What is wrong with these stars?

[michael8554]

5 hours ago, happy-kat said:

reading other posts suggest it doesn't impact.

There are many posts suggesting that adding a clip-in filter will correct Autofocus after an Astro Mod to a DSLR .

If it does (????),  it must be because of refraction, as vlaiv has demonstrated.

Michael

[Ags]

I had a go at M42 with an IR block filter added. From my garden I can only collect 20 minutes of data and the gradient is from a near-by street light. The stars might be slightly rounder, but I don't seem to get good focus with the IR block in place?? Next clear night I will have another try with my Wratten 29 filter. It should reduce the gradient and remove all the shorter wavelengths. I think in the ye olde film days Wratten 29 were used for photographing Ha regions.

Edited December 6, 2020 by Ags

[vlaiv]

3 hours ago, Ags said:

The stars might be slightly rounder, but I don't seem to get good focus with the IR block in place??

Well, yes, that is the problem with lens that have chromatic aberration - always some wavelengths of light out of focus.

[happy-kat]

Was the lens still at f2.8?

[Ags]

1 hour ago, vlaiv said:

Well, yes, that is the problem with lens that have chromatic aberration - always some wavelengths of light out of focus.

Not sure that's the reason - I have reduced the range of wavelengths so there should be a point of focus that is at least as good as without the filter, but with the (very cheap) Meade IR filter  I get irreparably soft stars.

@happy-kat the lens is still a F2.8, but I have some step-down rings coming.

[vlaiv]

50 minutes ago, Ags said:

Not sure that's the reason - I have reduced the range of wavelengths so there should be a point of focus that is at least as good as without the filter, but with the (very cheap) Meade IR filter  I get irreparably soft stars.

@happy-kat the lens is still a F2.8, but I have some step-down rings coming.

Look at these stars - there is little point in the center of each of them - that is part of spectrum that is in focus.

This is star from first image. Only difference is that you have wider "skirt" of the star because of UV and IR light that has not been filtered out. Filter out far parts of spectrum (reduces skirt) and stretch a bit more and you have the same star as above.

 

[Ags]

Hard to compare the images - Orion was binned and stretched much more and shot at different gain...

My step-down rings arrived today - I can now shoot at F3.8 and F3.3.

Looking at getting a Yongnuo 100/2.0. Apparently optics are good but autofocus which I don't need is bad. Also focusing at infinity may be easier with a non-macro lens (macro lenses are geared for fast focus travel over a very large range). If I can coax good performance out of my 100 Macro, then i can start dreaming of a dual camera setup....

[Ags]

Stopped down to F3.8 and shooting Red only looks much more satisfactory. These are longer subs (16 seconds).