Crayford Style SCT 2" Focuser... Dec 16, 2011 0:36:45 GMT -6
Post by Chicago Astronomer Joe on Dec 16, 2011 0:36:45 GMT -6
Upgrading to a 2" Crayford-Style Focuser for the C11 SCT
I have never been a great fan of the SCT type of scope - preferring the ease and robust performance of Newts and the sharpness and contrast of Refractors. I always avoided using the SCTs at the Adler for public events, and was a refractor man for the longest. It's the mirror tilt/image shift that bugged me...
But I do appreciate the light gathering abilities of SCTs 8" and up and when the C11 decided it wanted me... I changed my mind and think they are great. But it's that damned mirror shift that haunts all SCT users - delicate and finicky.
Methods have been devised to keep this to a minimum. From mirror locks, to dual mini 10:1 focusers on the main focus knob and motor focusers. I have a JMI focuser already on the C11 - and it works nicely....but with it's two button action controller...one often passes-by the prime focus and has to come back. The delicate touch is lost with devices like this and although the battery life lasts a very long time, it may run out during a imaging session.
And also, during my testing of the Binoiviewers on the C11 recently, the coarse focusing of the main knob was too crude and sloppy. I needed something more precise, sensitive and hands-on - not using the heavy mirror for the focusing, but the eyepiece instead to find the focal point - like a newt or refractor. SCTs work, but not efficient nor precise.
In comes the SCT Rear Cell Crayford-style type of focusers.
Researching for performance and most importantly - price...I came down to two types. These level of auxiliary focusers are practically all made by the GSO company in China and all the retailer does is stamp their name on and re-sell it. Pockets that are shallow, (like mine), must be content with under $125.00 models and not the very cool type costing $300.00 +
First the Zhumell focuser -
I can get this down to about $103.98 with promo code and rewards. Free Shipping.
The OPT Focuser -
I can get this down to $98.75 with OPT membership reward.
$13.64 shipping = $112.64
Now the differences:
The Zhumell has a ratio of 11:1 while the OPT a nick below of 10:1 -
Dust cap on the OPT, while the Zhumell does not indicate one -
The Zhumell has a tube travel max length of 30mm, while the OPT stops at 25mm -
The OPT has Millimeter focus laser engraved marks on the tube, while the Zhumell has no indication of any -
Both have a 1.25" adapter included -
The Zhumell is a hefty 3lbs while the OPT is 1lb 10.5 oz -
The Opt has anti-reflection baffles, while the Zhumell does not indicate any -
Both have compression rings -
I have thought about this purchase for a few days now and weighed all the differences, pros and cons...and this wasn't an easy one.
I'm going with the OPT.
Because of the lighter weight, tube markings and the much more detailed data sheet and pre-purchase info. And even with the free shipping, the Zhumell seems like a mass produced faceless item, while the OPT offered a Chicago Astronomer/OPT rewards discount of 5%.
Wish I could evaluate both, but for now...It's the Opt.
Full review soon.