Massively speeding up polar alignment

[tc.astro]

Most people have their polar camera fixed to their mount. So why do we have to rotate the mount every time we do a polar alignment?

Seems to me that the first time we do a polar alignment SharpCap knows where the camera is pointing and where the mount is pointing. So it can store the angle between them for next time. Next time, when you do the first plate solve SharpCap looks up the angle and then knows where the mount is pointing. No need to do a rotation.

Seems that this method could make polar alignment even more accurate. When you affix the camera you can do a rotation calibration. Then choose three or four points of rotation instead of just two. That must surely make for better accuracy than a single rotation which will test the mount in just two positions.

Another speedup would be to plot the suggested adjustments as a function of time - plot say the last 30 seconds worth. The human eye would smooth out the seeing artifacts and get a better appreciation of the adjustments required rather than the current method which tends to chase the seeing a bit.

[admin]

Hi,

Unfortunately the approach of remembering the angle and then doing a one-step alignment each time only works well if you can guarantee that the alignment camera hasn't been disturbed at all from one used to the next. It's become clear that even the weight of the dangling USB cable is quite enough disturb an alignment camera enough to knock the polar alignment slightly off. Any sort of contact or impact between sessions could easily have the same effect and you would never know until your stars started trailing.

If you happen to rotate them out slowly one time in small steps of about 15° when doing your polar alignment you will notice that you get several red crosses appear that are measurements of the apparent centre of rotation for different angles of rotation. These are almost always perfectly aligned unless you have some sort of mechanical flexure in the mounting of your alignment camera – in fact those crosses are there to help people detect that sort of problem.

Cheers, Robin

[tc.astro]

It's useful to know that even USB cables can affect polar alignment when using SharpCap in the normal way.

Some people, though, have their camera permanently attached to their mount in ways whereby the camera will never move. Some mounts have the camera inside the mount itself in place of a polar scope - they are so buried that the lens cap contacts the mount and not the camera. Those people would benefit from not having to rotate the mount every session.

[RazvanUnderStars]

Hi Robin,

Quick question: what setups (i.e. mount + OTA combinations) were affected by the weight of the USB cable? I'm curious at what level they no longer matter.

Thanks.

[admin]

Hi,

I could make it happen myself with a good quality 50mm guide scope (with better than average mounting) and a standard ZWO/Altair/qhy guide camera. Typically the mounting of the camera in the guide scope and the mounting of the guide scope on the main scope have at least a bit of flex in them. With a dangling cable this was enough to lead to noticeable changes in the centre of rotation point as the angle of rotation changed. With a properly secured cable there was no problem.

Cheers, Robin