Focuser Question

[KansasDon]

Picked up an old Colter 13" reflector for $50.00. Cleaned up real well and have had a lot of fun with it over the last year.

 

Bought a new used 2" Zhumell focuser for it, got it mounted and learned I should have gotten a low profile focuser. (insert head smacking smiley) Focuser is too tall by about an inch (25mm).

 

I'm not real keen on how the primary mirror adjusts for calibration anyway on this cheap telescope. If I remounted the primary mirror say 1 1/2" closer to the secondary mirror would this new focuser then work properly.

 

Thanks.  ....Don 

 

 

[Peter Drew]

Short answer, yes. You do have to bear in mind that if the secondary mirror is minimal size to cope with the light cone then moving the mirror closer could cause vignetting.     

[CraigT82]

As above... remounting the primary a little further up the tube is the usual answer (other than selling on the focuser and buying another, lower profile one).

Peter mentions that moving the mirror up may cause some of the light cone to 'miss' the secondary and shoot back out the front of the tube, but this can actually be advantageous if the primary mirror isn't too well figured at the edges (a common condition), at the expense of the scope operating at a reduced aperture.

[KansasDon]

Thanks Peter and Craig.

 

I will move the primary closer to the secondary and see how it works. I can always undo it and get a low profile focuser if I don't like the results.

 

I think it will work though, this scope has a 76 mm secondary mirror minor axis and IF I've done the math right I should be able to move the primary mirror 26 mm closer and still be ok.

332 mm aperture / 1500 mm focal length = .221 per mm

Old focuser to the secondary mirror minimum is 267 mm from the center of the mirror.  + 38 mm (for the leading edge of the diagonal secondary) = 305 x .221 = 67.4 mm light cone width at the secondary.

New focuser to the secondary mirror minimum is going to be 293 mm from the center of the mirror. And adding 38 mm for the leading edge of the diagonal secondary =  331 x .221 = 73 mm (light cone width at the secondary)

But I really have no knowledge of telescope design fundamentals so my math is probably fubar?

 

Thanks again,

Don