Guiding Issues with Mach1 GTO


Tom Wade
 

Hi.  I have recently (2 months ago) purchased a Mach1 GTO that was made in 2007.  This mount was basically unused in all that time and sat in the gentleman's closet for 10 years.  It looks to be in great condition and sounds perfect.  I have it mounted on a pier in my observatory and am confident of excellent polar alignment.


That is all the good news.  The bad news is that I cannot get consistently good guiding no matter what I try.  I have tried with a guidescope and an OAG, I have tried with PEM turned on and off, and I have been very specific about my settings in PhD2 (never had problems using it before).  Nothing seems to help and I'm sure this mount is supposed to provide very accurate guiding.  I am going to attempt to attach a picture of my guiding results from the other night as example.  Any help or pointers would be greatly appreciated.


Thanks,

Tom



Roland Christen
 

This is what I would suggest for anyone who just got a new mount, whether it is an AP or any other brand. Do a simple analysis as outlined below so that you know what you are starting with:

First pointer is to turn off all backlash compensation in PHD. Then set your guide speed to 1x. Next is to get a good accurate calibration at the celestial equator near the meridian.

Polar alignment is desirable so that Dec does not drift more than about 1 arc sec in 5 minutes. If uncertain about polar alignment, turn guiding off initially and watch the drift in Dec on a star near the zenith. Adjust the azimuth until you get no drift of the Dec axis (best done near the celestial equator, but works at the zenith also). This also will give you a baseline error component due to seeing that will be the minimum level that you can expect to achieve during guiding. Watch the guide star as it slowly drifts away from the Dec line and turn guiding on with aggressiveness set to the absolute minimum at first. Increase aggressiveness slowly until the star moves to the zero line, but don't go any further. You want stability rather than setting it to 100% and hoping for the best. It will probably head towards the zero line at between 50 and 70% aggressiveness.

Next turn off RA guiding and watch the graph for about 6 - 7 minutes with PEM off, and let the guide star drift up and down and take an image of the result. The peak to peak should be under 10 arc sec and could be below 5. If you have any drift in RA, you would need to subtract this from the peak-peak drift of the periodic error to get a good idea of how well the worm gear is tracking. Of course it would be nice to have no overall drift, which could be achieved with proper altitude alignment (polar RA drift align).

Once you have the baseline RA tracking error curve, turn PEM on. Do the same test again, this time with the RA axis. Since this is a used mount, it could very well have a totally hosed up bogus curve in it, courtesy of a previous user who did not know what he was doing. So, if the PE data is good, the result will be a better tracking graph than the PEM-off data. If it is not better or is very much worse, then you have two choices: 1) use the mount with PEM set off, or 2) create a new PE curve, load it in and run the mount with PEM on.

The above steps will allow you to characterize the mount and give you all the information you need to determine how well your mount actually works and will allow you to make good settings to your guider program. You will also be able to communicate this info to others in the user groups to help determine what may be preventing the mount from guiding accurately. Just showing a guider graph is not helpful without a complete understanding of the mount's actions.

Rolando



-----Original Message-----
From: tomwade123@... [ap-gto]
To: ap-gto
Sent: Fri, Dec 22, 2017 10:35 am
Subject: [ap-gto] Guiding Issues with Mach1 GTO



Hi.  I have recently (2 months ago) purchased a Mach1 GTO that was made in 2007.  This mount was basically unused in all that time and sat in the gentleman's closet for 10 years.  It looks to be in great condition and sounds perfect.  I have it mounted on a pier in my observatory and am confident of excellent polar alignment.

That is all the good news.  The bad news is that I cannot get consistently good guiding no matter what I try.  I have tried with a guidescope and an OAG, I have tried with PEM turned on and off, and I have been very specific about my settings in PhD2 (never had problems using it before).  Nothing seems to help and I'm sure this mount is supposed to provide very accurate guiding.  I am going to attempt to attach a picture of my guiding results from the other night as example.  Any help or pointers would be greatly appreciated.

Thanks,
Tom




Tom Wade
 

Rolando -
Thanks.  I will follow your steps down to the letter on the next clear night.  Thank you for taking the time to provide all that information.  I will update my results.  Thanks again.

Tom