Measuring effects of CA reduction techniques for achromatic refractor

[vlaiv]

I decided to measure two common and least expensive techniques for CA reduction. Wratten #8 filter and aperture mask.

I guess this is not overly scientific way to measure it, but hope someone will find it helpful for understanding the effects of each and combination of them.

Equipment used: SW 102/500 f/5 achromatic refractor, GSO yellow #8 filter, aperture masks in following diameters: 50mm, 66mm and 80mm

Measurement technique: Alnitak was imaged at appropriate exposure settings (10ms - 40ms), 900 light frames / 900 dark frames for each combo, stacking was done in AS2!, further processing in ImageJ. Focus was on green component (at least I tried to the best of my ability)

Aperture masks give following values:

50mm, F/10, CA index ~5 (Conrady)

66mm, F/7.5, CA index ~3 (Sidgwick)

80mm, F/6.25, CA index ~2

102mm, F/5, CA index 1.25 (base line)

Results:

Image shows different combos of aperture mask, use of filter and stretch level, columns being in order: 102, 102+filter, 80, 80+filter, 66, 66+filter, 50, 50+filter and rows representing different stretch levels (extreme, a bit less, and even less than second )

Note: Images show that this particular scope sample has some astigmatism (red out of focus component appears to be a bit oval shaped in horizontal direction)

 

[Peter Drew]

About what one might expect overall.  Shows that if you are bothered by CA it's best to get a long focus achromat or a reflecting system.

[John]

Or a chromacor

 

 

[vlaiv]

The more I look at this image and the data, the more strongly I believe following conclusion can be made regarding short achromat:

It is really quite nice all-rounder low budget/beginner scope that can be used for deep sky wide field (base role, no alteration), planetary (use with 80mm aperture mask and any kind of planetary filter - yellow if one wishes a kind of "filter free" view - up to x160 mag) and a short focal length/wide field beginner imaging scope (use 66mm aperture mask and yellow filter) with small sensor budget guide camera (appropriate model which can later be used as guider setup base along the refractor if one decides to go that route). Granted all of this can be said for 130mm F/5 newton, apart from former setup being slightly more portable and collimation free.

[laudropb]

I would agree with your conclusions. I do not like the presence of CA so prefer to spend that bit more for an ED scope.