M31 with Samyang T1.5 85mm

[vlaiv]

ASI178mcc is rather harsh on this lens as it has 2.4um pixels, but lens does have some chromatic aberration stopped to F/2.

Here is the same image processed in two different ways. One is RGB compose in Gimp after calibration, stacking and some processing of linear data in ImageJ (background wipe, simple color balance).

Second is RGB composition in ImageJ. For some reason (I think I know why), it has rather intense orange color cast, but luminance layer is much better processed - I used green for luminance here and did RGB ratio color transfer (I need to rewrite background wipe to account for color image - as is, I did "per channel" background wipe, but I need to do it combined to keep color balance).

In any case, second image is much better looking with respect to star shapes and overall sharpness. Green channel is rather nice.

Here is comparison of R, G and B channels on a few stars:

I also took some images at F/1.4 (fully open) with intent of making mosaic and binning them to size where this sort of issues were not objectionable, but everything dewed up after an hour so most of that data is not good. I'll process it at some point just to asses star shapes when fully open.

[vlaiv]

Btw, I did not give tech info - above is one hour of exposure - 60 x 60s from red zone (SQM around 18.5). Hutech IDAS LPS P2 filter used. Mount is AzGTI. No guiding.

[Adreneline]

16 minutes ago, vlaiv said:

Here is comparison of R, G and B channels on a few stars:

This is interesting Vlad as I had similar experience with my Samyang 135 and 85 lenses when used with a dslr.

The blue channel stars were quite bloated compared with red and green. I found that by fitting an inexpensive Hoya UV filter to the lens these problems were significantly reduced. When I used the 'filtered' lens with my ASI1600 and rgb filters I found the fwhm figures were far more consistent across all the filters whereas without the lens filter blue was significantly bloated. I suppose if I lashed out and purchased the UV/IR filter the red might have been tamed as well.

Adrian

[vlaiv]

1 minute ago, Adreneline said:

This is interesting Vlad as I had similar experience with my Samyang 135 and 85 lenses when used with a dslr.

The blue channel stars were quite bloated compared with red and green. I found that by fitting an inexpensive Hoya UV filter to the lens these problems were significantly reduced. When I used the 'filtered' lens with my ASI1600 and rgb filters I found the fwhm figures were far more consistent across all the filters whereas without the lens filter blue was significantly bloated. I suppose if I lashed out and purchased the UV/IR filter the red might have been tamed as well.

Adrian

That is interesting info. ASI178mcc is supposed to have UV/IR cut filter built in - according to ZWO:

And I used Hutech IDAS LPS P2 - which is also supposed to act as UV/IR cut filter as it has following response curve:

In fact, it is even somewhat minus violet since it does not pass wavelengths below 410-415nm according to above graph.

Next time I plan to try it with Baader Contrast Booster filter - it has similar response to IDAS LPS between 500 and 600nm but cuts into violet part of spectrum more aggressively:

I bought that one for use with achromatic refractor to eliminate both a bit of LP and violet halo.

Should be getting artificial star tomorrow, so I'll be able to run extensive tests on different aperture settings and filter combinations to see what sort of stars I can expect. I also got very low profile rotator and quick changer - this will allow for RGB type of imaging with ASI1600 - Maybe doing individual channels and focusing for each - R, G and B will yield better results.

Btw, this is what stars look at F/1.4:

Interestingly, all have well defined "core" but also have significant "skirt"

Profile plot of one star in each channel

[Adreneline]

My Canon dlsr cameras have built in UV/IR cut filters but that didn't stop the bloating experienced with the Samyang lenses (or indeed my Canon 200mm) - the HOYA UV filter definitely made a difference. Maybe it's just a feature of the lenses!

9 minutes ago, vlaiv said:

Should be getting artificial star tomorrow

Interesting!

Adrian

[vlaiv]

Just a small update, could not get color to work for some reason - too much bloating in both blue and red, but here is mono version - which is just green channel.

Stretched a bit more to show all that has been captured: