Obstructions everywhere

[thefrogulox (Al)]

Hi all

I have been attempting to drift align in my backyard using a modded webcam and K3CCD Drift explorer

The main problem I have is that from the only realistic spot I can use in my backyard has obstructions (trees, houses etc) in almost every direction. The worst is a block of apartments directly to my south which is +- 10 metres high and about 10 metres away. By some rough calculations, this blocks out over 45 degrees from the southern horizon.

I also have a massive tree to the east which blocks out most of the sky directly to the east.

Every tutorial I have read about drift alignment suggests using stars as low on the horizon as possible.

Is the main problem with using stars high on the horizon is that you have to wait longer for them to drift thereby decreasing accuracy of the alignment and increasing the time taken to get into alignment?

Does anyone have any general suggestions for drift alignment when you have obstructions in almost every direction?

Thanks for the assistance.

[[1ponders] (Paul)]

& thefroulox

What sort of view do you have to the west? You can comfortably use 20 deg altitude east or west. Once you start to get too much higher Azimuth adjustments start to have an impact on you readings.

This isn't to say you can't go higher but you will likely need more iterations between Altitude adjustment and Azimuth adjustment.


You are certainly going the right way about it using K3. Its all I ever use and it will give you an accuracy as good as if not better than any other method.

but then maybe I'm biased

[thefrogulox (Al)]

Thanks for the prompt reply.

My view to the west is better, but still not ideal. You must be thinking that I have no sky at all!

From what I think you are saying, does using a star in a higher altitude in effect mean that it is difficult to isolate the alt/azimuth adjustments from each other?

Meaning that I basically have to move between making adjustments in alt (in east/west) and azimuth (north/south) more times before I can get an accurate alignment?

[[1ponders] (Paul)]

Not at all. My previous house had no west at all (two story wall straight up) and around 45 deg to the east. It simply meant that I had to adjust elevation/altitude and azimuth through multiple iterations.

I would roughly adjust altitude using the scale on the mount. Then using a star near the meridian at Zenith I would adjust azimuth until it was around half the original deg drift according to K3 (the pink reading in drift explorer). (Note: for ease of adjustment I would make sure the webcam was orientated so that when I pressed the up dec button the star moved up and parallel to the K3 reticule). Its important to record/remember the amount of Dec drift in both Altitude and Azimuth readings in this process.

As in any drift alignment ignore RA drift completely. It doesn't enter into any adjustments.

Once you have adjusted half the azimuth adjustment out slew down to your lowest elevation (you can easily allow +/- 5 deg n/s off 0 deg dec). Now adjust your Altitude adjustment so that you remove half to 2/3 of your dec drift error.

Slew back to your meridian star and again remove half your drift. Then back to Altitude adjustment. Then back to Meridian and remove the remaining adjustment. And finally back to Altitude to finish removing its error. You may find that you will need to do another one or two iterations, but no more.

Using this method you should be able to manage sub-arcsec drift/min in a very reasonable time. It is a very accurate method. If you make sure you mark your tripod leg spots when you setup next time you will should only need a minor tweek of your settings.

Hope that helps.

[thefrogulox (Al)]

Thanks Paul, that's very useful.

If my lowest elevation towards to N/S horizon is greater than 5 deg from 0 dec, do I have problems?

[[1ponders] (Paul)]

You won't be using a N or S horizon. It has to be east or west (0 deg declination. Basically 90 deg to your RA axis). you won't be able to drift align point towards the north or the south, basically point towards where the sun comes up or goes down and that will get you close enough.

I don't bother using the dec circle on my mount, I simply turn my dec axis is 90 deg to the RA and that will do it

[thefrogulox (Al)]

Thank you very much for all that Paul.

I tried it tonight and made substantial progress but was cut short due to pretty extensive cloud cover...

How long do you let the drift explorer run between adjustments before retargeting? I was doing 1 - 2 mins each hit as I found I got inconsistent readings (ie drift per minute would move up and down) if I did less (I'm guessing due to clouds passing over head and below average seeing - it was very humid out).

Is this too long / too short?

Hope my question makes sense!

[[1ponders] (Paul)]

Initially if there is a large drift I may only let it run for 15-20 sec. As your polar alignment accuracy gets closer and closer I'll let it run for longer but rarely longer than a min or two. If you find you get a drift and then it settles out reselect the start or hit the reset icon in the drift explorer to zero it a again. As I'm sure you're aware you are aiming for the pink graph to sit on the zero line (+/- seeing effects), and the pink Dec drift readout to read 0. I normally settle for anything less that 1 arcsec drift. A bit of dec drift is ok. It keeps all the dec adjustments during autoguiding in one direction and helps to eliminate the need for backlash compensation.

[thefrogulox (Al)]

Paul, I'm a little confused over star selection for the drift alignment.

I'm happy with the star on meridian near zenith for Azimuth adjustment, that is clear enough to me.

However, I'm not really sure which star to use at low elevation on the eastern horizion.

You've said to use Dec 0 +/5 degrees (ie celestial equator).
However, as I go higher up on the eastern horizon to get a clear view and find a bright star, I move away from the celestial equator (as the line of 0 dec heads towards the north horizon). Is it better to stay on the equator (+/- 5 degs) or stay pointed East?

[[1ponders] (Paul)]

If you find you are having trouble finding a star low and close to the celestial equator in the east, increase your exposure time. I use a standard ToUcam 840 and don't have a problem finding a star at 1/25sec @ 5 fps. It doesn't need to be really bright, just as long as you can see it on the screen and K3 can select it.

[Moon (James)]

Al,
It looks like you're on the right track - so I'll just throw a few random comments out there:

1. Keep at it. Practice makes perfect and you will develop your own technique in no time at all - and then it becomes second nature.

2. I always had trouble finding stars with a web cam. I found the QHY5 more sensitive and therefore easier to find stars just about anywhere in the sky.

3. I prefer to use an eyepiece for drift alignment - of course its only a personal preference - it just seems easier to me. The theory is the same, but with less technology to go wrong. It does have the disadvantage that you need to replace the eyepiece with a camera at some stage :-(

James

[thefrogulox (Al)]

Last night I managed to get the drift down to around 0.3"/min after around 3 hours of mucking around!

The seeing however was terrible and as the star was bouncing around, I had to let it drift for longer than I imagine I would usually have to in order to get an accurate reading.

Once again, thanks for the help Paul, really appreciated.

[[1ponders] (Paul)]

Yeah that sounds about right for a beginner Don't worry, it will get much easier and much much quicker once you get the process down pat. On a new site with no interuptions I can get to 0.5"/min drift in under half an hour (usually much less)from finishing setup and balance. It does take practice and knowledge on how your gear responds to adjustments etc.