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


Henry Kwok
 

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.


Setup:

Mach1GTO

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 via 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, but 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!


Henry



Andy Galasso
 

Henry,

Feel free to post your PHD2 Guide Log on the PHD2 Forum if you would like the PHD2 experts to analyze it for you.  It would also help to post one of the 15-minute subs (raw FITS format) showing the issue.

Andy


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.
 
Rolando
 
 

-----Original Message-----
From: henry.ck.kwok@... [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.

Setup:
Mach1GTO
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!

Henry




Eric M
 

He mentioned he's using an OAG, so there shouldn't be any flexure. Strange problem, you've seen nothing like this when using your Atlas? I ask because with the numbers you gave, your worst guiding with the Mach1 was the same as the best guiding with the Atlas. 


Roland Christen
 

Off-axis guiding should be free of differential flex, but it depends on the actual mechanical coupling and the weight of the camera. The test method I outlined should eliminate it as a suspect or perhaps show that it is. The best approach to solving problems is to rule out any and all variables, one at a time.
 
Rolando
 
 

-----Original Message-----
From: eric.marlatt@... [ap-gto]
To: ap-gto
Sent: Wed, Aug 31, 2016 5:24 pm
Subject: Re: [ap-gto] A bit disappointed with my Mach 1 / What am I missing?



He mentioned he's using an OAG, so there shouldn't be any flexure. Strange problem, you've seen nothing like this when using your Atlas? I ask because with the numbers you gave, your worst guiding with the Mach1 was the same as the best guiding with the Atlas. 


Henry Kwok
 

Thank you for all the suggestions. I was hoping to do more test as per Roland but that will have to wait for bad weather to pass. In the mean time I have posted my PHD logs as dropbox links  as Andy suggested - hope you can have a look and comment too. (I was stupid enough not to save any subs from these two nights.

First night, not so well polar aligned:

Second night, better aligned:

Thanks

Henry


Roland Christen
 

It really is difficult to wade thru miles of numbers and try to figure out anything from them. In fact, I can't. When I want to see the guiding performance I simply look at the guider graph. So, a screen shot of the guider graph of 10 - 15 minutes of guiding will tell a lot.
 
Rolando
 
 

-----Original Message-----
From: henry.ck.kwok@... [ap-gto]
To: ap-gto
Sent: Sat, Sep 3, 2016 7:03 pm
Subject: [ap-gto] Re: A bit disappointed with my Mach 1 / What am I missing?



Thank you for all the suggestions. I was hoping to do more test as per Roland but that will have to wait for bad weather to pass. In the mean time I have posted my PHD logs as dropbox links  as Andy suggested - hope you can have a look and comment too. (I was stupid enough not to save any subs from these two nights.

First night, not so well polar aligned:

Second night, better aligned:

Thanks

Henry



bw
 

Hi Henry.  I think you should move this conversation over to the OpenPHD2 forum as Andy suggested.  Your last 24-minute guiding session showed a tracking/guiding RMS of 0.77 arc-sec with nearly equal stats for both RA and Dec.  This should produce nice round stars and was probably seeing-limited.  In other words, it looks to me like the mount is doing exactly what it's supposed to.  If you see problems in your images, those are probably coming from something else.  If you post on the OpenPHD2 forum, we can probably help you figure out what your issue is.

Good luck,
Bruce