Re: A bit disappointed with my Mach 1 / What am I missing?

Roland Christen

If your guider graph gives consistently 1 arc sec or less RMS guiding, then it doesn't matter how long your exposure is, the result will be round stars. In other words a 5 minute exposure will look just the same as a 30 minute exposure. If the guider graph is basically flat in both axes and not drifting up or down, then the mount and your guiding software are both doing the job properly.
The mount cannot anticipate how the stars might be drifting on your imaging camera chip. It may be that the off-axis guider and your imaging camera are slowly moving with respect to each other as the exposure advances (differential flexure). Thus a 5 minute exposure produces round stars, but a 30 minute produces trailed stars. You can do a simple test to see if this is the case. Take a series of 5 minute exposures (maybe 5 to 10) where the stars of each exposure are round. Do not turn on drizzle or move the guide star during each exposure. Take each of the exposures and place them one on top of the other, but do not register the stars, rather register the edges of the format. If the imaging camera is differentially moving with respect to the guide camera, you will see a position shift between the first and last 5 minute exposure. You can also measure the centroid of a star in each exposure to see how far it has shifted.

-----Original Message-----
From: [ap-gto]
To: ap-gto
Sent: Wed, Aug 31, 2016 4:03 pm
Subject: [ap-gto] A bit disappointed with my Mach 1 / What am I missing?

I have taken my second hand Mach1GTO (7 year old) out for the last two nights first time since I bought it in July and taken it back home half way across the globe to New Zealand. I am a bit sad to say I am a little disappointed by the guiding performance. Don't get me wrong, I think its performance is still better than my old Atlas, but it is just not as good as I expect it to be. I am not sure if my expectation is too high. Having said that, being a new mount to me I suspect it is more of user/setup error rather than with the mount per se. I would appreciate if some advice can be given on areas I should check to get better performance out of the mount.

OTA is 6" RC
Guiding is with OAG using a Lodestar Mk1
Reducer is AP CCDT67
Focal length is 1050 mm or thereabout at f/7 ish
Control is v ia PHD2, SGPro, AP ASCOM driver

Before the Mach1GTO, I have an NEQ6 (Atlas), I get consistent 5 minute subs and can get 10 minute sub with about 25% loss due to guiding error. RMS error has reported by PHD2 with my Atlas is about 0.7-1.1". Some of this is due to the constant poor seeing in Auckland, NZ due to the coastal unpredictable weather, and some of it is the mount as one part of the sky is better than another. I upgraded to the Mach1GTO in the hope of achieving longer subs as I wish to get into narrowband imaging.

On the first night I did a quick polar alignment with PoleMaster double side taped to the mount housing. I have not had good result with PoleMaster but I wanted a quick test so thats what I used. I get guiding rms error better than my Atlas at about 0.7-0.9" rms. However looking at the guide graph there are many guide corrections - similar to what I got with my Atlas. I got good 5 minute subs, b ut at 10 minutes there is some minor egging of stars and at 20 minutes the stars are obviously oblong. At the end of the night the guiding deteriorated so I did a test of my polar alignment and noted significant DEC drift of 40 arc min, so I put it down to poor PA.

On the second night I spent some time drift aligning using PHD2 tool. Both Azimuth and Altitude axis reported less than 1 arc min error. However, guiding graph shows similar pattern with lots of corrections. The RMS error gets down to 0.4" and up to 0.7/8" so it is well within the pixel scale of 1 arc sec / pixel of my setup. 5 min sub is good; 10 min sub is acceptable but very slightly egged if I pixel-peep and 15 minute sub gives me "trianglular stars" (no, the optic is not pinched, stars are round at 5 minutes).

I have gone through my usual set up routine as before. I have balanced as well as I could. Could not really set East-Heavy as I simply could not tell, but I understand it does not matter with AP mount. Everything is bolted tight. There is no cable drag...

So in summary the Mach1GTO can achieve what I had with my Atlas but it does not seem to enable me get longer subs. Where should I be looking to get better performance? 

- Could the mount need adjustment since it has travelled from San Francisco to San Diego by UPS then by commercial flight back to NZ? If so how can I tell if it needs adjustment? PHD2 reported a DEC backlash of 600 ms whatever that means - which is miles better than my Atlas by the way.
- Better balancing with a fish scale?
- Better polar alignment (will have to be software assisted, as I am in Southern Hemisphere)?
- Non mount related issue?

Many thanks!


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