Filter Wheel and focal length issues with CPC800

[PeterCPC]

Hi All

Some clarification wanted if anybody can help.

I am using a CPC800 with the Celestron 6.3 focal reducer. I recently acquired a filter wheel to use between the reducer and my Canon 550d. 

I have obviously increased the focal length of the scope by doing this as the filter wheel and the adapter adds about 70mm to the length that the camera is away from the telescope. Does this mean that the telescope is still f6.3 with the reducer and filter wheel or what is it now? Any clarification would be appreciated.

Peter

[Zakalwe]

Dont forget that you will have had to rack the mirror to achieve focus. So you might be "robbing peter to pay Paul"?

[Zakalwe]

AFAIK, what will alter the focal length is changing the distance from the FR to the sensor. You should aim to have that correct, which for the Celestron FR is (I believe) 108mm and 85mm for the Antares FR.

[brantuk]

IIRC the camera chip needs to be 55mm from the lens of the reducer/flatener. Hth

Nope - I'm wrong - try 105mm (see below).

[PeterCPC]

AFAIK, what will alter the focal length is changing the distance from the FR to the sensor. You should aim to have that correct, which for the Celestron FR is (I believe) 108mm and 85mm for the Antares FR.

I reckon the distance from the FR to the sensor is about 104mm.

Peter

[brantuk]

Scratch my previous comment - I just found this on Celestron's q/a pages:

"

Celestron’s focal reducer/corrector is optimized to change the focal ratios and effective focal lengths of f/10 Schmidt-Cassegrain telescopes (SCTs).

A focal reducer is an accessory that screws onto the threads on the back of your SCT’s optical tube assembly (OTA) to give a smaller and brighter image at prime focus. It yields lower magnifications, a wider field of view and a faster photographic f-number than the original f/10 configuration of the OTA. To do this, it shortens the converging cone of light from the main optics, bringing the focal plane closer to the back of the telescope. There will be vignetting at the lowest powers or for prime focus photography.

Optically, focal reducers (also called telecompressors) are positive lenses of two or more elements described in terms of the magnification factor. Celestron makes a focal reducer that has a magnification factor of 0.63. (Since this number is less than one for reducers, it's often called the compression factor.) The resulting f-number with the reducer on the scope for a SCT that has f/10 optics is this factor times the f-number, f/6.3. This reducer is often called the f/6.3 focal reducer.

However, the actual degree of compression is dependent on the spacing of the lens from the eyepiece or camera image plane. The compression is less (higher effective f-numbers) for short distances, greater (smaller f-numbers) for longer distances.

For the Celestron reducer with a focal length of 231mm working at f/6.3, the spacing is 105 mm. For Celestron accessories simply screw the reducer onto your scope and view at f/6.3. For aftermarket uses the compression ranges from about 0.7 at 50 mm spacing to about 0.5 at 225 mm spacing."

http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=2243&nav=0,251

Hope that helps.

[PeterCPC]

That's quite reassuring then. As the spacing is about 104/105 I should still get about f6.3.

Thanks for your input.

Peter

[ollypenrice]

You can use plate solving to measure the true focal length...

Olly