#1  
Old 18-07-2008, 10:29 PM
StardaddyEd (Ed)
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Mount

All

I am kind of new to imaging and most of the time I can get decent images from my guided scope. A few weeks ago I could not get a decent image due to tracking errors (elongated stars, double stars). I use PHD to guide a ST80 on the main scope (9.25 SCT) on a ASGT. This problem got me to thinking (you can tell by the smoke coming from my ears).

1. How do you evaluate the quality of a mounts ability to track by itself and under guiding?
2. On a CG5 can we see the periodic error? Can we compensate for it.
3. What is a realistic maximum exposure time I should expect with quality star roundness (if that is even the right measure)?

Any thought are welcome!

Ed
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  #2  
Old 19-07-2008, 07:35 AM
Alchemy (Clive)
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a couple of things come to mind

1. a larger mount might help, find out what the load capacity of the cg5 is , usually for astrophotography its better to be on the safer side, what will pass for visual may not for imaging.

2. try using a focal reducer

3. most mounts are no good without guiding, you might get away with a paramount but not many can do it

4. the guiding is for compensating the periodic error

5. there are methods for imaging the periodic error, phd can also log the corrections which will give you an idea of what is happening

6. Maximum exposure time varies with mount and scope, a longer focal length shows more error and is less forgiving, a higher quality mount has less error. its a bit vague i know but you could aim for 3 min exposures for globs and 5 plus for other stuff.

7.ive noticed on this forum most beginners start imaging with an ed80 and then move up from there, long focal length is challenging i do 1500mm and generally cant go past 6 min subs. theres ways to do longer but most solutions come with a price tag.
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  #3  
Old 19-07-2008, 07:56 AM
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gregbradley
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A common advice with astrophotography is the most important component in the imaging setup is the mount.

Not all mounts are the same of course.

There is a site where the periodic errror of different mounts is shown in graph form. A really excellent mount does under 5 arc seconds. These would only be a few brands.

Takahashi, Paramount, Astrophysics, Micron, Mountain Instruments, Losmandy Titan. I am sure there a few others.

But of course that all comes with a price tag. Its not so much the periodic error it is the rough sudden jerks that can cause problems. So it is also the smoothness of the errors.

I have a Tak NJP mount and you can get software that will log the periodic errors as a graph after autoguiding. I have seen graphs as low as 1.95 arc seconds with my NJP. That was better than usual. 3-5 arc seconds is common.

CG5, EQ6 etc are probably more in the range of 20 arc seconds or more.
A fork mounted scope is often 30 or more. They also can vary.

Autoguiding has lots of tricks. I often do not get the same guiding errors night after night. Seeing plays a part as well. Focus, balance, focal length of the guide scope, flexure, focal length of the imaging scope, even your choice of guide star and choice of guide camera, choice of guiding software all play a part. Settings in the software, how well you callibrated yadda yadda yadda. So its get complicated really quickly.

But as said, smaller focal lengths are easier than longer focal lengths.

For my setup I have always gotten the best results with the shortest guide exposure I could get. 1 second guide exposures work the best for me unless it is poor seeing where I see better guiding at 2 or 3 seconds. You can also stay with 1 second and set the minimum move to a higher value so not every correction is made unless it is significant.

Getting a 1 second guide exposure is another story. Sbig cameras have internal self guiding which is a great feature but not of much use if you want to do narrowband imaging as you are guiding through the filter.

Greg.




Quote:
Originally Posted by Alchemy View Post
a couple of things come to mind

1. a larger mount might help, find out what the load capacity of the cg5 is , usually for astrophotography its better to be on the safer side, what will pass for visual may not for imaging.

2. try using a focal reducer

3. most mounts are no good without guiding, you might get away with a paramount but not many can do it

4. the guiding is for compensating the periodic error

5. there are methods for imaging the periodic error, phd can also log the corrections which will give you an idea of what is happening

6. Maximum exposure time varies with mount and scope, a longer focal length shows more error and is less forgiving, a higher quality mount has less error. its a bit vague i know but you could aim for 3 min exposures for globs and 5 plus for other stuff.

7.ive noticed on this forum most beginners start imaging with an ed80 and then move up from there, long focal length is challenging i do 1500mm and generally cant go past 6 min subs. theres ways to do longer but most solutions come with a price tag.
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  #4  
Old 19-07-2008, 09:17 AM
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multiweb (Marc)
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I was in the exact same situation 8 months ago. I bought a second hand Losmandy G11 and never looked back. In my opinion a solid good quality mount is a must. Worry about your scopes later. Get the best mount you can afford. Buy the only one you'll ever need.
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Old 19-07-2008, 10:33 AM
pjphilli (Peter)
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Hi Ed
I also have a CQ5 which I bought as an EQ4. My EQ4 had upgrades which I think then designated it as an EQ5. This was a nice little economical mount and I was able to do some limited imaging with it. However, a couple of months ago I reluctantly came to the conclusion that my ability to image was severely limited by the following:
1. The mount has a maximum capacity of 10Kg but I was loading it with about 11.5Kg.
2. I measured the periodic error and it came to 50arcsecs. This would not have been so bad with guiding but the transition from the negative side to the positive was very steep and PHD guiding had problems adjusting to it.
3. I had the cheap paddle/tracking motors fitted. Although I was able to adjust the backlash in the mount itself down to a low level, the backlash in the gearing of the stepper motor/gearing arrangement was very bad and
PHD tended to get "lost" in the backlash gap.
So I bought a HEQ5PRO mount which is in an altogether higher order of class compared with the CG5. I have measured the periodic error at 30 arcsecs but the transition from the negative to the positive limits is pretty smooth and PHD has no problems in keeping up with it. I have set the mount Set Backlash to 1 arcmin so that PHD sees virtually no backlash which is important for DEC guiding. As with most hobbies you are tempted/led to more and more purchases. The problem is knowing when to stop!
Cheers Peter
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  #6  
Old 22-07-2008, 11:46 PM
StardaddyEd (Ed)
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Thanks Guys

A couple of you mentioned software to measure your error. What software do you use?

This discussion has given me a couple of ideas. The first is that PHD does a great job of tracking in RA. The graph has very little wiggle to it but the DEC graph has a periodic spike in it. It corrects and is on the line then slowly rises for a little, then spikes up followed by a spike down to the line and the process starts over. I wonder if that is from the backlash in the gears and if I can adjust the spike out with the software (NexRemote) or I have to adjust the mount itself?

I have asked about the CG5 in the past and I usually get the "upgrade" your mount response. I've determined that you get what you pay for and while the CG5 is a modest mount it will provide the quality of images I am interested in producing right now. I have just started imaging so I have a ton of photo-technical things to before I can produce consistently high quality images. I could see myself upgrading to an Atlas or CGE in the future but for now I think I should be good for my skill level. Besides if I suggest to my wife that I want to buy a new multi-thousand dollar gadget for astronomy I will have plenty of time to use it!
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  #7  
Old 23-07-2008, 12:27 AM
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wmzaphod
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Quote:
Originally Posted by gregbradley View Post
A common advice with astrophotography is the most important component in the imaging setup is the mount.

Not all mounts are the same of course.

There is a site where the periodic errror of different mounts is shown in graph form. A really excellent mount does under 5 arc seconds. These would only be a few brands.

Takahashi, Paramount, Astrophysics, Micron, Mountain Instruments, Losmandy Titan. I am sure there a few others.

But of course that all comes with a price tag. Its not so much the periodic error it is the rough sudden jerks that can cause problems. So it is also the smoothness of the errors.

I have a Tak NJP mount and you can get software that will log the periodic errors as a graph after autoguiding. I have seen graphs as low as 1.95 arc seconds with my NJP. That was better than usual. 3-5 arc seconds is common.

CG5, EQ6 etc are probably more in the range of 20 arc seconds or more.
A fork mounted scope is often 30 or more. They also can vary.

Autoguiding has lots of tricks. I often do not get the same guiding errors night after night. Seeing plays a part as well. Focus, balance, focal length of the guide scope, flexure, focal length of the imaging scope, even your choice of guide star and choice of guide camera, choice of guiding software all play a part. Settings in the software, how well you callibrated yadda yadda yadda. So its get complicated really quickly.

But as said, smaller focal lengths are easier than longer focal lengths.

For my setup I have always gotten the best results with the shortest guide exposure I could get. 1 second guide exposures work the best for me unless it is poor seeing where I see better guiding at 2 or 3 seconds. You can also stay with 1 second and set the minimum move to a higher value so not every correction is made unless it is significant.

Getting a 1 second guide exposure is another story. Sbig cameras have internal self guiding which is a great feature but not of much use if you want to do narrowband imaging as you are guiding through the filter.

Greg.
That's great info Greg thanks - On the point of Guide Scopes - I have a cheap MaKCas 80mm that I "can" mount on my 80mm refractor, do you think that's adequate or over the top for a guide scope?

Peter
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