Full Moon test of a doublet

[Camelopardalis (Dunk)]

Folks, was almost full Moon last night and I was curious about some of the CA I appear to have in some of my DSO shots, so I took the scope outside as I figured the Moon would be a challenging target for the little scope (WO Zenithstar 71mm)...

The saved from RAW shot is on the left (native resolution), and a gratuitous stack of 7 frames on the right with a little heavy handed magic wavelets

From the raw shot, if you zoom right in you can see a faint blue ring around the outside, but doesn't look like much to me. I've noticed then when trying to focus on a star that I see more of the blue ringing effect on one side of focus than the other, so obviously I try to approach it from the less side

This scope is "what I've got to work with" at the moment, so it'll have to do either way, but is this more or less what I should expect from a doublet/ED? Or is the Moon a bad target for this sort of test? Or is the CA more evident in longer exposures?

[Starlite (John)]

Taken with a Nikon Coolpix P600 sitting on top of SW 102 refractor

[dannat (Daniel)]

id take a long exp of a star field & look for blue stars
in you full moon pic the image looks either ever so slightly out of focus / a tad soft -the stack looks fine

[LewisM]

I really think CA is over-rated sometimes - I wouldn't worry a ZOT about any CA in those moon images. It's not an achromat, so it will control the CA well enough. I did image with a 150mm achro once, and the results were disgusting.

I still use a Vixen and Tak fluorite doublet for imaging alongside my quad FSQ85. I don't find it an issue at all.

[cometcatcher (Kevin)]

That's not a knife. THIS is a kn... I mean CA. Taken with the infamous Long Perng ED90 doublet.

Be sure too your camera settings don't contribute to it. At first I thought my SW ED100 F9 had CA, but it was actually a setting in the camera that was causing it. Since I sorted that out, CA isn't an issue. Flare on the other hand....

I'm not familiar with that model of WO scope, but I have read that some of the doublets do have a little residual CA. If you push them hard enough, most refractors will show it. If it's minor CA like yours, a fringe killer will deal with it, as will a Semi-Apo filter. The Semi-Apo is a good filter for nebula as well.

[Camelopardalis (Dunk)]

OK cool thanks chaps...

I see what you mean about the soft focus Daniel, I had focused it with live view rather than the bahtinov mask

I've tried some longer exposures recently (4 minutes) and it only seems to be the brighter stars that get a bit of a blue halo. Interestingly, it seems to get diluted during stacking, along with all the other colours!

Will look into a filter too, I think the reducer/flattener has a 2" filter thread scope-side.

[raymo]

Wow Kevin, that pic is far far worse than my SW 80mm achro would
produce.
raymo

[LightningNZ (Cam)]

Originally Posted by cometcatcher

That's not a knife. THIS is a kn... I mean CA. Taken with the infamous Long Perng ED90 doublet.

Be sure too your camera settings don't contribute to it. At first I thought my SW ED100 F9 had CA, but it was actually a setting in the camera that was causing it. Since I sorted that out, CA isn't an issue. Flare on the other hand....

I'm not familiar with that model of WO scope, but I have read that some of the doublets do have a little residual CA. If you push them hard enough, most refractors will show it. If it's minor CA like yours, a fringe killer will deal with it, as will a Semi-Apo filter. The Semi-Apo is a good filter for nebula as well.

There's a reason why your stars lose colour when stacking - you are increasing the total range within the image so the variation in levels at the very top end is too small to see without stretching this region. You can recover this just fine in tools that let you increase vibrancy and saturation. You're not actually losing anything, you just have to know how to make it available again.

Hope this helps,
Cam

[cometcatcher (Kevin)]

Originally Posted by Camelopardalis

Will look into a filter too, I think the reducer/flattener has a 2" filter thread scope-side.

I've found the Orion FF introduces a little CA when used with my ED80. There's definitely less without it. I wonder if the Televue model is more CA free? Only problem is it's 3 times the price.

[cometcatcher (Kevin)]

Originally Posted by raymo

Wow Kevin, that pic is far far worse than my SW 80mm achro would
produce.
raymo

That's why I sent it back.

[Larryp (Laurie)]

Originally Posted by cometcatcher

That's why I sent it back.

I sent mine back too!

[LewisM]

I think all who cannot see CA in the Long Perng 90's (and some other sizes) is colour blind. I was shocked by the CA in the 90 I once looked through, though thankfully the owner only used it for terrestrial use (though had asked if it was suitable for astronomical use - to which I gave it a yes with stipulations)