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Hoping for advice on imaging near Polaris, please

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#1 Scott Sloka

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Posted 19 September 2019 - 01:47 PM

Hi all,

 

I was hoping folks knowledgeable with imaging near Polaris could please share their experience with me. After some time off, I am back to imaging and want to retry imaging at higher latitudes. I once tried to image right near Polaris, but experienced significant field rotation so I never went back to it, but I have since acquired a Polemaster, which is one of the best pieces of kit I have ever owned (mostly because I truly sucked at polar alignment). I am hoping that will help improve my chances of success.

 

My system is an FSQ106 (fl=530), PhD guiding (have not upgraded to PhD2), Skywatcher EQ-6 Pro mount, and a KWIQ guider.

 

My questions are:

 

1. Is there any advantage to upgrading to PhD2 from PhD? It seems to work okay for most of my needs but I can upgrade if it will improve my chances of success, although I dread a night long of pulling my hair out in the field if the transition is not smooth.

2. I recall a parameter somewhere in the "brain" icon for PhD that I need to increase or decrease to improve the calibration within PhD. Do I need to adjust any parameters to get better/feasible calibration?

3. I have read some older posts about imaging "high up" and some have suggested calibrating guiding at a lower latitude and then moving up to the higher latitude. Will PhD be able to correct for the change in latitude?

4. Any suggestions on where to pick a guide star? Some older posts have suggested to pick on in the middle of the field of view, but given my setup I am not sure how to make that happen as I cannot tell where I am with my guide scope (there is a camera on the back of it).

5. I tend to acquire 10min subs for luminance. Will this be possible, or will I have to lower my expectations and back this down to 5min to avoid elongated stars.

 

I am hoping the addition of the Polemaster helps tremendously.

 

Thanks for any guidance you can give as I begin to kick the rust out of my acquisition methods.

 

Scott



#2 the Elf

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Posted 19 September 2019 - 02:13 PM

Scott,

 

how close is close to polaris? Which one is the dominant problem in your setup, periodic error or polar alignment error? Do you have PEC?

If your mount suffers from huge PE and your polar alignment is perfect you might want to guide RA only and use a guide star far away from the NCP, at the corner of the guider's field of view. If your mount is tracking well and your polar alignment is poor (maybe it was perfect, but the mount sinks in the ground during the night) you might want to guide dec only. I guess you have to do some testing first.



#3 Scott Sloka

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Posted 19 September 2019 - 02:42 PM

Those are great questions and good advice. I would actually image Polaris if I could.

 

I would say historically, polar alignment was my dominant problem until the Polemaster. It still could be an issue, although my alignment has been much better with the Polemaster.

 

I have never given thought to periodic error, but most nights I can get good star shapes even at 80 degrees now. I do not know how to look for this or measure it. Do I need PhD2 to look for this? I have seen some graphing capabilities with PhD2 that I have not found in the older PhD that I am using.

 

I have heard of shutting off dec guiding. I might try that in the field if I cannot get good star shapes.

 

Scott



#4 555aaa

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Posted 19 September 2019 - 03:02 PM

Well, at the actual pole you can't actually guide. For example, consider you are at the pole and when you get there, the hour angle of your mount is at 0. Now the star drifts east by one arc second across the sky perhaps because of some flex in your mount or some misalignment. To make that move, the mount has to rotate 90 degrees in RA so that it is at hour angle = 6 and then make a move in declination. THe star has really drifted in dec but the mount has to move itself in RA to make the correct move in dec. Tools like PHD don't understand that and the whole process collapses because the moves are so large. The pole is really a point of ambiguity where all hour angles mean the same thing (the HA and RA of the pole is ambiguous). That's why when you are up very high in declination things start to fall apart in guiding.But the saving grace is that the stars don't really move on their own at the pole.


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#5 Scott Sloka

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Posted 19 September 2019 - 03:25 PM

Thank you 555aaa for your insights. Does this mean that not guiding at all would be the best approach for imaging Polaris? It would mean that my polar alignment would have to be bang on.

 

Scott



#6 555aaa

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Posted 19 September 2019 - 06:47 PM

Well Polaris is very bright so the exposure for that is going to be a fraction of a second, but for faint objects near the pole the optimal approach is going to be sensitive to how you are misaligned on polar alignment. Suppose the true pole is in your field of view and it is say 10 arc minutes misaligned with the mechanical pole of the mount. A star at the true pole will trace a complete circle on your detector in 24 hours, and the circle radius will be ten arc minutes, or a length of 2pi times 10 or 62.8 arc minutes on the image scale of your detector. So in one hour it will be a length of (62.8/24) or 2.6 arc minutes and in one minute that would of course be 2.6 arc seconds. So that gives you an idea of how much trailing you would get, with the drive on and working correctly but not being polar aligned but imaging the "true" pole. I put it in quotes because refraction means that it's not really the true pole but the refracted pole in this case since I think refraction would be constant.


Edited by 555aaa, 19 September 2019 - 07:19 PM.

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#7 Scott Sloka

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Posted 19 September 2019 - 09:13 PM

I went back to look at an old attempt in 2016 and I am not impressed with the oblong stars. This was with 10min subs. I might have to experiment a bunch and maybe "blow" a night learning the trick to it, although from my perspective, there is never really a blown night as I am just happy to get out again and each step is a moving forward.

 

Will share any learning that happens, although I am still happy to receive more advice before I try again. I am still uncertain if I can calibrate PhD guiding at a lower altitude and then use it higher up and maybe shut off dec. May have to go to 5min subs too, although I am still hoping 10min are possible. Nobody has suggested that I switch to PhD2, which simplifies my life.

 

Scott



#8 Scott Sloka

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Posted 19 September 2019 - 09:14 PM

Thanks again 555aaa for your detailed advice.

 

Scott



#9 the Elf

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Posted 20 September 2019 - 01:06 AM

Short instruction for home made PE measurement, no software needed at all:

- manually polar align your scope, not too precise

- intentionally misalign az about 1 deg

- pick a star on the equator, (right now Altair early evening, Betelgeuze early morning)

- Set the lowest ISO, camera in portrait orientation

- let your mount track as usual with siderial speed

- Take a 25 - 30 min single shot

-> the strong polar error in az results in a strong star drift in North-South direction, up or down in the image. A pefect mount would give you a straigt line. You will see something like a sine curve with 8 to 10 minutes per oszillation, depending on how the mount's mechanics is designed.

- either calculate your arcsec per pixel or get it from plate solving a test image (astrometry.net) to translate the amplitude in pixels to arcseconds.

If your mount supports PEC, teach it and try again. Here is my experiment with the SW EQ6-R:

 

PE-compare.png

 

I inverted the image and added the red lines and put two image in one for better comparison. Basically this is the actual star trail. The image is rescaled so that 1 pixel is 1 arcsec now.


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#10 Scott Sloka

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Posted 20 September 2019 - 09:05 AM

Very cool. Thank you very much the Elf. I am going to be spending the next while reading about PEC as I suspect that I might have to shut off either DEC or all of guiding for capturing close to Polaris. In the past, I could be lazy about this as guiding seemed to make my stars look better. I will have to pick a night and invest in fooling around with PEC. I have an older EQ6 mount which supports PEC, so I am now reviewing guiding and PECPrep and am now downloading PhD2 onto my older laptop to try to run it separately in the field. Thank you very much for giving me a nudge in a good direction.

 

Scott



#11 the Elf

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Posted 21 September 2019 - 05:16 AM

Some mounts support PEC .... and forget the data when you turn off power. Others keep them in permanent memory. In that case it is sometimes called PPEC - permament periodic error correction. The EQ6-R (as well as the AVX) keep the data permanent. Check if your mount does or if you have to upload the data every time you want to use it. Imho the simple mount function to record and replay is much better than nothing. For the perfectionists there are tools that extract cleaner data from a set of several curves. I use the simple one in addition to the auto guider. To record use the auto guider and pick a bright star near the equator like Altair or Betelgeuze or anything with Dec within +/- 20°. Make sure you stay away from the mount. Any walking on the ground nearby will disturb the data and standing in front of the scope blurring the air by body heat does so as well. Check the result as I described. Good luck!




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