Re: Mach2 - Elongated stars


Roland Christen
 


My suspicion is that if DEC balance is off, then DEC might creep under gravity, even PA is perfect. So you might see DEC continually drift  then gets corrected by PHD2 periodically.
Do you have a picture of your guide chart? The Dec axis would not creep under gravity if the clutches are tight. I just checked a mount with 3 lb out of balance in Dec and see zero creep or any other guide issue. The main thing that happens when you have polar axis error in azimuth is that the guider will always be off in Dec. The guide star is constantly being pushed back towards zero when it reaches the Min Move setting, so it will always have a minimum elongation error of your Min Move setting. So, if the Min Move is set high in Dec, you will get a high error in Dec. Best practices is to zero out your Dec drift and use a low Min Move setting. In my normal settings, my Min Move is .02 seconds, which translates to 0.3 arc seconds at 1x guide rate (0.2sec x 15arc sec/sec = 0.3 arc sec).

I always check my Dec drift to make sure that it is reasonable. You can do this by turning off guide pulses to the mount while recording the drift in the two axes. I do this near the meridian with scope on the west side of the mount pointing within 1 hour of the meridian. In my camera I use MaximDL in focus mode with a crosshair and watch the slow drift of a guide star in Dec (Dec is Up-Down on my camera). If the star moves down, I turn the right hand azimuth knob which moves the star to the right on my camera image. You can also do it with PEMPro and with PHD2 by watching the graph for a few minutes. In my method, if the star moves 5 arc sec in 2 minutes the amount of adjustment in azimuth is approximately 1/2 turn of the Azimuth knob. I can usually get the drift down to less than 1 arc sec per 5 minutes. I can do this in the daytime on a star as faint as magnitude 5 using my H-a filter.

Rolando

-----Original Message-----
From: yanzhe liu <liuyanzhe@...>
To: main@ap-gto.groups.io
Sent: Sun, Mar 7, 2021 12:11 pm
Subject: Re: [ap-gto] Mach2 - Elongated stars

Roland,

I will check my mount serial number later as I just packed everything due to rain forecast.

My camera's orientation was 0/180 degree, so I assume top to bottom elongation would be DEC.

I dont have any picture of my setup, I just tore it down since it will be rainy this week. My setup is FSQ106+OAG+16803, image scale is 3.5".
I was imaging Rosetta Nebula, after it was just past meridian for 2-3 hours.

I used PHD2 drift for PA, it reported <5' PA error, I am waiting for polemaster adapter so I can confirm I get good PA next time. 

DEC was out of balance because my setup was rear end heavy, if it was in balance position then I could not rotate my CCD in all directions. My guess is that the DEC balance was off by less than 1lb. In the next test, I can balance DEC to see if the issue still exists.

My suspicion is that if DEC balance is off, then DEC might creep under gravity, even PA is perfect. So you might see DEC continually drift  then gets corrected by PHD2 periodically.

Yanzhe

On Sun, Mar 7, 2021 at 9:35 AM Roland Christen via groups.io <chris1011=aol.com@groups.io> wrote:
Couple of questions:

1 - what's the mount's serial number
2 - which axis is the elongation
3 - which direction was the scope pointing (RA-Dec)
4 - what scope and focal length or what is the arc sec per pixel.
5 - do you have a picture of your setup?

One comment about balance - it should be easy for you to completely balance the scope with the clutches loose. Can you estimate how many lb out of balance your scope is in Dec? Normally i would not think this would have any effect, but doesn't hurt to check this.

Roland Christen
Astro-Physics Inc.



-----Original Message-----
From: yanzhe liu <liuyanzhe@...>
To: main@ap-gto.groups.io
Sent: Sun, Mar 7, 2021 12:44 am
Subject: [ap-gto] Mach2 - Elongated stars

Testing new mach2 for a couple of nights. It works great out of the box and I am very pleased.

One issue bothering me is that I got slightly elongated stars across the entire image (15min Ha sub).
The exact same telescope/CCD were used with another mount and stars were round, I am wondering what I should check (I think there was due to some setup issue, but I could not figure it out).
- Used OAG for guiding, so there should not be any flexure.
- CCD orientation was 0/180 degree. The stars were elongated top to bottom.
-  I did polar alignment with PHD2 drift align. PA error should be < 5'. The field rotation of each sub is ~0.01 degree, much larger than the other system (but elongated stars do not look like field rotation, they were same across the image)
- Both RA and DEC guiding curves looked smooth, had similar RMS. DEC correction was mainly to one direction.
- DEC balance was slightly off, camera heavy. Is Mach2 DEC sensitive to this? Will the guiding/image be affected?

I dont think any of the above is of particular concern, except DEC balance. Any comment/help is appreciated.

--
Roland Christen
Astro-Physics

--
Roland Christen
Astro-Physics

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