Focal Reducer - Beginner Question

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Focal Reducer - Beginner Question

Postby leedsgreen » Wed Jul 01, 2009 3:50 pm

Dear All,

I have recently bought a SkyMax 127 OTA (second-hand) and notice the back barrel (into which is fitted my 1.25" diagonal) actually has a t-thread. What this means is that I can attach my Canon DSLR via t-thread adaptor directly to the back of my SkyMAx 127.

I would like to get a shorter focal length (say 750mm) for attempt at DSO imaging, however, I am unsure about what focal reducer I should buy. Does anyone know if there is a 0.5x focal reducer available that has t-threads so it can fit directly between my DSLR and SkyMax 127 mounting tube?

If not, can anyone else provide some useful ideas. A 1.25" 0.5x reducer I tried caused some vignetting, so I'm keen to try another way.

Thanks in advance,

Nick
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RE: Focal Reducer - Beginner Question

Postby brianb » Wed Jul 01, 2009 4:06 pm

I suspect the best way forward is to obtain the adapter which converts the thread on your Skywatcher Maksutov scope to standard SCT thread, then use the standard SCT 0.63x focal reducer. If you increase the projection distance slightly you will get a larger reduction, the 0.63x works quite well between 0.75x and 0.5x.

Edit: But be warned that some vignetting is inevitable, the sensor diagonal of your camera is almost as large as the hole in the baffle tube in your scope. A SCT format reducer will still work better than the small 1.25" threaded reducer.

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RE: Focal Reducer - Beginner Question

Postby leedsgreen » Wed Jul 01, 2009 4:40 pm

Briaan,

Thanks again for your speedy help! Can I check a couple of things?

1. How does the SCT 6.3 focal reducer attach to DSLR t-thread?

2. Would vignetting be as noticeable on C5 SCT? I imagine the baffle tube in these scopes are similar width to those in SkyMax 127?

Cheers,

Nick
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RE: Focal Reducer - Beginner Question

Postby brianb » Wed Jul 01, 2009 8:16 pm

[quote]1. How does the SCT 6.3 focal reducer attach to DSLR t-thread?

2. Would vignetting be as noticeable on C5 SCT? I imagine the baffle tube in these scopes are similar width to those in SkyMax 127?
[/quote]
1. The SCT f/6.3 reducer comes with a T threaded extension tube.

2. I don't think there would be much difference. The C5 will of course have a larger field of view due to the smaller f ratio.

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RE: Focal Reducer - Beginner Question

Postby leedsgreen » Thu Jul 02, 2009 10:26 am

Brian,

Thanks again for response. A few more q's:

1. Are there any SCT f/6.3 reducers to avoid and any that are known to offer superior qaulity?
2. If I've got this right, changing the length of the t-thread extension tube will chage the amount of reduction (as you say from around 0.5x to 0.75x)?
3. For a given focal length (based on the physics of point 2), would the image through a Mak with a SCT reducer equal that of an SCT?

Thanks very much for any further help(!),

Nick
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RE: Focal Reducer - Beginner Question

Postby brianb » Thu Jul 02, 2009 12:23 pm

[quote]1. Are there any SCT f/6.3 reducers to avoid and any that are known to offer superior qaulity? [/quote]
Much of a muchness apart from a rogue batch of Meades released a few years ago which are actually fine but need less extension than most others.

[quote]2. If I've got this right, changing the length of the t-thread extension tube will chage the amount of reduction (as you say from around 0.5x to 0.75x)? [/quote]
Yes, longer extension means shorter focal length ...

[quote]3. For a given focal length (based on the physics of point 2), would the image through a Mak with a SCT reducer equal that of an SCT? [/quote]
... but the field is still limited by the baffles in the tube. The size of these is about the same in 5" Mak as 5" SCT but as the Mak has a longer focus the true field is smaller in the Mak. A focal reducer just reduces the size of the illuminated field, unless it contributes some vignetting of its own; which a SCT focal reducer in a small scope is unlikely to.
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RE: Focal Reducer - Beginner Question

Postby leedsgreen » Thu Jul 02, 2009 1:30 pm

OK, that all makes sense. Thanks again Brian, much appreciated.

Nick
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