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Major star streaking in CEM25P

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#1 Tom3

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Posted 15 May 2021 - 08:02 AM

I am getting star streaking of 94 arc-sec in 10 second exposures. I used to be able to expose for 60 sec.(unguided)  without streaking.  I polar align with PoleMaster.  I think I have a problem with one of my exes.How do I tell which axis is giving me problems?

Tom



#2 Tom3

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Posted 15 May 2021 - 08:50 AM

I just checked the position of my scope at the zero position and it was 2° below the pole, so, could my streaking be due to cone error?

Tom



#3 MillHey

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Posted 15 May 2021 - 03:06 PM

Hi Tom,

 

The amount of streaking depends on the declination.  If you were imaging M81 at around 69 deg DEC, with a 2 degree misalignment, you would drift just over 11 arcsecs per minute, so it does not sound as though this streaking is due to polar misalignment.

 

Here is a link to a calculator for you to try with the declination that you were tracking  http://celestialwond...ftRateCalc.html

 

Chris



#4 Tom3

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Posted 15 May 2021 - 05:14 PM

Doing the calculations on the calculator you mention indicates that cone error is worse than polar alignment error at least in my case.  I will be working on reducing the cone error in a couple nights.  I was imaging at +55°DEC. I was trying for M101 but because of my error problems I missed it.

 

Tom



#5 michael8554

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Posted 16 May 2021 - 06:21 AM

"How do I tell which axis is giving me problems?"

 

Compare your images with a star map or planetarium, or Platesolve.

 

Then you'll know if its drift in RA or in Dec.

 

Which Scope ? Which Camera ? How did you measure 94arcsecs ?



#6 Tom3

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Posted 16 May 2021 - 12:01 PM

I am using an Explore Scientific ED 80 (480 mm fl) with a ZWO ASI294MC pro.  From astrometry.net I find 1.95 arcsec/pixel, and I use ImageJ to measure the length of the streaks (in pixels).  I can try plate solving one of the streaked images, but I am now leaning to cone error as the root of the problem, so I am installing different shims to try to eliminate cone error.  The streaks are at -59°, but I can't figure out if it is in RA or DEC.  The DEC center of the image is at +54° 46' .

 

Tom


Edited by Tom3, 16 May 2021 - 12:17 PM.


#7 michael8554

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Posted 17 May 2021 - 06:09 PM

Doesn't the Platesolve solution show RA and Dec directions ?

 

Here's a 3 degree view of M101 with RA left to right, and Dec Top to bottom.

 

Compare with your image to find which axis is drifting. Your image may be laterally or vertically flipped compared to this view:

 

M101_B.JPG

 



#8 Tom3

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Posted 18 May 2021 - 12:37 AM

"Doesn't the Platesolve solution show RA and Dec directions ?"

If it does, I can't figure it out.  I do my plate solving at Astrometry.net.

 

Tom



#9 MillHey

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Posted 18 May 2021 - 05:28 AM

I use All Sky Plate Solver and it has the option to superimpose an RA/DEC  grid on the solved image



#10 Tom3

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Posted 18 May 2021 - 09:46 AM

I just checked your suggestion about All Sky Plate Solver and found it's a.EXE which means I can't use it because I am running on a Mac. What I can do though is ,the next time I am imaging, is note the direction that stars move when I move the arrows on the hand controller in DEC and RA.

 

Tom



#11 dx_ron

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Posted 18 May 2021 - 10:09 AM

Untitled.jpg

 

I ran an image through the web version of astrometry.net. My image was in landscape orientation when I uploaded it, so where it tells me that "Up is 272deg E of N" lets me know that the long axis of my image is N/S (=Dec) and the short axis is E/W (=RA). If you are also using the web version, the diagrams tell the same story - long axis is Dec, short is RA.



#12 Tom3

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Posted 18 May 2021 - 01:45 PM

Streaking.png Well, it looks my streaking is in both axes.

The image uploaded is too small to show the streaks, but they run at about 45°

 

Tom


Edited by Tom3, 18 May 2021 - 01:46 PM.


#13 michael8554

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Posted 19 May 2021 - 10:18 AM

If the streaking is in both axis you'd have big fat round stars.

 

So if the stars are elongated, the drift is a lot more in one axis than the other - which is it ?

 

Does -59 degrees means 59 degrees anticlock  from the long edge of the frame ?

 

In that case I would say that would be Dec :

 

19thMay.jpg



#14 Tom3

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Posted 19 May 2021 - 11:13 AM

The night I noticed the streaking, my electronic focuser was not working so the stars were somewhat bloated so I can't tell really whether it's RA or DEC. Last night I didn't take any pics that took long enough to show streaking but I did get my focus motor working again so in the future I should be able to figure this out. I am also working on reducing my con error.

Tom



#15 Tom3

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Posted 25 May 2021 - 11:22 AM

I did some testing, and I believe my streaking is in DEC.

Tom



#16 dx_ron

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Posted 25 May 2021 - 01:25 PM

Looking back through the thread, there is no mention of guiding. If you're not guiding, the Dec axis should just be sitting there doing nothing, which allows/causes you to see Polar misalignment as drift in Dec (I used 'allows' because this is the basis for 'drift alignment' - turn off guiding and tweak az/al to minimize Dec drift).

 

I don't think that cone error will mimic polar alignment error. As long as the RA axis is correctly pointed at the NCP, I don't think it matters to tracking if the telescope is slightly off from the axis. Now, I am absolutely not an expert on the geometry of cone error effects, but I think they manifest more in errors in having a goto command land with the target centered. Do you have a way to independently check your Polemaster alignment result?

 

Meanwhile - is there any 'play' in your Dec axis? If you try to wiggle the telescope back and forth, is it solid? (just don't push so hard you cam the gears!)



#17 Tom3

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Posted 25 May 2021 - 02:49 PM

No, I was not guiding.  I have greatly reduced cone error, and I now do not notice any drift in DEC.  I now have two other problems: 1) I need to clean the contacts on my hand controller and 2)I think I need to re-tune my RA belt tension.

 

Tom



#18 fmeschia

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Posted 25 May 2021 - 04:09 PM

Cone error has no effect whatsoever on tracking. Assuming you’re talking about cone error of the main telescope.


Edited by fmeschia, 25 May 2021 - 04:09 PM.



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