Balance and polar alignment of a computerized EQ mount

[orfest]

 Hi,

I have a Advanced VX 6', which comes with a computerized equatorial mount with a GoTo.

I hope you can explain to me, what should I do when I put a DSLR camera (or a binoviewer, or a heavy eyepiece) on my scope and need to rebalance it. Should I turn off the mount, re-balance, turn it on, re-polar align? If so, how can you polar align the mount, with the camera on it instead of an eyepiece?

Is computerized alignment good enough for DSO imaging?

I usually deploy the telescope in the twilight. I polar align the mount, balance the scope get started looking at some bright objects. The polar alignment is not really good, but with 2 stars and 4 calibration stars the computer works well enough for observing and GoTo is rather precise.

Then night comes and I want to try some DSO imaging, and put a camera on my scope. The balance is off. I need to fix it. What should I do?

Usually I proceed as follows:

- Turn off the mount.

- Balance it.

- Turn on the mount.

- Re-do the polar alignment. This part is a problem, because it's difficult to aim the scope directly at a star when you don't have an eyepiece, but only have a camera with a rectangular screen. I also suspect that the center of the camera screen and the center of the eyepiece do not coincide.

These are my questions:

1. How to properly polar align a computerized mount? My recent attempts show star trails after as little as 30 seconds of exposure.

2. Assuming my current polar alignment is good, can I skip the polar alignment after re-balancing the scope?

2a. Is it even possible to re-use the current polar alignment? When I'm turning the mount off, I'm going to move it around with my hands, and I'm sure I will not move the scope to exactly 0 dec and 0 RA before turning it on, but rather to ±0.5 dec and  ±0.5 RA. This should not affect the tracking, but it will make GoTo function imprecise.

[Davey-T]

Have a look at this method.

Dave

http://www.cloudynights.com/page/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

[orfest]

That guide looks really helpful, will try it next time.

[Dr_Ju_ju]

Look for the set-up videos on here  https://www.youtube.com/user/astronomyshed/videos, there is one for drift alignment, which uses just a camera & software

[Pompey Monkey]

If it's an EQ mount and you have just polar aligned it with an eyepiece, it will retain the polar alignment until you move the mount.

After all, you don't realign every time you swap out the eyepiece.

The balance should not affect the polar alignment. If the mount is so shaky that altering the balance does this, then it's probably not good enough for AP.

Or do you mean a star-alignment for the go-to? That is a different kettle of fish. In this case do a star alignment with the eyepiece, but make sure that the finder is adjusted absolutely spot on to the main scope. That way, you'll have a fighting chance of doing a star alignment with the live-view on your camera.

[Owmuchonomy]

Before I had my obsy I would balance my setup in twilight with all the photo kit on it, guidescope etc.  I would then remove the DSLR and use the usual viewing set up without rebalancing for polar alignment.  Then when replacing the eyepiece with a DSLR you are ready to go for astrophotography.

[MarsG76]

G'day mate,

You don't need to do you polar alignment again if you just change from an eye piece to a DSLR. The only time you need to do your polar alignment procedure is if you move the mount. That said I think you're more talking about star alignment, then if you power down to attach your DSLR, you should do a star alignment to ensure your GOTO accuracy.

Personally I use the live view feature on my DSLR to do star alignment if I'm going to do astrophotography... Use that if your camera has the feature.

The polar alignment feature on your VX mount is excellent for viewing but it is not accurate enough for AP, hence why you have drift in as little as 30 seconds. Such a short exposure time is an indicator to your PA being quite a fair bit off. To align you mount you need to use drift alignment to zero in on the pole.

https://starizona.com/acb/basics/using_polar.aspx

As well as getting your polar alignment sorted out, you will still be limited to a maximum of only about 90seconds to 2 minute exposures due to periodic error, the imperfection in all gears which will start to elongate your stars... Once you get started into astrophotography and will want longer exposure, you'll need to have a autoguiding kit to correct for the gear (period) errors.

Hope this answers some of your questions.

[orfest]

Thank you all for the replies.

I think I understand it now:

Polar alignment depends on the position of the mount.

Star alignment needs to be re-done if the mount if powered down.

Balancing the scope with the DSLR attached and never rebalancing it again sounds good

I tried DARV last night, but was only able to align the scope on 1 star, which is South, 0 declination. That did let me have a 60-seconds exposure with less drift than I used to have on a 30-seconds exposure.

I could not do DARV adjustment on East or West horizons, because of trees and a hill respectively

Isn't it true, that if I adjust the azimuth of my mount using a star on South, 0 declination, then I can use any other star to adjust the altitude of the mount?