Polar alignment with Astrotrac

[Phillyo118]

Good evening everyone,

First time poster here. I've just bought an ASI533 to use on my astrotrac and I wanted to use Sharpcap to do an accurate polar alignment because the polarscope with the astrotrac is a pain (and I don't like lying on the floor to align). However, I'm a little bit lost as to how to do it.

My current set up would be as follows: Tripod with Manfrotto 410 Junior Geared Head, Astrotrac on top, Manfrotto Ball Head on that for framing with the ASI533 attached to a Samyang 135/2 lens with a few other bits. When I run the polar alignment in sharpcap, it's going to ask my to rotate the RA axes by 60 degrees or something. How do I do that on the astrotrac? I'm really lost! Is it even possible?

Thank you all in advance for any help and advice.
Phil.

[admin]

Hi,

I'm afraid I don't have any experience with this particular product – I hope that that someone else here on the forums who might know the answer. In recent versions of SharpCap I did reduce the amount of rotation required in the polar alignment routine to accommodate it working with devices that cannot rotate through large angles. There is now a checkbox near the bottom of step one in the polar alignment routine that will enable this mode – when this is enabled, you only need to rotate at least 15° to get polar alignment result.

Cheers, Robin

[Phillyo118]

Thanks Robin.

If it doesn't work it isn't the end of the world. I'll be upgrading within the next month or two to a better mount anyway I guess.

Phil

[SteveInNZ]

Does your ballhead have a panoramic base ?
If so, you can do the rotation with that as it's aligned to the polar axis. There are other options but you are rather limited if you only have that lens/camera combo.

Steve.

[Phillyo118]

Hi Steve,

Yeah I think the ball head has some kind of panoramic base. It's the Manfrotto 496RC2? I'll take a look and see if I can get that working.

Thanks,
Phil.

[Phillyo118]

So I got the polar alignment working well, accurately aligned now. But I do have another question regarding exposure lenght.

I live just outside Lincoln UK, in a place called Washingborough. My LP was measured around 6e/pixels/sec (or whatever the measurement is in). It's now telling me that my optimum exposure time is about 3-4 seconds. That seems ridiculously fast!! I was expecting around 60seconds maybe but really? 3-4 seconds?

Have I done something wrong with the sensor analysis maybe? This is with the ZWO ASI 533 with Samyang 135 at f2 which I know is fast but wow.

Phil

[admin]

Hi Phil,

firstly, good news that the polar alignment worked!

The light pollution measurement of 6e/pix/s would be equivalent to a relatively dark sky (at least for the UK) - something like Bortle 4.5 (I have a calculator for this online here : https://tools.sharpcap.co.uk/).

A rough guideline for the recommended exposure length is 10 * read noise squared / light pollution rate.

Given your high light pollution rate (which is largely due to the very fast lens), and the lower read noise of this camera particularly if you run it at gain 120 or above, this formula gives you the sort of suggestion that you're looking at in the 3 to 4 second range.

Now, you can always use longer exposures than the recommendation as long as you don't start getting to the point where your tracking becomes a problem – what SharpCap is telling you with the recommended exposure is that going to longer exposures past that point won't help you to see more fine detail in your final image (no matter how much advice to the contrary you might read from old-timers on other forums…).

Cheers, Robin

[Phillyo118]

Thanks for the reply Robin.

Coming from a CCD background when I used to image a while back, I was using my QSI583wsg with a 3nm Astrodon Ha filter and I was doing 15-20 min exposures. Seeing that I only need a few seconds just boggles my tiny mind.

I'll give it a shot though! Ha! With only needing 3-4 seconds, I doubt I'll need to even worry about polar alignment too much either. Win win situation in my eyes!!

Thanks again,
Phil.

[admin]

Hi Phil,

do watch out if going for very short exposures – some people find that they have trouble stacking those sort of images because the stacking software can't find enough bright stars. You also end up with vast amounts of data, so you may end up settling on a compromise exposure value in the 30 seconds to 1 minute range.

Cheers, Robin

[Phillyo118]

I managed to grab about 5 hours (312x60sec) on M45 last night so a fairly decent start there. However, the polar alignment was WAY off using Sharpcap to the point I had to do it manually with the polar scope.

Is it because I'm using such a short focal length at 135mm? (I think equiv is about 336mm on full frame) Or did I just do something wrong?

Thank you,
Phil.