Drift Alignment - The DARV method

[Sam]

After reading the sticky post about PHD I decided to do whatever I could to improve my guiding. My first step was to review my alignment. It did this with the DARV method for drift aligning. I hadn't heard of this method until I saw it as an option on APT so after some googling and reading I understood what was going on and decided to give it a go myself.

I'm in the Southern Hemisphere so I pointed the scope south at about 0 dec and focused. I then set up a one minute exposure. On the EQMOD control window I held down west for 30 secs then held down east for the rest of the exposure. I ended up with a very narrow V shape on the picture being drawn by a star moving in Ra corresponding to my 30sec button presses in each direction. So I adjusted the azimuth east a quarter turn. Then I did the same one minute exposure with the west/east thing for 30secs again. The V was a bit narrower so I did another wee turn on the azimuth to the east. This was the right direction as the V got a bit narrower. I repeated this until the V turned into a line. Then I took two minute exposures and held the west/east for one minute each button. The result was a very narrow long V, almost a straight line. I did this a couple of times, slowly tweaking the azimuth east until the V completely disappeared. If the V got wider then I would have to move the azimuth west but in my case east worked and I didn't overshoot.

The next step was to swing the telescope east and as close as possible to the horizon and adjust the attitude until the V disappears just like the process above. The result after all of that is an aligned mount. This process is very effective. I didn't worry about the orientation of the V as it would have worked anyway the camera was orientated so there was no need to twist it around to going sideways.

I did a couple of 5min exposures to test and the guiding was quite a bit more accurate. Job done!

[Pete Presland]

never heard of the DARV method, but anything that improves alignment has got to be worth doing!

[jgs001]

Interesting and nice proof Sam. I've been meaning to try it myself the next time I get the haq5 out

Sent from my GT-I9300 using Tapatalk

[Sam]

Basically it's just a way of visualising star movement so no real difference from using a reticule or any other method. I'm quite pleased how well it works and how quick, I would like to have known about this 5 years ago! The RV in DARV stands for Robert Vice - thanks Mr Vice!

[Steve Ward]

It's a very effective and simple method , I've been recommending it here for ages ... 

Much easier than all that squinting through a reticule eyepiece for hours .

You will need to to a couple of runs on each axis  , each time you adjust one it affects the other slightly , but each time you re-tweak the error gets less.

And the longer you make the test exposures the more accurate it is , even if the star goes way of the chip it will still return and give a trace to evaluate.

[wxsatuser]

Have tried it once and it gave good results.......it's simple and quite effective.

Here is one that went off the sensor but the return path is nicely on top of the original.

I did this when it was a bright but hazy cloudy night.

Next time I want to try longer ones just to double check.

[Physicist13]

Guess this may be stating the obvious but its good to know how well your polar alignment needs to be for what you indend to do. To avoid unecessary time and effort. ie do you need it good to 0.5deg, 10arcmins,1arcmin,10arcsec,1arcsec ??!

P

[ollypenrice]

I've tried this method and it is pretty attractive. However, we need to remember that it is only a study of drift for the length of your exposure. 2 minutes isn't long for observing drift. I normally make it ten minutes a time for an observatory alignment. So far as I can see the DARV method doesn't bring anything new to the table other than a very helpful visualization (as Sam has said.) However, it's only as good as your test subs are long. It doesn't matter if you slew right off the screen either. Just slew back onto it again later!

Olly

[lensman57]

After reading the sticky post about PHD I decided to do whatever I could to improve my guiding. My first step was to review my alignment. It did this with the DARV method for drift aligning. I hadn't heard of this method until I saw it as an option on APT so after some googling and reading I understood what was going on and decided to give it a go myself.

I'm in the Southern Hemisphere so I pointed the scope south at about 0 dec and focused. I then set up a one minute exposure. On the EQMOD control window I held down west for 30 secs then held down east for the rest of the exposure. I ended up with a very narrow V shape on the picture being drawn by a star moving in Ra corresponding to my 30sec button presses in each direction. So I adjusted the azimuth east a quarter turn. Then I did the same one minute exposure with the west/east thing for 30secs again. The V was a bit narrower so I did another wee turn on the azimuth to the east. This was the right direction as the V got a bit narrower. I repeated this until the V turned into a line. Then I took two minute exposures and held the west/east for one minute each button. The result was a very narrow long V, almost a straight line. I did this a couple of times, slowly tweaking the azimuth east until the V completely disappeared. If the V got wider then I would have to move the azimuth west but in my case east worked and I didn't overshoot.

The next step was to swing the telescope east and as close as possible to the horizon and adjust the attitude until the V disappears just like the process above. The result after all of that is an aligned mount. This process is very effective. I didn't worry about the orientation of the V as it would have worked anyway the camera was orientated so there was no need to twist it around to going sideways.

I did a couple of 5min exposures to test and the guiding was quite a bit more accurate. Job done!

Thank you for posting and very interesting , I have this facility in APT but as with many other variations of the Drift Aligning the pre -requisite is an uninterupted all around view of the horizon, something that the a great majoriity of back garden imagers are deprived of.

Regards,

A.G

[JamesF]

Thank you for posting and very interesting , I have this facility in APT but as with many other variations of the Drift Aligning the pre -requisite is an uninterupted all around view of the horizon, something that the a great majoriity of back garden imagers are deprived of.

I'm not absolutely certain you need a clear view of the horizon.  I think it will work higher in the sky, but may take a few more iterations.

James

[Sam]

I've tried this method and it is pretty attractive. However, we need to remember that it is only a study of drift for the length of your exposure. 2 minutes isn't long for observing drift. I normally make it ten minutes a time for an observatory alignment. So far as I can see the DARV method doesn't bring anything new to the table other than a very helpful visualization (as Sam has said.) However, it's only as good as your test subs are long. It doesn't matter if you slew right off the screen either. Just slew back onto it again later!

Olly

True, but not really difficult to do a ten minute exposure and make a similar pattern to visualise the drift - like pressing the west button for 10 secs every minute five times then the same with the east button. What it brings to the table is another way of making drift alignment easier.

[ollypenrice]

True, but not really difficult to do a ten minute exposure and make a similar pattern to visualise the drift - like pressing the west button for 10 secs every minute five times then the same with the east button. What it brings to the table is another way of making drift alignment easier.

That's a very good idea!

Olly

[SnakeyJ]

I'm not absolutely certain you need a clear view of the horizon.  I think it will work higher in the sky, but may take a few more iterations.

James

That's interesting James, I had discounted this due to my restricted western horizon which is probably 22deg, my southern is probably 15deg.    I will have a go and see if I can improve on my rather coarse/rudimentary alignment.

[wxsatuser]

That's interesting James, I had discounted this due to my restricted western horizon which is probably 22deg, my southern is probably 15deg.    I will have a go and see if I can improve on my rather coarse/rudimentary alignment.

You don't need the horizon the way I read it.

Point scope due south, and at 0 degrees dec.

The meridian and 0 degrees dec will be about 40degrees high in the sky.

Eastwards/Westwards as far as I know you point at 0 degrees dec, it does'nt say at the horizon.

So find a star thats on the equator towards the East/West, I understand you can go about 20 degrees

above the horizon for this.

[lensman57]

You don't need the horizon the way I read it.

Point scope due south, and at 0 degrees dec.

The meridian and 0 degrees dec will be about 40degrees high in the sky.

Eastwards/Westwards as far as I know you point at 0 degrees dec, it does'nt say at the horizon.

So find a star thats on the equator towards the East/West, I understand you can go about 20 degrees

above the horizon for this.

To the south I have clearance above 45 Degrees, to the West above 55 Degrees, to the east above 50 degrees, will give it a go and see what happenes.

Regards,

A.G