Crayford Focuser for SCT?

[Matty P (Matt)]

Will I benifit from purchasing a Crayford focuser for my SCT?

Do any SCT owners use them?

[browndog (Ian)]

I use a Bintel 10:1 Crayford focuser with my C8. I have found it a lot easier to get focus using it... compared to the standard focuser knob at the back of the SCT. I find the Crayford focuer is a lot smoother and less prone to image shifting .

There would be a lot better and more expensive ones available, but my budget was limited at the time of purchase. BUT the cheap one works well!!!

[Matty P (Matt)]

Thanks Ian,

Do you use the standard focus knob as well as the crayford focuser?

[Astrod00d]

Originally Posted by Matty P

Will I benifit from purchasing a Crayford focuser for my SCT?

Do any SCT owners use them?

I have a JMI NGF crayford.
For imaging, a crayford will help you focus precisely with minimal image shift.
However it adds a fair bit of length to the optical system. You may not have enough forward travel in the SCT mirror to reach focus with a DSLR. I cannot reach focus when I use the NGT with the C8 and an f6.3 flattener. Without the flattener the DSLR and NGT will reach focus but of then of course there are issues with field flatness.

Hope this helps.

Cheers,

Rob

[browndog (Ian)]

Yes - I use both the standard focuser knob and Crayford focuser. I use the standard focuser knob to get the coarse focus and then the the Crayford for final adjustments.

As Rob indicates it does add extra length to the optical system but I have no problem reaching focus, even using a Meade 6.3 focal reducer and a DSLR with my C8.

The set-up that I use is:
Telescope -> Focal Reducer ->Crayford Focuser -> 2" Camera Adaptor -> T-ring -> DSLR

I have read elsewhere (but I can not recall at the moment) that there is a preferred distance from the Focal Reducer to the Camera. I believe that the distance from focal reducer to Camera is longer than this preferred distance using the Bintel Crayford focuser.

[wysiwyg (Mark)]

Matty,

Crayford is defintely the go, especially a dc motor one if you have the budget.

As far difficulty in focusing, you can eliminate that by placing the flattener else where in the imaging train. Preferrably as close to you dslr as possible, as close, I mean approx 115mm from your imaging sensor. Unless your crayford focuser resembles a similar distance most people will experience difficulty in focusing when placing the flattener to the visual back of the SCT.

This way it wont matter what size focuser you use, you will always get focus and illiminate any vignetting also.

Hope this helps.

[Matty P (Matt)]

Thanks for your help guys.

That was exactly what I wanted to know.

[Matty P (Matt)]

Can anyone recommend a good quality crayford focuser for a SCT?

Thanks.

[wysiwyg (Mark)]

Matty,

William optics make very good focusers, that I have used and trust.
I would stay away from the DSO brand (WO copies), I have purchased one of these for one of my refractors and ended up sending it back, it was crap.

Moonlite also make Excellent focusers www.focuser.com and Star Light Instruments http://www.starlightinstruments.com/, they are pricey but very smooth and well built.

Or the Rolls Royce by Van Slyke Engineering (Huge Bucks)http://www.observatory.org/scfmacro.htm

Given the exchange rate you could get a very good deal.

Hope this Helps
Mark

[asterisk (Geoff)]

Originally Posted by Matty P

Can anyone recommend a good quality crayford focuser for a SCT?

Thanks.

You could also try:

http://www.optcorp.com/product.aspx?...-175-1041-1825

Email them first asking for a shipping quote - it will be much cheaper than the one they put on the web site (I think I paid about US$25 for shipping).

Cheers