can install to try and fix this but I'm not sure what I'm looking for to see the tilt in my image.

2 - Its probably a combination of both t.b.h. I'm right on the weight limit for my CEM25p. Max payload is 12.3kg and I'm rocking about 10.3kg. I know its a lot but the CEM mounts seem to be able to take a bit more than the GEM ones. (I'm planning on upgrading to a CEM40 which has a max payload of 18kg soon. Just need to sell the cem25p).
I'm guiding with a QHY5L-II and a 60mm guide scope. I'll see if I can dig up the guiding log from PHD2.
I'm pretty sure the collimation is a bit out as well. The scope took a big knock just after I got it, and although I have recollimated it with a Takahashi collimating scope I have always felt it was never quite right. But with RCs being rather difficult I have been hesitant to tweak it too much as I don't want to make things worse. I'll figure it out one day though.

3 - I am using a 0.75 focal reducer that has been known to cause reflections with bright stars as can be seen here (top left corner there is a reflection of the star in the bottom right. This is just a trash sub taken while testing).
In saying that, this image was taken without the focal reducer and if you really stretch it, it also has a 'dark ring' in the center.

The M78 image was stacked and processed in DSS, but I'll try using PI to calibrate and process it to see if its present in the subs (still learning PI so tend to gravitate to DSS for stacking for its ease of use).

And just out of curiosity, why should I not be using the bias frames?

1 - The star shapes change over the image from left top (worst) to right top (best). I think this is a very minor issue, and difficult to get better without special software (CCD inspector).

2 - Yes, as I said, I wouldn‘t fuss more with collimation. Check if stars are rounder in shorter subs, if yes, it is your tracking. Btw, I could not attach my guidescope rigidly enough to my CF RC, so switched to an OAG with much improved results.

3 - If this is reflection, it is from stray light bouncing off somewhere in the image train, not from bright stars. 
 

Bias: The bias on many CMOS cameras is unstable. Also, you would need it only for dark scaling, which won‘t work well due to some amp glow of that sensor. If you use matching flat darks for your flats and matching darks for your lights you will not need any bias frames. Make sure to turn „dark scaling“/„dark optimization“ off when calibrating and do not use bias, maybe this already fixes the ring.


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#6 DarkMain

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Posted 14 January 2021 - 07:33 AM

1 - The star shapes change over the image from left top (worst) to right top (best). I think this is a very minor issue, and difficult to get better without special software (CCD inspector).

2 - Yes, as I said, I wouldn‘t fuss more with collimation. Check if stars are rounder in shorter subs, if yes, it is your tracking. Btw, I could not attach my guidescope rigidly enough to my CF RC, so switched to an OAG with much improved results.

3 - If this is reflection, it is from stray light bouncing off somewhere in the image train, not from bright stars. 
 

Bias: The bias on many CMOS cameras is unstable. Also, you would need it only for dark scaling, which won‘t work well due to some amp glow of that sensor. If you use matching flat darks for your flats and matching darks for your lights you will not need any bias frames. Make sure to turn „dark scaling“/„dark optimization“ off when calibrating and do not use bias, maybe this already fixes the ring.

1 - Ac, I thought that was the result of an unbalanced image (secondary mirror being out of alignment causing off axis astigmatism) - http://www.deepskyin...ure_Ver_1.0.pdf

RCs really are tricky.

2 - I'll check some shorter subs and see what the stars are like.
I'm actually tempted by an OAG, but that will probably have to wait for a wee while till I can get the funds.

Cheers for your help. I'll look more into these things.

Edit: I didn't think the 168c was meant to have any amp glow. It it does QHY has been replaying the cameras (as seen here - https://www.cloudyni...-no-i-say-yes/)


Edited by DarkMain, 14 January 2021 - 07:40 AM.


#7 DarkMain

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Posted 14 January 2021 - 06:15 PM

 

* ... or it is because of high clouds (check the calibrated subs if you see it there)

So I have just aligned my lights in PI and done a blink and there is most definitely clouds in just about all of the subs.
I'll keep hold of all of the data (as everything's is a learning experience at the moment) but chances are these will all end up being replaces as I gather more subs on other nights.




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