Using an achromat for AP

[rmuhlack (Richard)]

I have recently modded a Canon 1000D by replacing the stock Canon UV/IR filter with an Astrodon Inside UV/IR blocking filter. First light was last night on Eta Carinae, and for something different I thought I'd also have a go at using my Long Perng 80mm f6 achromat for AP as well.

So, for anyone wondering how an achromat performs when used for AP, then wonder no more.

15 x 2min subs at ISO1600
Modded Canon 1000D
Long Perng 80mm f6 achromat
Williams Optics P-flat 88 field flattener

The first image is the full frame at 20% of full resolution, while the second image is a crop at 100% resolution to show the extent of the CA.

Processed with Pixinsight

[omegacrux (David)]

Nice for a small acro
you should be happy with that

David

[Gem (Grant)]

I used an achro for AP in the past. While some people don't like the colour - I never had a problem.

[bojan]

If you don't like the CA, just place the aperture in front of a lens, so the entry diameter is reduced to, say, 50 ~ 75%.
Yes, this will result in longer exposures (for the same result), but what the heck.. APO is 10x more expensive compared with achromat, and aperture cost nothing (you can make it by cutting the round hole in black cardboard disk).

Of course, for narrow band and RGB imaging with BW sensor, there is no problems with CA, as achromats are corrected for spherical aberrations.. and field flattener/coma corrector is necessary anyway.

[LewisM]

I used a 150 mm Sywatcher achromat for imaging a couple times. Yes, there was blue fringing, but the results were pleasant!

I find blue fringing less objectionable than diffraction spikes to be honest.