Simple AP setup - is my alignment off or my RA drive slow?

[JohnH]

Hello IISers - I need some advice - I have acquired a small EQ mount with RA motor (stepper) drive for use as a portable widefield AP mount mostly just for my Canon 20d with 50 or 135mm lenses and if I am lucky with .8 FR and WO66 SD.

I have hit what I think is a snag. Polar alignment is tricky, the setup has a PAS but I am not sure how accurate that will get me, here is what happened.

I aligned as best as I could with the PAS, focused and got tracking going then tried a series of shots at the Antares region. I got 60s shots with the 135mm lens without too much trailing (a few frames were rejected with trails but that is ok - the mount was only $200 and the lens less than $50 I am not expecting perfection!).

What I did notice though is that over the run antares gradually drifted down the frames so that after 3 hrs it was about 1/3 of the way down the frame. I guess this means my alignment was off in AZ quite badly but with this setup I will not be able to do a drift alignment readily (no scope!) so I am wondering what the best way forward would be.

I am also concrened my RA drive may be running slow, is there any easy way to check that - I guess it will depend on knowing the gear and worm ratios - of course I do not have those as the kit was bought s/h. I have timed the roataion at the worm spindle and it is 1 rev/8 mis approx.

All thoughts/advice welcome.

[toryglen-boy (Duncan)]

Originally Posted by JohnH

Hello IISers - I need some advice - I have acquired a small EQ mount with RA motor (stepper) drive for use as a portable widefield AP mount mostly just for my Canon 20d with 50 or 135mm lenses and if I am lucky with .8 FR and WO66 SD.

I have hit what I think is a snag. Polar alignment is tricky, the setup has a PAS but I am not sure how accurate that will get me, here is what happened.

I aligned as best as I could with the PAS, focused and got tracking going then tried a series of shots at the Antares region. I got 60s shots with the 135mm lens without too much trailing (a few frames were rejected with trails but that is ok - the mount was only $200 and the lens less than $50 I am not expecting perfection!).

What I did notice though is that over the run antares gradually drifted down the frames so that after 3 hrs it was about 1/3 of the way down the frame. I guess this means my alignment was off in AZ quite badly but with this setup I will not be able to do a drift alignment readily (no scope!) so I am wondering what the best way forward would be.

I am also concrened my RA drive may be running slow, is there any easy way to check that - I guess it will depend on knowing the gear and worm ratios - of course I do not have those as the kit was bought s/h. I have timed the roataion at the worm spindle and it is 1 rev/8 mis approx.

All thoughts/advice welcome.

Hi there!!

good polar aligment will allow you to have exposures up to around 2 mins, well i have found that from my experience. i drift align, but i doubt you could do that with this setup

maybe someone else will chime in.

[JohnH]

Originally Posted by toryglen-boy

Hi there!!

good polar aligment will allow you to have exposures up to around 2 mins, well i have found that from my experience. i drift align, but i doubt you could do that with this setup

maybe someone else will chime in.

Hmm - well it depends doesn't it - on the image scale you are using and if you are guided or not. In my case I am unguided and looking to image at 4.5"/pixel at worst, more often half 8-9" that thus assuming I can tolerate up to 2 pixels of movement in an exposure before I have to can the frame I can tolerate 20" of error using a lens - or 10" for the scope.

Now I wonder how closely aligned do I need to get to ensure my rate of drift is less than that for a 60s or 120s exposure, and can I do it? I realise the drift rate will vary with DEC of the target....I did not think I would be that far off hence my doubts about the RA drive speed...