Issues with ep projection and c8 with 3.3FR on. advice please

[myk68]

Hi all,

I had this in a different section but got no response so think i may have had the wrong place but lets see.

So I managed to get a few clear night to try different setups. by few I mean 2 and haven't had any more yet, lousy weather.

Now I was wanting to get a little closer to objects so I thought eyepiece projection would be the way, and all though it certainly made the object fill the chip more it was also more out of focus, I can sort that with more time getting to focus however I wanted to be sure it could also be the blurring of the stars on the edge of the frame that is saying the whole thing is distorted.??

I wasn't expecting this. Have I missed something in the set up or is it eyepiece quality or something. do i also need a field flattener ? or is it that I need to work out the correct distance from the ep to the ccd?

I have attached the images i am talking about. only the first has been darked and flat fieldand stacked the other two are just rough shots

The first is one i took a while ago and there is small trailing in the stars due to guiding at the time but its not bad.

The second is the EP projection attempt, again the focus may be out a bit but its more the starts at the edge I want to see if i can get back to being stars.

the third image is from my C8 with a 3.3 focal reducer on and although better the stars are still pulling a bit at the sides.

or would the best idea be getting the atil 450. same size chip as far as i can see but 4 times the pixels so i would guess more detail/closer image.

any ideas anyone.

Cheers

Mike

[michael.h.f.wilkinson]

I seriously wonder why you would want to use a 0.33x focal reducer combined with EP projection, because the latter is intended to increase the focal length. EP projection is something to use for planets and the like, not DSOs. Furthermore, normal EPs are corrected for production of a virtual image at infinity, but in projection mode produce a real image on the sensor. On-axis the performance is often OK (fine for planets), but depending on the design the performance may go pear-shaped off axis (or comet shaped in this case). By the looks of it, the EP projection did not magnify vastly, in this case, so the EP is working well out of its comfort zone. What EP did you use? What was the distance to the sensor (roughly).

Also, it is not clear whether the first one is with the FR 0.33x in place or not. The FR 0.33x was really only intended for small CCD chips, so it only has coma correction over a small area.

If you want more magnification a 0.63x reducer would be better, or use the full F/10 (only for very patient people).

[rfdesigner]

I agree with michael.. I've only had really woeful results from eyepiece projection. Prime focus is the way to go with either a dedicated reducer for wider field work or a barlow for really narrow field of view.

Derek

[michael.h.f.wilkinson]

I have had some decent results with Vixen and Celestron Plossl EPs, but I now use TeleXtenders and PowerMates to extend focus. I also intend to use the 0.63x focal reducer for DSOs again.

[myk68]

Hi Michael

Thanks for the reply

Apologies for being unclear. the 3.3. FR is only on the C8 then straight to the chip. But I wasnt expecting the distorted stars around the edges.

The EP Projection is only thru the 80ED to try and get the subject larger on the ccd and therefore get me more detail I was thinking. But then i get the 'comets' around the edges.

its a Plossel ep but I dont have the make, think it was a 25mm. do you mean the distance from the end of the ep to the sensor?