Monday, March 09, 2009

Cheap Equatorial Mount

I've been seeing these Barska 90060 telescopes at True Value over the past couple months. They always sell out really quickly but yesterday Dennis spotted a pair at True Value in Shangri-La Mall. I asked him to reserve one for me, and bought it last night. Cost around $110. This is in contrast to $70 to $100 in the US, however not factoring in the hefty cost of shipping a 20-pound box to the Philippines.

I really was only after the mount (or more specifically, the equatorial head).

I did not have high expectations (these things have 200-arcsecond periodic error, a wobbly aluminum tripod, and a poor-quality telescope). I did not even intend to unpack the telescope, instead I planned to mount my 80mm Vixen refractor on the mount.

A few conclusions are in order:
  • The 100-tooth Right Ascension gear has a significant amount of slop;
  • However the biggest source of inaccuracy is slop in the worm gear caused by an improperly-tightened end bushing. I intend to adjust this when I can get an Allen wrench set.
  • It uses a non-standard dovetail, however the entire EQ head is made of some pot metal so I enlarged the dovetail bolt holes with a screwdriver so I could bolt my tube rings to it.
  • The Celestron/Vixen 80mm f11 is way too heavy for the tiny counterweight.
The view of the moon at 28X (with a 32mm Vixen Plossl) and 90X (with a 10mm) was razor-sharp, with only a tiny hint of CA on the limb. The equatorial mount works as advertised, allowing hassle-free tracking (so long as the tube isn't so unbalanced by the tiny counterweight), even with only an eyeball polar alignment.

Due to the really heavy Vixen OTA, I decided to give the telescope tube (a 60mm f11) which came with the Barska a try.

To my tremendous surprise, the cheap plastic focuser is a 1.25" one, not the usual 0.965" on cheap telescopes. So I was spared the indignity of using the supplied Huygenians and Symmetrical Ramsden (17th-century technology!) eyepieces with the kit.

The view of the moon is decidedly less impressive than with the Vixen (as expected). The cheap plastic focuser doesn't quite reach focus with the 32mm Vixen Plossl, but the 10mm works fine and I could see the Straight Wall quite well near the lunar limb.

The 5x24 finder which comes with the kit has a very small eye relief, tiny field of view, and is pretty useless (I used the hole in the mounting stalk to aim the main telescope tube). Good thing I intend to use a Telrad or Rigel Quick-Finder (I have one of each) with the Vixen tube.

In summary, the Barska 90060 is a piece of crap which should not be purchased by newbie star-gazers, who will gnash their teeth in frustration at the many limitations of this mount and telescope. But it's a relatively cheap entree into equatorial mount land, and with enough tuning up and tooling, I'm confident that I can make it competent enough.

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