Para-Corr and Photos

I've been taking astrophotos with my TV85 for some time, and decided to move on to my Meade 10 inch F4.5 newt to gather more light and get more focal length.

I knew coma would be an issue, but intended to use my Para-corr on the scope.

Mine is an older Para-Corr (I.e, not a "tunable" ParaCorr). It has a two inch barrel that fits into the focuser drawtube and holds the optics. At the other end of that is a set of t-ring threads that screw accept a two inch barrel that receives the camera adapter, camera, etc.

Now, visually (using an eyepiece), this Para-Corr reduces the coma just as I would expect. However, I have no idea what it was doing this weekend when I attached the camera. I had horrible coma trying to focus the stars with DSLR focus. I rechecked collimation, looked for possible bright spots to flare (and look like coma) and all the regular things I could think of.

Anybody have any idea what is happening here?

Alex

Okay folks--for those who may be thinking about this problem.

I called Tele-vue. David helped me out (isn't Tele-Vue Great!)

In order to work right, Para-Corrs need a specific distance (55mm) from the para-corr to the field stop/film plane/chip, etc. That is why they made the "tunable top" on the newer models. You can move it in and out to accommodate the different positions of the field stops/etc. in your eyepieces.

The old Para-Corr's (top screwed off and left a T-adapter) were meant to be used attached directly to the camera adapter. This gives the correct distance.

Well, I was attaching my camera through a camera adapter to the Para-corr's eyepiece holder. SO, the distance was way off. Hence my bad coma.

ON the other hand, my scope would not reach focus (not enough travel) if I connected the camera directly to the Para-corr's t-threads. Therefore, I had no choice if I wanted to use the Para-Corr at all. (The coma was so bad, I decided to remove the para-corr entirely and just suffer the regular coma from the 4.5 mirror.

Now, I forget whether it was lack of infocus or lack of outfocus travel that hung me up last weekend. Hopefully, it is lack of out travel--in which case I merely have to add a spacer. If it lack of intravel, I to either move either the mirror or the focuser. (or maybe just change collimation screw position if it is just a bit!).

I'll see what I have to do this weekend, and try it all over again.

Alex