Focusers

What's the difference between using the focuser that comes on the ota and adding a seperate focuser to the setup, say something like a crayford. I know that the crayford has two settings for focusing one finer than the other but is this the only difference?

Joanna - typically the focuser that comes with your OTA is of average performance/quality. Some OTA's *do* come with quality focusers but that's usually not the norm.

To improve the accuracy/precision of focusing, you can purchase a replacement focuser of better quality. The Crayford is a particular design and, if manufactured well, will give better precision/accuracy. Helical focusers are also probably better than rack-and-pinion focusers.

Also, some after-market focusers allow you to attach more weight to it than a standard focuser - mostly of use to astrophotographers.

You can also choose to go electronic and get a temparature compensating focuser - though again, that's really in the realms of astrophotography. With software, these electronic focusers can even alter the focuser position (automatically) depending on which filter you are using...

Overall, you'll get what you pay for both in terms of design and also build quality (and hence performance).

HTH

Dave

Hi Joanna,

Here is a link for what you need. I'm in the market for one myself.


www.iankingimaging.com/show_products.php?category=82

Hope this helps you.

Joanna --

A rack-and-pinon focuser is based on gear teeth (one set on the round pinion and a complementary set on the flat rack). A helical focuser is based on threads, much like a bolt screwing into a nut.

One problem with both these designs is backlash. If you get the teeth/threads too tight then they bind, but if they're too loose then you get no movement for a moment after switching directions.

The Crayford overcomes this problem by using a friction drive -- a smooth shaft pressed tightly against a flat surface. There is inherently no backlash in this design, but the high pressure requires ball bearings for smooth operation. Some cheaper "Crayford-style" focusers use friction pads (such as teflon) instead of ball bearings, but those aren't "real" Crayfords.

Another issue with focusing is precision. Some focusers are built with multiple speeds so that you can do coarse (but quick) focusing, and also very fine (but slow) focusing. This is often true with both Crayfords and rack-and-pinion focusers -- but the two (or more) speeds isn't actually part of what makes them either a Crayford or rack-and-pinion. The helical focusers, because the teeth are much finer, usually don't need the slower speed, but I have seem some complain that they don't have a fast speed for large changes such as when using a Barlow.

Bottom line: any of the designs can be made well with sufficient speeds and limited backlash. The rack-and-pinion is the cheapest to make poorly, while the Crayford is probably the easiest to make reasonably well at moderate cost. Interestingly, when you get up to the premium units in large sizes, they often go back to the rack-and-pinion design.

-- Jeff.

Hi Joanna,
Can you say what scope you are talking about.?