Achro false 'colour'

[Macavity]

I sense there is a whole area of refractor design featuring doublet... even singlet objectives that might (once did) bear constructive investigation. Checkout the Dialyte design...The fabled (and practical) Hanwell 30" refractor etc. But it seems rather sensible to perform basic "light gathering" via simple®, large diameter, objectives - With the job of colour correction (if needed) etc., done downstream, by smaller intermediate lenses.

P.S. My bracketed "r" yields a "registered" sign!

[Moonshane]

another factor here is that there's another optical aspect to the imaging rig, possibly two. I presume people are using a camera and possibly an eyepiece? what effects do these have/introduce (or even improve??) when imaging the moon?

[RikM]

another factor here is that there's another optical aspect to the imaging rig, possibly two. I presume people are using a camera and possibly an eyepiece? what effects do these have/introduce (or even improve??) when imaging the moon?

You are right, the optical train is different. In my case I used a DSLR camera body screwed straight onto the back of the focuser drawtube, so no extra glass between objective and sensor. I tried to match the image to the visual view I had using an 1.25" Celestron mirror diagonal and Pentex XW 14 mm eyepiece. That was the closes match in terms of image scale, the ST80 is only 400mm FL, so not much magnification.

Through the eyepiece, the purple fringe was not quite as bad as shown in the first image but the lunar disc was much sharper and with more contrast than the second image. I had the whole lot balanced on a standard phototripod, so not realy ideal. the idea was to try and get a picture that shows how CA looks through the eyepiece. I didn't quite manage it in one exposure, though I could probably photoshop the two images together and get a better approximation.

My experience is that your eye/brain does a good job of compensating for optical defects when observing. Cameras can be unkind to telescopes and tend to show off optical defects to greatest effect. A large part of image processing is correcting for these defects.

[russ.will]

You are right, the optical train is different. In my case I used a DSLR camera body screwed straight onto the back of the focuser drawtube, so no extra glass between objective and sensor. I tried to match the image to the visual view I had using an 1.25" Celestron mirror diagonal and Pentex XW 14 mm eyepiece. That was the closes match in terms of image scale, the ST80 is only 400mm FL, so not much magnification.

Through the eyepiece, the purple fringe was not quite as bad as shown in the first image but the lunar disc was much sharper and with more contrast than the second image. I had the whole lot balanced on a standard phototripod, so not realy ideal. the idea was to try and get a picture that shows how CA looks through the eyepiece. I didn't quite manage it in one exposure, though I could probably photoshop the two images together and get a better approximation.

My experience is that your eye/brain does a good job of compensating for optical defects when observing. Cameras can be unkind to telescopes and tend to show off optical defects to greatest effect. A large part of image processing is correcting for these defects.

I should perhaps of specified that in the OP - Only prime focus with a camera attached directly to the draw tube is applicable - No other glass/mirrors allowed.

I did mention that exposure should be adjusted to give the best subjective match for the CA alone. As you say, the brain does a lot of image processing on the fly, that filters out the effects of atmospheric instability and motion tracking, that a camera freezes in a snapshot.

Russell

[SnakeyJ]

Mine here, though it was shot through a Baader CCD Luminance filter (not IR-PASS) as stated and I do have an Astronomik CLS filter protecting the sensor (two bits of glass) - otherwise prime focus/no barlow.

There is a faint band of CA around the limb of the moon.

[John]

Some amazing performing achros here folks

The ones I've owned, with the exception of the F/10's (TAL and Vixen), have all shown more CA to my eyes than the photos here. Looks like I'm cursed with hyper-CA sensitivity or lack the ability to see past it