Sidereal versus Solar tracking

[rawhead]

My TAL mount is currently fixed to the Solar rate. I assume this relates to a complete once round in exactly 24 hours. A Sidereal day is however (as I’m sure you’re all aware) 23 hours 56mins and 4.1secs…

This equates to the stars whizzing through the night sky at (only) 99.72685% the speed of my mount.

Assuming a perfect polar alignment, imaging M42 at midnight tonight using a mag of X20. How long before I would notice trailing? (IE: Is it significant?)

Thanks for any help.

Jon

[Merlin66]

The 4 minute difference in the rates is almost one degree/ day.

So the "error" is 2.5 min of arc/hr, 2.5 sec of arc/ min.

If you have good seeing conditions of say 2" star images, this infers that they would appear as an oval 2" x 4.5" in a one minute exposure - that's a noticable elongation.

Hope this helps.

[brendant]

Hi Jon,

The only way to correct the Tal drive rate is to use a drive corrector as they use a synchronous A.C motor. Here is a link to a review on Cloudy Nights http://216.92.113.163/item.php?item_id=826, not sure if you can still obtain them or not, but it probably makes more sense to upgrade the mount if imaging is the desired result.

For the Tal-2 you will need a HEQ5 as a minimum due to the size and weight of the OTA, I have mine on an EQ5 and whilst fine visually it is not up to imaging.

Brendan

[rawhead]

Merlin66:Thanks for the techie translation. I was getting acceptable results for 30 sec subs then it was starting to drift. I'm sure I'm also getting some periodic error which I think I could improve, but it's not really worth the effort when I'm tracking at the solar rate.

Brendan:Thanks for the advice. I have read up on the correctors. The cheapest I can find is 99 Euros. Not sure it's really worth paying as much again as I paid for the whole Tal 2 setup, that's a good way towards a 2nd hand EQ5.... (which you manually guide very well by the way!)

Enjoyed drift aligning for the first time last night...

Cheers

Jon

[brendant]

Jon, very seldom use the Tal now as I mostly image with the ED80 on the EQ5, its just so much easier lugging it out and setting it up, love the views through the Tal though, they are really great instruments visually for a 6" and built like a tank. The most useful mod I done to it was fit a skywatcher autofocus motor, just makes finding focus so much easier. If imaging is a route you would like to take with the Tal, an EQ5 is not big enough, I regret not getting a HEQ5 or EQ6 mount.

An HEQ5 or EQ6 with the Tal and 80ED side by side or piggyback would make a nice setup, guide with one and image with the other of vice versa .

Brendan

[rawhead]

The most useful mod I done to it was fit a skywatcher autofocus motor, just makes finding focus so much easier.

Agreed Have done the same mod myself....