9 replies to this topic

[RAKN]

I just invented the EPA (Elementary Polar Alignment) toolkit : It's a piece of transparent plastic with two small holes in it and a whiteboard marker as seen in the pictures.

With a permanent marker pen I prepared the plastic sheet with positions of some stars near NCP, and also two crossing lines that connect a few of them and NCP. This was done with a picture I took from that area, combined with a screen dump from Stellarium with the J.now equatorial grid (not j.2000). This piece of art is scaled to match what I see with my (Android) camera tethering app, dslrdashboard, since I use no pc in the field.

 

This is how I use it :
- First a rough traditional alignment with the mount's PA scope.

- Slew the scope (I do that manually to save time and battery), and rotate the imaging camera, to roughly align the Live View image with the EPA star pattern.
  Then set the RA setting circle to "0".

- Open the camera shutter (10-30 sec) while rotating in RA at least 180 degrees, and check for a nice star trail of Polaris.
  Return the RA axis to "0".
  Wait for more sky darkness, then take a picture of the faint stars (this time without trailing).

- Open the first picture, align the best fitting EPA circle as close as possible to the Polaris circular trail.
  Then put a small whiteboard pen dot in the center hole. This is the Center Of Rotation (COR) and it is just a temporary mark.
  See pic #1.

- Open the second picture and align the EPA lines and star marks with actual stars in the picture.
  Then copy the COR mark (on top of the temporary one). See pic #2.
  The crossing lines is NCP, so we need to move COR the distance and direction we have to NCP. This is done in the next steps.

- Moving COR to NCP equals moving the stars the same distance but in opposite direction.
  Rotate the EPA 180 degrees and align the last dot made on Polaris (or any star), then put a final dot in the hole at the crossing lines. See pic #3-4. This is where you should put that star.
  So, during Live View, move the chosen star with the AZ & ALT bolts to that last dot.

 

If your Live View window and the "open image" window have identical physical dimensions, you are lucky. That is not so with Dslrdashboard, so I have to adjust the star moving distance to 80 % of the original distance.

This procedure can be done differently, but if done as described here you do not need to hold any plastic sheet on the display when you adjust the ALT & AZ bolts. Both hands are free to do the adjustments.

And how does it work then ? I have only tested a couple of times, but it much improved both accuracy and time needed (after doing a little practicing). Field rotation is almost gone. I used to do the SynScan PA utility with random results and now I just nail the PA (almost), then do a 1-star align and that's it.

This is done with a 700mm FL APO and a aps-c dslr (Nikon D7000). I was lucky with the imaging scale but I guess this can be done with a guidescope camera connected to a pc as well. And how long will it take before someone implements this in software... ? I would love an Android version.

 

Ragnar

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[syscore]

Have you heard of Polemaster?

[RAKN]

Have you heard of Polemaster?

Like I said, I use no pc in the field. And I guess 300 $ matters to some people.

[KenS]

Nice simple solution.

I've been experimenting with a similar approach although I use a computer. In the southern hemisphere there is no bright pole star so using a polarscope is difficult even in dark skies.

Main differences with my method are that I calculate the Centre of Rotation from three positions on the arc. And rather than plotting the positions of the circumpolar stars I plot the circles they rotate around. Placing three stars on their respective circles is enough to get alignment within a couple of arc minutes (confirmed with drift alignment).

In due course I'd like to code it into PHD2 or something similar.Maybe you could work out how to print out the plastic sheet to the right scale?

I did a more comprehensive writeup, which might give you more ideas, at: http://www.iceinspac...d.php?t=143297 

[RAKN]

Thank you Ken,

 

That is some impressing work you have done combining different software and ending up with a high precision PA method.

My method can not compete with that precision, but it sure beats the SynScan PA routine. I just came in from doing a test in my backyard, and during a 20 min exposure, I cannot see any dec drift at all. But then, my OTA is only 700 mm FL.

 

 

[KenS]

If you want to get more accuracy you can keep applying the same approach but zoom in closer (if you can). I estimate you have a pixel scale of 1.4"/pixel whereas I was testing at around 11'/pixel. Sou you could zoom in digitally almost 10x without any problems.

You'd need to identify some fainter stars nearer the pole to align on.. One compensation in the southern hemisphere is that even though we don't have a bright star like Polaris, as you get closer to the SCP there are several stars around mag 7 or 8 within about 10' of the pole making a distinctive asterism. That close to the  NCP the stars are much fainter, around mag 10.

[FirstC8]

If you want to get more accuracy you can keep applying the same approach but zoom in closer (if you can). I estimate you have a pixel scale of 1.4"/pixel whereas I was testing at around 11'/pixel. Sou you could zoom in digitally almost 10x without any problems.
You'd need to identify some fainter stars nearer the pole to align on.. One compensation in the southern hemisphere is that even though we don't have a bright star like Polaris, as you get closer to the SCP there are several stars around mag 7 or 8 within about 10' of the pole making a distinctive asterism. That close to the NCP the stars are much fainter, around mag 10.

Ken, since you do use a PC, have you thought about Polemaster? And how does your method differ from it, other than the extra $300 cost?

I understand the appeal of figuring out stuff oneself, even though there might be devices available to do it for you. Sometimes it is not about money.

Edited by FirstC8, 25 April 2016 - 01:04 AM.

[KenS]

I mainly did it for a bit of fun, as it often leads to learning new techniques. Normally I go straight into a drift alignment with PHD2 so no need for a Polemaster. But I thought it would be good to have an alternative method for, say, when the horizon is too hazy.

Obviously, my method (and the OP's) is based on similar principles to Polemaster which in turn seems to be based on this paper by Rick Kellogg http://www.syracuse-...nment_Scope.pdf. The unique aspect of the Polemaster would appear to be the adapter for mounting it in line with the RA axis.

The breakthrough for me was when I realised I don't need to mount the camera on the RA Axis and could just use the guide camera or even the imaging camera, as long as it could be roughly collimated with the RA axis. I believe some Polemaster owners have also been successful using them in a similar way.

Edited by KenS, 25 April 2016 - 02:54 AM.

[RAKN]

If you want to get more accuracy you can keep applying the same approach but zoom in closer (if you can). I estimate you have a pixel scale of 1.4"/pixel whereas I was testing at around 11'/pixel. Sou you could zoom in digitally almost 10x without any problems.

You'd need to identify some fainter stars nearer the pole to align on.. One compensation in the southern hemisphere is that even though we don't have a bright star like Polaris, as you get closer to the SCP there are several stars around mag 7 or 8 within about 10' of the pole making a distinctive asterism. That close to the  NCP the stars are much fainter, around mag 10.

Sure, I could zoom in digitally in Live View and also when opening the actual picture taken. But from what I have found so far, I get very good results from one single operation which takes maybe 5-10 minutes. And I also want to keep things as simple as possible.

One thing I had in mind when doing this was portable devices, like Vixen Poalrie etc (I don´t have one, so just guessing). The PA can be hard to do accurately with these, and with just a smartphone and the EPA (modified for the actual focal length) maybe it could improve things. 

The deep space season is over here now, so I really look forward to the next season - and images free from field rotation .

[BenKolt]

This is a neat field method you've come up with.