SCT Reducer spacing

[haitch]

Just a quickie - am I right in thinking the Long Celestron t adaptor they sell will give the correct spacing to the sensor on my canon DSLR when used with a standard canon eos t ring? (Until I want to start sticking guilders and filter wheels in the train of course)

[Davey-T]

Unless you're using a focal reducer it doesn't matter.

Dave

[haitch]

Sorry - mentioned the reducer in the heading but not the body of my post. Yes, I meant with reducer.

Cheers

Paul

[Davey-T]

Needs to be around 110mm from the back of the reducer to the sensor which is 55mm back from the front edge of the "T" ring, it's not that critical though I've used from around 80mm to 200mm with good results.

Dave

[Astroboffin]

No to be picky, but it is 110mm from the centre of the FF/R lens assembly to the sensor, so measure 110mm from the bottom of the threads on the focal reducer to the camera sensor, as that is where the centre of the lens assembly is, (around 103mm to the rear of the reducer, flattener glass ) there are two sets of two lens in the FFR one set is the reducer and one the flattener, the reducer being first nearest to the scope, so you need to measure from the middle of the two sets, after the reducer but before the flattener.

If you measure 110 from the rear glass that will be around 117mm from the back of the reducer and give a less flat field and will push the reducer past its best working distance, but it will still work....

Sorry if that sounds a bit picky, but it is important to get the best from that celestron FFR, I use one with that spacing and is perfect and gives f6.3 which gives optimal performance and the flattest field.

Regards

AB

[Davey-T]

Being picky, it only gives 6.3 depending where you need the mirror to focus, as you wind the mirror in / out the focal length changes

Just dug mine out (Celestron part no:93633) and it makes the measurement 110mm to the centre of the tread on the rear of the reducer and 105mm from the rear of the reducer.

As you say Paul it gives you the correct distance.

Dave

[haitch]

Cheers all - especially Dave with the specifics on the adaptor.

Actually, to add my own pickiness, wouldn't celestron design the f/l into the reducer + spacing? If the sensor is the correct distance behind the reducer which is screwed to the back of the tube they should be able to work out the f/l of the SCT in producing the correct focal plane and therefore the effect of the reducer.

[haitch]

Just need to get a celestron t adaptor now - let one go when I got rid of the C8 and didn't see far enough into the future that I'd be getting a C11! :-/

[Davey-T]

Cheers all - especially Dave with the specifics on the adaptor.

Actually, to add my own pickiness, wouldn't celestron design the f/l into the reducer + spacing? If the sensor is the correct distance behind the reducer which is screwed to the back of the tube they should be able to work out the f/l of the SCT in producing the correct focal plane and therefore the effect of the reducer.

They have designed it for DSLR spacing but it can be used with other cameras, an f10 SCT  is actually only f/10 when the mirror is in one particular place and the focal length changes as you focus.

Dave

[haitch]

So if you screw the reducer to the rear of the tube and the sensor is a given distance behind it you have defined the position of the mirror haven't you? (+/- a small amount for temperature compensation) I would have thought this was what Celestron used in the calculation of the f6.3

[Astroboffin]

So if you screw the reducer to the rear of the tube and the sensor is a given distance behind it you have defined the position of the mirror haven't you? (+/- a small amount for temperature compensation) I would have thought this was what Celestron used in the calculation of the f6.3

Exactly!

[Astroboffin]

Being picky, it only gives 6.3 depending where you need the mirror to focus, as you wind the mirror in / out the focal length changes

Just dug mine out (Celestron part no:93633) and it makes the measurement 110mm to the centre of the tread on the rear of the reducer and 105mm from the rear of the reducer.

As you say Paul it gives you the correct distance.

Dave

105mm is pretty much spot on, and will give excellent results,

AB