focus Ceestron c8

[keppar (Graham)]

HI all
Having trouble focusing with a Celestron SCT C8, dslr, filter and a focal reducer i,m not sure if I need more travel (longer adapter tubes) or ths camera needs to be closer to scope. Any thoughts ?
Thanks Kepp

[Omaroo (Chris Malikoff)]

Hey there Kepp

Sounds like the distance between the rear of the reducers back lens and the cameras focal plane is too great. I had a similar problem on my C8 until I found a t-threaded adapter that was a lot shorter than the standard ones sold by Meade and Celestron. I ended up using the barrel off of a 2" GSO SCT diagonal, which breaks down into a tube and thread just the right length. Call Mike at Bintel in Sydney - he's got a box full of these barrels on the shelf.

[keppar (Graham)]

Thanks Here is a photo of setup

[Omaroo (Chris Malikoff)]

Here are a couple of mine - including the actual adapter. You can judge by the length what is required on a C8.

[Matty P (Matt)]

Hi Kepp,

You shouldn't have any problems focusing with that setup. I used to have my setup in the same orientation as yours minus the skylight filter.

What type of focal reducer are you using. F/6.3 or F/3.3?

[Omaroo (Chris Malikoff)]

Originally Posted by Matty P

Hi Kepp,

You shouldn't have any problems focusing with that setup. I used to have my setup in the same orientation as yours minus the skylight filter.

What type of focal reducer are you using. F/6.3 or F/3.3?

It also matters what type of camera you use Matty. The distance from the bayonet to the actual sensor plane (denoted by the circle with a line through it stamped on top of your body) differs between models and brands. My 350D is 49-odd mm whilst my Nikon's is only 41mm. My 350d won't work on my f6.3 reducer (which I suspect is what Keppar has) unless I use the shorter tube, whereas my Nikon's fine.

[keppar (Graham)]

Matt 6.3 .I thought I read somewhere that it is critical to have the correct distance from the focal reducer to the camera chip

[keppar (Graham)]

Gents I have a canon 40d

[Omaroo (Chris Malikoff)]

Kepp - what's that distance I spoke of on your camera - from the bayonet facing to the sensor plane mark?

[Matty P (Matt)]

Originally Posted by Omaroo

It also matters what type of camera you use Matty. The distance from the bayonet to the actual sensor plane (denoted by the circle with a line through it stamped on top of your body) differs between models and brands. My 350D is 49-odd mm whilst my Nikon's is only 41mm. My 350d won't work on my f6.3 reducer (which I suspect is what Keppar has) unless I use the shorter tube, whereas my Nikon's fine.

Ah... I see. You learn something new everyday.

Thanks Chris.

[Omaroo (Chris Malikoff)]

Originally Posted by Matty P

Ah... I see. You learn something new everyday.

What's the 450's distance Matty?

[Matty P (Matt)]

Originally Posted by Omaroo

What's the 450's distance Matty?

Approx 45mm.

I also just measured the distance from the sensor to the focal reducer as well. Which is 210mm. Might help?

[Omaroo (Chris Malikoff)]

What marker did you use on the casing of the reducer? Distance (I believe) is usually measured to the rear of the rear lens surface - which you can't see when it's all screwed in place.

[Matty P (Matt)]

I measured to the rear of the reducer (facing to the sensor). The line where it connects to the next adapter.

If that makes sense?

[keppar (Graham)]

Chris Mine measures 47mm

[Omaroo (Chris Malikoff)]

Originally Posted by keppar

Chris Mine measures 47mm

OK - so 2mm shorter than the 350D. Your tube must be only a smidgen too long then. Did you give Bintel a buzz and ask about the tubes they have?

[keppar (Graham)]

Yea they are sending one that is only 20mm long the one I have now is approx 50 mm

[Matty P (Matt)]

While we are on the subject of distances. What is the ideal distance from the FR to the DSLR sensor?

I remember reading about it but don't recall.

[keppar (Graham)]

Matt dont quote me but I believe it was between 50 -75 mm