M13 from last night- set up critique please

[Swoop1]

M13 from last night, RVO Horizon 72ED + field flattener, ASI290MC, HEQ5PRO. 30 X 30 sec lights, 9 X 30 sec darks, 10 X 0.0001sec bias, 20 X 10 sec flats.

Stacked in DSS, histogram stretched in SIRIL, levels tweaked in GIMP.

Exposure and post processing need work but, this is my first use of a field flattener- have I got that right?

Thanks.

 

[WolfieGlos]

Not sure what you mean by “have I got that right” by using a field flattener? 

For 15 mins of exposure it’s a pretty good image. Colour seems a bit “white” to me on this screen though?

 

[Swoop1]

14 hours ago, WolfieGlos said:

Not sure what you mean by “have I got that right” by using a field flattener? 

For 15 mins of exposure it’s a pretty good image. Colour seems a bit “white” to me on this screen though?

 

Wolfie,

I believe that incorrect flattener to sensor distance can cause abberations in images so I wanted to check that I had the correct spacing with the set up I used.

I am sufficiently not knoweldgeable enough re imaging and my eyes are insufficiently accurate for me to decide for myself!

[tooth_dr]

I like it as an image. Has that natural feel about it. It’s hard to say if the distance is correct, a single sub frame would be more useful.  

[WolfieGlos]

Oh I see. Like tooth says, if you post a single sub (uncropped) then it can be seen.

[Swoop1]

Here is one of the original lights frames from this run-

 

Light_ASIImg_30sec_Bin1_21.6C_gain0_2023-09-04_224104_frame0001.fit

[Clarkey]

The 290 is a relatively small sensor so the exact back-focus distance is unlikely to be critical. The stars do look round which suggests it's about right.

[Swoop1]

Thanks Clarkey

[tooth_dr]

2 hours ago, Swoop1 said:

Here is one of the original lights frames from this run-

 

Light_ASIImg_30sec_Bin1_21.6C_gain0_2023-09-04_224104_frame0001.fit 4.05 MB · 1 download

Looks pretty good TBH, stars look nice and round throughout the image.  I'd not change anything with your setup.

 

[ollypenrice]

Hmmm, I'm a little dubious. What seems odd is that, the bigger the stars are, the softer they are. Yes, this is normal enough but, here, it seems exaggerated. We seem to jump from small, tight stars to large soft ones. Was there any haze when you shot this?

On the basis of just this image I wouldn't, personally, sign off the optics as all OK just yet. I'd try another dense star field.

Olly

[Swoop1]

@ollypenrice- transparency wasn't perfect and, later in the evening, when I switched to lunar, there was a definite softness to the sky.

[ollypenrice]

2 hours ago, Swoop1 said:

@ollypenrice- transparency wasn't perfect and, later in the evening, when I switched to lunar, there was a definite softness to the sky.

In which case I'd give the rig another run under a crisp sky.

Olly

[vlaiv]

@Swoop1

Did you use UV/IR cut filter? If not - star bloat can be because of that. ASI290MC does not have one inbuilt.

[ollypenrice]

3 minutes ago, vlaiv said:

@Swoop1

Did you use UV/IR cut filter? If not - star bloat can be because of that. ASI290MC does not have one inbuilt.

That would make sense.

Olly

[vlaiv]

2 minutes ago, ollypenrice said:

That would make sense.

Olly

Yes, but I don't think it was the case - here is per channel comparison of the largest star:

There are 2 green, one blue and one red images of that star extracted from bayer matrix of posted sub.

It looks like similar level of bloat in green (1 and 4) and red (3 - based on background level). Blue seems tightest and my guess would be that this is due to correction of the scope and focusing.

[Swoop1]

2 hours ago, vlaiv said:

@Swoop1

Did you use UV/IR cut filter? If not - star bloat can be because of that. ASI290MC does not have one inbuilt.

No filters used. 
I have a light pollution filter and will include that in the imaging train later, just taking one step at a time at the moment.

[Swoop1]

2 hours ago, vlaiv said:

Yes, but I don't think it was the case - here is per channel comparison of the largest star:

There are 2 green, one blue and one red images of that star extracted from bayer matrix of posted sub.

It looks like similar level of bloat in green (1 and 4) and red (3 - based on background level). Blue seems tightest and my guess would be that this is due to correction of the scope and focusing.

I don’t understand the significance of this information @vlaiv . Would you mind expanding for me?

 Thanks 

[vlaiv]

16 minutes ago, Swoop1 said:

No filters used. 

Ok, so first order of business is to include UV/IR cut filter.

14 minutes ago, Swoop1 said:

Would you mind expanding for me?

I was just trying to assert if you used filter or not.

Doublet scopes, even better corrected ones like ED scopes can only bring two wavelengths of light into a focus.

Above curve represents "defocus" depending on wavelength of light. I drew two red lines - examples of where you can focus your scope (literal focus position by moving focus in / out). If you focus at bottom line then green will be in focus while red and blue parts of spectrum will not.

If you focus on upper line - then things will be a bit more balanced - green will be a bit out of focus but so will red and blue.

Anyway - by examining stars per separate channel - you can see how much defocus there is in each channel and you can sort of guess things - like how well corrected scope is, if one used IR/UV cut filter and so on.

This is because above curve is not symmetric - for same focus shift - blue tends to be more defocused then red.

This asymmetry is bigger if you don't use UV/IR cut filter.

In any case - I tried interpreting what was going on in your case, but I got it wrong. There are several variables and given that star bloat is similar in different channels (wavelengths of light) - I assumed that you in fact used some filter and that any small difference is due to how the scope is corrected (above curve can be tilted more towards short wavelengths or long wavelengths) and how well you focused.

Sometimes it is very hard to find best focus spot with doublet scopes - precisely because there is no best focus - you have to judge visually or let computer calculate based on stars in the image and take some "best" average of star profiles.

Btw - here is image of different modes of optimization of doublet scope:

So you can have C-e line brought to common focus, C-F line or d-F line for example - which tilts this curve differently (and makes different bloat per channel).

 

[vlaiv]

@Swoop1

Here is another example of such comparison on some stars that I did some time ago:

This shows very strong chromatic aberration (unlike ED glass) - from left to right: red, green and blue channel. You can see that red is a bit more bloated than green and blue is even more bloated (even higher defocus).

When you combine such channels - you end up with this:

See how you can see chromatic aberration in stars - there is purple halo around big one and others have reddish ring around them.

[Swoop1]

Thanks @vlaiv.

So, your examples illustrate how my glass behaves with different light wavelegths and demonstrates that 'perfect' focus may not be achieveable?

I am never going to be a high quality astrophotographer so, as long as I can achieve results that please me, I will be happy 👍

[ollypenrice]

There is a lot you can do in post-processing to reduce star bloat.

If you use Star Xterminator you'll get to the stage where you are ready to replace the stars and have them as a top layer over the starless image. Simply increasing the contrast on the star layer can drop the bloat below the level of the nebulosity in the bottom layer so it won't show. The stars themselves become brighter but then you can adjust the global brightness of the star layer. The key thing is contrast in the star layer, which you can adjust using Photoshop's contrast slider or have more precision using an S-Curve in Curves.

Olly

[vlaiv]

1 hour ago, Swoop1 said:

So, your examples illustrate how my glass behaves with different light wavelegths and demonstrates that 'perfect' focus may not be achieveable?

I am never going to be a high quality astrophotographer so, as long as I can achieve results that please me, I will be happy 👍

Second example that I gave is from optics that is quite a bit less corrected than yours. By adding UV/IR cut filter you can improve things considerably.

Sure - ED doublet will never be as corrected as APO triplet, but it can come really close and star bloat can be kept at unobtrusive levels.

Don't look at my technical ramblings above in light of your work - but rather as explanation of what I did and why - main point was to determine where the star bloat comes from, and conclusion is - from lack of UV/IR cut filter.

[Swoop1]

56 minutes ago, vlaiv said:

Second example that I gave is from optics that is quite a bit less corrected than yours. By adding UV/IR cut filter you can improve things considerably.

Sure - ED doublet will never be as corrected as APO triplet, but it can come really close and star bloat can be kept at unobtrusive levels.

Don't look at my technical ramblings above in light of your work - but rather as explanation of what I did and why - main point was to determine where the star bloat comes from, and conclusion is - from lack of UV/IR cut filter.

That is exactly the kind of information that I can use to improve my output so, no problems at all vlaiv- thanks for taking the time to explain in such detail!