jmi ev2cm focuser with 3.3 focal reducer

[eye in the sky]

hello all

If someone could help me with the following problem i would be extremely grateful. I have a meade lx90 with the focal reducer screwed directly onto the scope and the focuser screwed behind that. When i use my ccd imager (dsi pro 2) the picture distorts the further you go from the centre. Is there any special accesory i can get for my focuser to stop this. I dont want to remove focuser as i can focus on computer in house as its motorised. Any advise is grateful.

thankyou

dave

[roundycat]

I'm not sure about this. I have the exact same focuser on my LX200 and I have used it with an f6.3 reducer, no problem with an ST10 which I believe is a bigger chip. The back focus for the 6.3 reducer is about 105mm which is exceeded if I mount the reducer behind the focuser. If I mount the focuser direct onto the scope and then the reducer/camera the back focus is less than optimal. Either way, apart from the fact that I do not get the designed reduction, it all works ok. No distortion worth mentioning.

Your problem is most likely down to the 3.3 reducer needing to operate very close to its optimum back focus. How does it work if you swap positions for the focuser and reducer.

Dennis

[Merlin66]

The x0.33 reducer needs to be close to 60mm infront of the CCD chip.... how does that compare with your set-up???

This reducer was originally designed for the early Meade Pictor cameras which had very small, I think 1/3" CCD chips; doesn't work to well ( maybe not at all!) with larger modern cameras....

[eye in the sky]

Hello folks

Thanks for your replies.

So what your saying merlin is if i screw my 3.3f/r onto the back of my scope the distance from the ccd chip to the f/r should be 60mm. I think my distance is more like 80mm to 85mm thats the problem then. So whats the best way to cure this?

Can i screw on focuser first then screw f/r onto focuser then use a 60mm tube or whatever their called then my filter wheel &ccd imager or should i get a 40mm tube because filter wheel is about 20mm. Is that correct?

Because i am putting focuser on scope first and f/r behind does that affect focal ratio? ie will it still be 3.3

cheers

dave w

[Merlin66]

It doesn't matter to the reducer whether or not it's infront off/ behind filter wheels, focuser, or spacer. The ONLY thing that impacts on the focal reduction is the physical distance between the reducer and the CCD chip. Getting as close as possible to 60mm ( for the x0.33; 110mm for the x0.63) will optimise the performance.

Hope this helps.

[eye in the sky]

hello merlin

I understand the gap between has to be 60mm & 110mm for each f/r what i dont understand is if you put the f/r on the scope then put on focuser & made gap 110 between f/r & ccd wouldnt you get a different focal length if you put focuser on scope first then f/r then tube to make 110 from f/r to ccd. The f/r is working further away from scope changing focal ratio. Am i correct or not i just want to get them in the best possible position for my imaging.

thankyou

[Merlin66]

With the SCT there's no optical difference between the methods. You just refocus to accommodate the OVERALL change in back focus required. Relative to the focal plane on the CCD the reducer is still sitting in it's design position.

Do a quick sketch of the set-ups you're considering and you'll see that the focused beam through the reducer is the same.....

[eye in the sky]

hi merlin

I believe you mate i just wanted to make sure there was no difference as i thought to focal length would change. Going out now to try orion starshoot 2 ccd with 6.3 f/r i will post on site if they are any good. (i doubt).

cheers

dave