Trouble with Abberation

[Bikeman55]

I have an Orion 8" f/3.9 newtonian astrograph. I am relatively new to imaging but am giving it my best shot. I have been struggling with abberation since day one and have become quite discouraged. I realize especially with fast newtonians that comatic abberation is to be expected. I am using a Baader Planetarium Mark III coma corrector and collimation, to the very best of my knowledge, is pretty dead on. I also use a bahtinov mask for focusing so focus should also be pretty dead on.

Below I have attached a recent photo I have taken as an example, very minimally processed. I am using an unmoded DLSR so please disregard the noise, it is only a stack of 8 images before the clouds rolled in. Basically it is an example of what I am dealing with. If you are able to zoom in (you may have to save it to fully zoom in and see what I am talking about) you can see that my stars are not round, in fact they are fairly distorted but the distortion is exactly the same across the entire field.  I thought coma was really only noticeable on the outer edges and corners of photos where this image shows the distorted stars across the entire image all exactly the same. Same in the edges and corners as in the very center. Is this coma or another abberation? 

 

Any help, advice or knowledge would be greatly appreciated. 

[Ben the Ignorant]

Hi.

You're overreacting, your pic is quite good. The little imperfection looks like a tiny bit of trailing but t's not coma. Just try without the coma corrector and you'll see: coma is nil in the center, and progressively worse toward the edges. It makes stars look like eggs, with the sharper part of the egg always oriented to the center.

You're only assuming the problem is optical aberration, but it can't be, optical aberrations are never consistent over the whole field, only tracking error is.

[cletrac1922]

Hi

When you imaging, you also taking the same number of dark  images

IE Lens cap on

When processing then zip with both dark and light images

That will also help eliminate noise

 

[The-MathMog]

Looking at the smaller stars, it looks more like tracking errors to me. Where it exposes on some pixels, moves slightly (backlackish or bump), and then exposes on some just next to it. Even if it was only in one frame, then it would still show through when only stacking 8 images. But if it is tracking-errors, it might be periodic errors causing them.
But just to fill in some other info. Are you guiding? And what mount do you use?

[Stargazer33]

+1 for tracking. Also focus is ever so slightly soft.

How long were your subs? Did you use guiding or did you rely on polar alignment alone? If you used long subs and don't use guiding then that will explain the trailing stars.

Despite your reservations that is still a very good image.

[alacant]

Hi. +1 for tracking. I had a go at fixing the trailing in software. It's just on the limit of what you can fix without it looking 'photoshopped to death'. HTH and good luck.

[vlaiv]

To me it looks like multiple sources of problem for star shapes. I would personally:

1. check the collimation just to be sure

2. check coma corrector distancing and specs (I think f/3.9 is a bit fast for MKIII to fully correct) - there seems to be a bit of field curvature left and some astigmatism (that is probably due to corrector rather than mirror).

3. Of course check tracking / PA

[alacant]

2 hours ago, vlaiv said:

f/3.9 is a bit fast for MKIII to fully correct

Hi. At 3.9 on aps-c, the only affordable way I found to get round stars into the corners is the 4 element gpu although surprisingly, the cheepo 2 element 0.9cc does a good job too and you get a wider fov. HTH.

[Bikeman55]

Thanks all for the great advice. I'll try to respond to all your questions but it looks like I'll be spending some time with my tracking next time I'm able to get out.

8 hours ago, cletrac1922 said:

When you imaging, you also taking the same number of dark  images

I did take an equal amount of darks/flats/Bias for this photo and stacked with DSS but with only 8 frames it's still pretty noisy.

6 hours ago, The-MathMog said:

But just to fill in some other info. Are you guiding? And what mount do you use?

Yes I am guiding and I am using a Skywatcher EQ6 mount.

 

4 hours ago, Stargazer33 said:

How long were your subs?

My subs were 4min. Can't seem to get much longer than that with my light pollution. A filter might help in that regard but I haven't gotten there yet.

 

Thanks again for the responses. I will work on my guiding. Hearing it from you all makes a lot of sense but for some reason I thought it was 100% optical, guiding issues weren't even on my radar. Thanks so much. I'll look into it.

 

Also on a side note...

5 hours ago, Stargazer33 said:

Also focus is ever so slightly soft.

What makes you say focus is slightly soft? I'm not being critical, I am truly curious as I am still learning and would like to correct it.

[vlaiv]

48 minutes ago, Bikeman55 said:

What makes you say focus is slightly soft? I'm not being critical, I am truly curious as I am still learning and would like to correct it.

Problem with field curvature can often be interpreted as soft focus, which it essentially is - stars are in different focus across the field.

Apart from possible tracking issues, I would also check for other problems - like correct CC spacing and possible tilt (that can cause field to be uneven - not curved but it will still produce different focus points across the field - only linear in nature).

If you closer inspect star shapes in corners it might give you indication that tracking can't be only issue with the system:

Top left corner exhibits sort of coma shape stars - but instead of pointing towards the center of the frame they are oriented down - I suspect that collimation might be off for this to happen.

Bottom left corner shows clear signs of astigmatism - whenever you get cross shape in stars it is clear sign of astigmatism - this can be due to inherent astigmatism of optics, but I doubt it is - more likely to be caused by CC.

Right side also shows astigmatism artifacts - but a bit strange, like coma in top left corner it is not pointing the right way - astigmatism usually also points to center - shorter part of cross should point towards center of FOV and longer should be perpendicular to it, often curving a bit with center of curvature also pointing to center of FOV.

Now if all of this was just caused by one factor it would be somewhat easier to tell - just field curvature - there ought to be place where stars focus nicely. Astigmatism - should not be present in center of FOV, Coma - also not in center.

So I believe it is combination of three factors, like I've already said - first check and recheck your collimation - you have a fast system and collimation needs to be spot on prior to introducing CC into the mix. If you have good collimation than only obvious aberration of the system should be coma - and only in corners (well, progressing in magnitude from a certain distance from the center outward).

If you don't have that but you think your collimation is good - next to check for tilt. This will be somewhat easy to see - there should be "linear" gradient of aberrations in image - notably one side or one corner will be most affected when having tilt.

When you are happy with system without CC - proceed to sort out spacing of CC and beware - adding CC can cause additional tilt in optical train even if you had none previously. CC spacing is important, and note also that this CC might not clear up field completely and you might after finding correct distance be left with some aberrations in the image - but usually small amount and only visible in far corners (at least try to get them symmetrical to optical axes).

[MattJenko]

Image is pretty flat, with some slight curvature at the edges, mainly the left and lower hand side. I suggest, as others have, that the less than round are from tracking/guiding drift.

[bobro]

A couple of other points :

The posted image is 3607*2383 pixels i.e. about 8.6 Mpixels. Is the image cropped or just resized?

Using astrometry, the line of RA runs more or less top to bottom of the image. Stars are smearing in this direction, showing RA tracking error.

To help work out the other issues of apparent coma and astigmatism, collimation needs to be checked. A straightforward way to do this is to stretch one of the flats - the resulting image should be symmetrical and something like this if collimation is ok:

 

 

 

[Bikeman55]

50 minutes ago, bobro said:

The posted image is 3607*2383 pixels

The posted image is cropped slightly then made smaller for uploading on to the site so anybody looking at it could load it quickly. The original image is 4928x3264 pixels.

I will check more on my collimation and as for guiding I am researching as we speak to see what I can do to improve and try again, possibly Friday night, weather permitting. 

 

Thanks again!!

[bobro]

I'm hoping to image on Friday too - it's been ages due to the Moon/clouds!

It could be useful if you take a single short (say 1-2 min) sub of stars without the CC and post this without any processing (no flats etc) other than scaling (to make file size smaller) and jpeg compression. Don't crop it as that makes inspection difficult. With this single sub collimation and other issues will be easier to comment on.

Good luck!