Focus

[keppar]

I have celeston c8 with a 6.3 focal reducer and I wish to image with a 1000d I'm using the exact set up as per pic. I cannot get focus no matter if I screw the mirror in or out.Scope is working fine and collimation I believe is fine. Any thoughts

[Kai]

Do you get anysort of "blob" on the screen ???

If you do then does it get bigger of smaller when you rack the focus out ?

[dark knight]

Not sure about this one, but you may need to use extension tubes to move the camera further away from the mirror/telescope. I use a tube which is telescopic and therefore allows the camera position to be moved further away from the flip mirror. I must admit it's a bit of trial and error to find the correct position for the camera. I'm sure someone on here will have used a similar combination and will help you sort this out.

Carl

[dark knight]

Kai, can you please explain the relevance of the blob getting bigger or smaller as one racks out the focus. I presume it indicates whether more or less distance is required between camera and telescope, but which way round is it. If racking focus out and the blob/star etc gets bigger, then I presume the camera needs to be closer to the telescope, so the focus would need to be racked inwards. It maybe that there is just not enough inwards travel to achieve focus.

[Psychobilly]

The lens to sensor distance for the f6.3 should be around 110mm this is made up of a 55mm SCT-T adaptor and the other 55mm coming from the t-mount on the scope...

However you should still be able to get focus even away from this "ideal" setup all you will get is a slightly different "ratio" and distorted field...

Looking at the pic my guess is that in your case the SCT-T adaptor is too short

heres my setup...

[dark knight]

Peter is there a way to calculate the distance required between any given dslr's sensor/focal plane with the front lens mounting flange and a telescope in order to get prime focus? for example I have a Nikon D80 and a skywatcher 200p newt, the distance between focal plane/sensor and lens mounting flange is 46.5mm.

PS sorry if this is hijacking the thread but presume it is relevant to the original question.

Carl

[Psychobilly]

Hi Carl,

I suspect there is but I don't know it...apart from "measuring" it ... I suppose you could use a piece of greaseproof paper as a viewing screen fixed to a cardboard tube and use that to view the image from the scope get the image in focus on the screen then measure up...

Billy ... Peter... perhaps i should change my login to SchizoBilly...

[Merlin66]

On an SCT the focus is a "variable" position depending on the movement of the main mirror, so it's usual just to hang everything on the back and twiddle until you get focus. The only difference is when you use a focal reducer... the distance between the reducer and the CCD must be close to "design" as Billy-Pete (B-P?) says, of the x0.63 this distance is 110mm.

On a newtonian and a refractor the position of the focus is fixed relative to the tube and needs to be found to give a measure of the "available back-focus". We've all heard about having to move main mirrors to achieve focus and using extender tubes on refractors.

One of the best ways of finding the focal plane ( on any instrument) is by using a RONCHI ( Ron-Key) grating. This has a series of fine lines, usually 100 line pair per inch, and is placed over the end of the focuser tube and the scope pointed to a reasonably bright star; as the grating comes closer to the focus the number of line visible when you look through, reduces until, when you are exactly at focus, the whole view goes grey... You can measure and make note of this position for the future when adding up the dimensions of your CCD camera, filter wheel, focuser adaptor etc etc.

Hope this helps.

[keppar]

There is a blob? and it seems to get sharper when I move the mirror. Which way is inward and which way is outward / clockwise , anticlockwise etc

[Merlin66]

Depends on your set-up. On my Meade 10" SCT clockwise is closer focus, and needs about 40 turns from one extreme to the other, you should quickly see which way it's going in or out.

Finding the true focus is sometimes difficult and may only take 1/10 of a turn of the knob.

Are you focussing on a star? Does the "blob" appear to have a dark central area ( Like a doughnut?) or just a hazy lump??

Maybe you're using too high a magnification, try it with your longest focal length eyepiece first say 20mm or 25mm FL. When you can get a focus then put the camera back into place.

The distance from the x0.63 reducer to the CCD should be about 110mm before you even start.

Hope this helps.

[Helen]

The blob with the central black spot will get smaller and brighter as you get closer to focus

Helen