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1200 GTO pointing accuracy

Started by Mike Shade, 05/16/2002 10:28AM
Posted 05/16/2002 10:28AM Opening Post
Well the new 1200GTO arived yesterday and I spent several happy hours last night drift polar aligning it in the observatory(I hate drift aligning). Got a reasonably good polar alignment in that there is no Dec drift in 15 minutes that I can see through the C-14 with 12mm guiding eyepiece (about 325X). This thing is attached to a pier in the observatory and I used a GPS to get Lat and Lon and one of those "atomic time" clocks to get accurate time. I used the same guiding eyepiece for initial "alignment" for the first star of the evening. The pointing accuracy was reasonably good in one half of the sky say from Vega to M57 and from there to M8 and so on. However from there to say M51 it sort of missed the target. Was near the center of the finder but not in a low power eyepiece. Moved it and synched and it was happy. Also noticed this same problem during long slews when hooked up to The Sky, crossing the meridian and so on. Once it was aligned it put things almost in the center. Thus I have a few questions:

What is the expected pointing accuracy?
How important is the "time" setting for the handpad and in The Sky (set time in handpad to the nearest minute, NOT in The Sky as it was off by perhaps three or so minutes)?
What are some tricks for improving pointing accuracy, if any?
How much influence might mirror shift/flop and that sort of thing inherent in C-14s have?
I am thinking that Tpoint might help quite a bit as I'm planning on using this thing for CCD work with a tiny chip so it needs to put things in the center of the field.
Posted 05/23/2002 05:27AM #1

The pointing accuracy of the AP mounts is excellent. As long as your optical axis is orthogonal with the mount you should have no problem acquiring tarkets from one end of the sky to the other. From reading your post I would guess you have orthogonality issues with your SCT optics not coinciding with the main axis or the mount.

To correct this problem you are going to need to lock down the mirror so that it is impossible to flop. Then you are going to have to correct for orthonality and ensure that the main optics are coincident with the axis or the mount. This is all explained in the instruction manuals that came with your mount.

If you don't want to do the suggested modifications to your scope, BTW I negleted to mention your going to focus using an after market outside the tube focusser. Then your going to have to live with this inaccuracy. What you do here is sync with an object or bright star every 20 or so degress of movement and when you cross the meridian or the zenith you will have to resync the mount.

Another solution is get rid of the tube assembly and buy yourself a scope where the optics are locked down and don't move.

How do I know this? I own an AP 900 goto and an AP 155 F7 and I also have a Celestron 9.25 SCT yep even that one mirror flops. With my AP 155 F7 being its a refractor and doing a drift alignment allows me to have extremely accurate pointing with this combo. BTW I can have the mount and scope setup and drift aligned in 1 hour and 10 minutes that is my current record. My goal is trying to get it down to an hour! If I put the SCT optics on the mount. Now the pointing accuracy goes out the window. I've looked into mirror lock downs and mods and so on and i've decided its nothe worth the hassle. My answer was to purchase a AP 10 inch Mak which does not seem to suffer from orthonality issues or mirror flop.

One last little detail. The act of recollimating the secondary can through your optical axis off. SCT's are a real pain!

My recomendation is just live the mirror flop issue and just resync the mount as mentioned above. Its easier :-)

Clear Skies
Dwight L Bogan
Posted 07/26/2002 05:51AM #2
I have been using a 900GTo for about a year and at first was frustrated by the same problems. At the bottom of it all is a poor polar alignment and deflections in the optical alignment. I recently bought a Casio E125 PDA om for $200 and loaded Pocket Sky with Tpoint on it
(another $100). I have achieved (easily) polar aligment of 1 arc minute. The slew nails the object every time. I consider my problem solved. By the way, I do this without using the AP keypad. The PDA takes its place, it's incredibly convenient. Setting time to the second helps. I use the time service call up provided with the Sky 5.0, but if you can find AtomTime on the web, that is useful too.