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# Measuring Binaries with a Tracking Alt-Az

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### #1 Cotts

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Posted 08 April 2021 - 11:11 AM

I'm hoping to be able to measure binary stars, via "Video Lucky Imaging" with my not-arrived-yet 20" Dob.

The mount is an Alt-Az so the main problem I foresee is calibrating the camera frame's angle with the sky.  Field rotation from pointing here and there in the sky will mean that I will have to calibrate for PA before every video.

Here is a possible workflow for acquiring the imagery:  Using a Canon 60Da DSLR and Backyard EOS....

1.  Slew to target pair.  Centre.  Tracking ON.

2.  Place pair near East edge of the field.

3.  Tracking OFF and open shutter to record a drift line across the frame.

4.  Tracking ON, re-centre the pair.

5.  Acquire 30 seconds of video

6.  Repeat steps 2 and 3. The mean of the two  drift lines will be the  'actual'  drift line and be incorporated in the calculation of the actual PA.

Repeat 1-6 for each successive pair...

What am I missing?

thanks,

Dave

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### #2 astrokeith

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Posted 08 April 2021 - 12:10 PM

You could take stills, and then use plate solving to determine very accurately image scale and image rotation.

### #3 JamesMStephens

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Posted 08 April 2021 - 12:18 PM

Dave,

Do you have an astrometric eyepiece of any sort?

Jim

### #4 gregj888

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Posted 08 April 2021 - 01:17 PM

Dave,

Your plan should work if your timing is consistent.  I would probably do 2 sets of captures with another drift in between.  That gives you several ways to test the orientation solution.

So  drift, capture, drift, capture drift for each target.

You can also de-rotate the images and plate solve if wide enough field.

The best answer is probably a field de-rotator combined with your drifts.

Instead of Lucky, look at speckle or triple correlation... IMHO far better.  You can get Reduc or Speckle Tool Box for free.  Both do auto correlation, Reduc also does cross correlation and STB does Bi-Spectrum/Triple correlation.   I think Reduc will take video, STB want fits files/cubes.  Reduc will take Fits files and make a Fits cube.  With a 20" and 290m camera or better, you should be flirting with .25" separation and a limiting mag of 13-15.

BTW, you can combine Lucky with speckle but you want a lot of frames, like 10,000 plus.  Load into Reduc (also does lucky), crop for a power of 2, grade and select for lucky then run auto correlation.

Greg

### #5 Cotts

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Posted 08 April 2021 - 01:47 PM

I do have an astrometric eyepiece but its accuracy pales compared to video lucky imaging... I get consistent results to the second decimal of an arc second allowing me to publish to 0.1" and 1Âº......

The drift images will be full frame, about 40' x 26'.  These can be analyzed in Astroimage J to find pixel (x,y) values at each end of the trail and use Pythagoras math to get the angle .  Or a Still of about 30 seconds can be taken and analyzed by astrometry.net.....  I'll try both methods and compare results...

then for the video I switch to 640x480 crop mode, field of view about 4'50" x 3' 40" and grab about 30 seconds of video = 1800 frames.  Stack in Lynkeos and analyze in Astroimage J, to get apparent PA and Sep. in pixels...   Apply the image scale and the PA correction.... I have a spreadsheet for all of this - works great.   But I've not done measures with an Alt-Az before...

The drift -capture-drift-capture-drift methodology - I'm not sure what the extra drift will do....

I'm stuck with Mac computers so speckle, REDUC etc. is out of my reach..

With my DSLR and  20-inch I'll be reaching about 11th magnitude and separations down to about 2", depending on seeing.   There is a huge number of pairs needing measures that are wider and brighter than these limits....

Dave

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### #6 gregj888

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Posted 09 April 2021 - 11:00 AM

Dave,

All good...

drift -capture-drift-capture-drift  allows you to solve for 2 sets of data and the PA should be the same (error analysis).  You can also use the first and last drifts, calculate the orientations then compare with the middle drift, which again should match closely.   Genet and company use a minimum of 3 data sets when submitting... that would be 3 of these sets (drift -capture-drift-capture-drift).

What it really does is allows a better error analysis and helps (or should) data integrity.

You should be able to run Reduc on a virtual machine if you ever want to try it.

As I said, all good.

### #7 Carbstone

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Posted 10 April 2021 - 03:36 AM

Hi Dave,

Attached a publication (JDSO, Spring 2008) from prof. Thomas G. Frey about "Visual Double Star Measurements with an Alt-Azimuth Telescope".

Thomas used a micrometer instead of the 'Lucky Imaging' method. The time he spent on one measurement is certainly longer than what the 'Lucky Imaging' method takes. You can probably follow the same procedure.

Cheers,

Dirk

### #8 Cotts

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Posted 10 April 2021 - 08:39 AM

The micrometer eyepiece is not nearly as accurate as measuring the centroids of stacked video images.  Not even close...

My only problem is how to deal with field rotation, not during my short, 30-second videos where it would be negligible, but from pointing to various parts of the sky which radically changes the E-W baseline....

Dave

### #9 astrokeith

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Posted 10 April 2021 - 10:13 AM

The micrometer eyepiece is not nearly as accurate as measuring the centroids of stacked video images.  Not even close...

My only problem is how to deal with field rotation, not during my short, 30-second videos where it would be negligible, but from pointing to various parts of the sky which radically changes the E-W baseline....

Dave

I repeat - try plate-solving.

I have an 18" Dob and it gives me field rotation to many decimal figures. Its also very quick as I know within 30 arc min where I am pointing.

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### #10 Cotts

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Posted 10 April 2021 - 11:54 AM

Keith, I am aware of the data you get from astrometry.net and the like when plate solving.

The difficulty for me is that the field of view for my video stacks is 4'50" x 3' 40" and usually only contains the images of the binary star target.   So plate solving simply does not work due to lack of stars...

Thus, I drift a star across the field and get the camera angle from that..

It would take a star about 20 seconds to drift across the field with the drive off.  Then drive on, re-centre for the stacking video then drive off for another drift.  I could double or even triple the iterations of this for checks on repeatability and error analysis...

Three drifts and two videos could be done in under 5 minutes, I suspect...

I could take a full frame single image with a 1-2 minute exposure to get enough stars for a plate solve but that opens up an entirely different can of worms - auto guiding would be necessary..... takes too long....

Dave

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### #11 astrokeith

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Posted 11 April 2021 - 03:41 AM

Keith, I am aware of the data you get from astrometry.net and the like when plate solving.

The difficulty for me is that the field of view for my video stacks is 4'50" x 3' 40" and usually only contains the images of the binary star target.   So plate solving simply does not work due to lack of stars...

Thus, I drift a star across the field and get the camera angle from that..

It would take a star about 20 seconds to drift across the field with the drive off.  Then drive on, re-centre for the stacking video then drive off for another drift.  I could double or even triple the iterations of this for checks on repeatability and error analysis...

Three drifts and two videos could be done in under 5 minutes, I suspect...

I could take a full frame single image with a 1-2 minute exposure to get enough stars for a plate solve but that opens up an entirely different can of worms - auto guiding would be necessary..... takes too long....

Dave

Ok, that is a small field of view. I was guessing around 2-3 times that. I can usual get 4-5 stars in a 2-5 second exposure, which I find is enough for a plate-solve using fine indexes.

How accurate do you need the angle? It can be calculated from RA & Dec assuming your scope is set up level. Or failing that, if measured by drift at one position then it could be extrapolated by calculation to a 'nearby' position.

Ive just added an electronic finder to my dob, again using plate-solving. I can now almost automatically position the main scope to within an arcmin. I can get the finder image rotation which is fixed for any session with respect to the main scope. Would this kind of set up help your workflow?

Keith

### #12 Cotts

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Posted 11 April 2021 - 08:48 AM

Keith, I have done the drift-line method when measuring binaries with equatorially mounted scopes in the past. (You still have to get the camera angle correction because you can't put the camera in the scope accurately or repeatably)

I am very familiar with it and it is very accurate...as accurate as plate solving....

Old dog here who eschews new tricks unless they would significantly improve my accuracy...

Dave

### #13 astrokeith

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Posted 11 April 2021 - 10:11 AM

Dave,

I can see accuracy is key for you. I was wondering whether it could help with speed?

My own current interest is working through the Arp list of peculiar galaxies. Many are on or beyond the limit of my 18" and finding them requires a few minutes of averted vision - which makes even finding their location difficult!

I found the electronic finder removed up to 15 minutes of field checking, changing eyepieces, etc.

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