Crayford focuser

A crayford focuser? on a SCT? why for?

Well, Crayford focuser can be very useful for SCT but, I strongly believe, it won’t replace the built it focusing system which works by moving the primary mirror. Why? because, if you’re like me, you can do loads of different things with your telescope, such as:

  • use it for visual observing with eyepieces, barlow lenses etc…
  • use it for planetary astronomy with a webcam or a webcam style camera on top of a barlow lens.
  • use it for deepsky astrophotography, in which case you have an Off axis guider, a CCD for imaging and a CCD for guiding

And the focuser cannot do all that.


The main benefits from a Crayford focuser is that it allows you to adjust focus extremely accurately. So, really, in an ideal world, it’s useful in all cases mentioned above. But, as usual, there is a catch. First of all, even though you do get focusers with different travel courses, the overall length of your focuser is important, especially for deepsky imaging where it is critical, especially for an Edge HD, where the field is flat 127 mm back and nowhere else. Also, the deepsky imaging setup weight to about 4 pounds, which is slighly too much for most Crayfords focusers. As a final nail to the deepsky imaging coffin, the only way to position the main imager to the correct back distance, is without crayford, as it’s too far back otherwise. Which is why, in my case at least, I can only use the crayford for visual observations and planetary imaging. The crayford is great in those cases, because, it allows me to adjust the native focus knob  to a point which is mid-range to both applications, and then, lock the primary miror in place so that nothing moves, especially collimation. Also, if your telescope is subject to shifting, as in, the image moves while adjusting the focus knob, then using a crayford will cure that.

As you can see, a Crayford focuser can not do it all on a Celestron telescope, so, thinking is required before buying one. Also, it has to be said that if you buy a cheap one of poor quality, you might endup with a focuser that is worst than the original arrangement.

VN:F [1.9.7_1111]
Rating: 8.2/10 (5 votes cast)
VN:F [1.9.7_1111]
Rating: +1 (from 3 votes)
Crayford focuser, 8.2 out of 10 based on 5 ratings

Leave a Reply