HEQ5 Standard Unguided Tracking

[gfletcher]

Hi Guys

With the wealth of experience out there, would you have any idea what length exposures i could take on the Standard HEQ5 through my C8N reflector at prime focus using a 300D (unguided) before star traillng becomes a problem.

Assuming polar alignment is pretty accurate.

thanks

Graham

[jgs001]

The HEQ5 with my C80ED was good for nearly 4 minutes when my polar align wasn't an hour out ... I guess the longer and heavier scope would reduce that time, but couldn't say how much by.

[gfletcher]

John

Was that 4 mins with the Non Goto , Non Syntrek, standard HEQ5 ?

Graham

[jgs001]

oh, sorry, that was the Syntrek version. I thought there were only 2, the syntrek and synscan... I guess not

[Ben Ritchie]

It depends on what you mean by star-trailing becoming a problem. Resampling a DLSR image to 50% or 25% can hide a lot, whereas if you want point stars at 100% resolution the limits are far tighter.

I've measured around 20" peak-to-peak on a HEQ5 pro, averaged over five worm periods with PEMPro (it's a 10min, 38s worm period). Looking at the PE curve the worst case is a tracking error greater than 5" PE in around 30s, the best is around 2 minutes with PE variations of less than 5". So clearly some subs are going to be better than others, and it's a case of what you're happy with.

The predominant tracking error is rapid, aperiodic error of +/- 2" or less, which is mainly an issue for autoguiding at high resolution (and not downsampled), otherwise the predominant error is first-order from the gear being slightly oval and happens twice per worm period.

[gfletcher]

John

The type i have is the Original Black HEQ5 mount

[Ben Ritchie]

Here's the PE curve

[Ben Ritchie]

For an 8", f/5 Newt + 20D (which I assume is roughly the same pixel size as a 300D?) you're working at a native 1.3"/pixel, resampled to 25% size (i.e. an image about 900 pixels across) that's a bit over 5"/pixel. So based on the graph above, a 10 minute unguided exposure with a perfectly aligned mount, resampled to 25%, would show star trailing of around 4 pixels from tracking error.

[blinky]

With my HEQ5 polar/drift aligned and my 8" Newt I could only ever get maybe 2mins tops before trailing was evident on my 350d