Telescope Reviews: ETX 80 DSO M27

I've found polar mode to be pretty sucky after quite a while attempting to tweak the system. In my opinion the scope is not well designed for the loads placed on it by putting it equatorial position. The entire platform loads up one side of base when it's tilted over and I could never escape having problems with it dragging a bit while tracking.

Looks fine for a bit...then drift starts as the base bearing hangs up somewhere and it slows down the works, then the motor loads it enough to pop it free from where it's hung up. Repeat. Smooth...stick..jump. Smooth..stick..jump.

I pulled it apart numerous times messing with the clutch and RA nut tension, and trying all kinds of lubricants on the rim of the base. Some worked better than others, but I could never get it reliable.

The other issue I continually fought was the clutch...when the system is tipped over to polar mode, any slewing in RA which not near the meridian means the clutch must be tight enough to rotate the whole works without slipping.

With a DSI on there it's a fair load and balancing it with some lead helped a lot, but every other night it would slip halfway through a slew and bollix up my Autostar alignment.

I worked all these issues concerning a polar setup for some months and also broke the stupid plastic RA/Clutch nut along the way. (What a lousy part to choose to make from plastic). I learned a lot about this system along the way. Somewhere I ran across a hack for the RA axis on these things which involved cutting some small slots in the base circle and installing teflon pads as friction bushings to make the RA axis better, this may work.

I finally tired of it and went back to Alt/az where none of these issues crop up. Having the base properly tightened and clutched and lubed will result in a fine Alt/az unit for one made so...modestly, but I was simply never happy with polar mode. With a smooth running alt/az setup and Drizzle, you should be able to take some pretty decent shots even without polar mode. I know I did. At this image scale I found I could run 15 sec subs in alt/ax mode just fine, and with Drizzle to derotate, you can do a lot. You can definitely take OK astrophotos in stock mode, as long as you know the limits.

What essentially ended my experiments in polar mode for good was the aquisition of a 4.5" Starfinder 4504 Newtonian for cheap off of Craigslist. I gave away the OTA, redid the motor mountings, set up the worm tension right, and got a $50 german equatorial in return that works pretty danged well with the ETX tube assembly on it. GOTO's are excellent and I routinely get 30 sec subframes that are pretty good.

Below is an example from the current setup (DSI-C, ETX70 OTA, 4504 GEM), I didn't have the pole dialed in all that well yet so there is a slight bit of trailing. 30 second subs x 34 frames. Also, I screwed up and was running in BMP mode so the pixel depth is not what it should be. Still, it gives an idea of what is possible.

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"Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use." Mark Twain

Edited by MtnGoat (11/10/08 12:48 PM)