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Equipment Discussions >> Mounts

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Stargazer78
professor emeritus
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Reged: 03/05/13

Loc: Susanville California
Re: Polar Alignment new [Re: orlyandico]
      #6057083 - 09/01/13 04:15 AM

Quote:

You must be using the AT8IN?

If so.. then yes 60 seconds unguided is out of reach.

If you're absolutely sure your polar alignment is dead-on (i.e. no drift in DEC if you turn off DEC guiding in PHD) then you're hitting the large periodic error of the CGEM and CGEM DX.

Try doing a PEC training. This still won't help a lot of your 8/3 error is too large.




So what am I supposed to do? I bought this mount for astrophotography. The manual doesn't tell me how to do a pec training and honestly I have no idea what that is? I am beyond frustrated at this point!


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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: Polar Alignment new [Re: Stargazer78]
      #6057090 - 09/01/13 04:30 AM

Try guiding. This will cope with the uncorrected PEC and the dec drift if you aren't perfectly polar aligned.

How much Astrophotography have you done before? Are you just starting or are you moving to a longer focal length?

Chris


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Stargazer78
professor emeritus
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Reged: 03/05/13

Loc: Susanville California
Re: Polar Alignment new [Re: cn register 5]
      #6057093 - 09/01/13 04:42 AM

Lol. That's my issue I cannot guide at all. That's why I believe I need the beta fix.

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A. Viegas
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Reged: 03/05/12

Loc: New York City/ CT
Re: Polar Alignment new [Re: Stargazer78]
      #6057273 - 09/01/13 08:58 AM

Ok... Have you used PHD before? Check your settings or post them here. Next verify your polar alignment by re-aligning on two different alignment stars and calibration stars and then check your polar alignment display... Then if necessary redo the ASPA on a star close to the meridian and celestial equator... celebrai or rasalthague should do fine...

Now when you are running PhD are you seeing large spikes in the guiding graph? If so then the guiding problem can be more software than hardware... If your guiding graph does not show big deviations and yet the guiding is still poor with oblong stars as short as 60 seconds then you could have a bad CGeM. If It's still under warranty, send it back, if not then try the beta motor control software...

Oh last comment, what FL are you shooting at? If you are imaging at under 1,000mm then it is a real shame and your CGEM could hve a really bad 8/3 and cogging issue...

Al


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Stargazer78
professor emeritus
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Reged: 03/05/13

Loc: Susanville California
Re: Polar Alignment new [Re: A. Viegas]
      #6057285 - 09/01/13 09:14 AM

setup is a AT8IN 800 mm focal length with a orion 80 mm short tube guide scope using an orion auto guider. I start off doing the 2 alignment stars followed by the 4 calibration stars.after that is done I slew the scope south to nunki. I start up the polar alignment routine. Instead of using a reticle eyepiece I use my autoguider with phd guide. I turn on phd with the bullseye. I follow the routine for pa and place nunki right in the center. I will then observe for any drift. I observe for 15 minutes. Once there is no drift. I then confirm the polar alignment routine. I then check my polar alignment display and it will read 00 00 00 for both AZM and ALT. I then go to my target and fire up phd again. Once im guiding I check the phd graph and I get a saw tooth pattern and at times the graph jumps off the chart.i tried unguided and I cant even get 60 second exposures with out trails. I dont understand how I can see no drift when I polar align but my guiding is terrible! Its not making any sense and I am so frustrated!

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Stargazer78
professor emeritus
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Reged: 03/05/13

Loc: Susanville California
Re: Polar Alignment new [Re: Stargazer78]
      #6057287 - 09/01/13 09:15 AM

Using all default settings also for phd

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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: Polar Alignment new [Re: Stargazer78]
      #6057300 - 09/01/13 09:24 AM

You have DEC cogging it sounds like. The beta firmware may fix this but may introduce other problems. You could try swapping the ra and dec motors as only some of the motors have this problem.

One fix you could try is to be off a little in your PA. That way the guiding is always in one direction and greater than the cogging effect. Those that have a problem free CGEM will never understand, as the CGEM is very nice when working correctly, so take it to heart.


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Polar Alignment new [Re: Stargazer78]
      #6057309 - 09/01/13 09:32 AM

well... if you are sure there is no drift in declination then turn off DEC guiding. see from there.

EDIT. You say all default settings in PHD. These won't work. Go into the brain icon and change the Calibration Step to 2000ms. Normally it's much less than that. I think what is happening is your calibration step was so small so during PHD calibration, PHD had to send a lot of DEC guide commands to overcome the cogging/stiction/whatever so when actually guiding it over-corrects.

But still try to post your PHD guide graph. If you're getting sawtooth in DEC that's either cogging or stiction.

PHD should be able to smooth out even very bad RA periodic error.

Also try using longish guide exposures (say 4 seconds) to smooth out the seeing. Also.... when you calibrated PHD did you do it on a star close to what you want to image?

theoretically you should re-calibrate PHD ("Force Calibration" button in the brain menu) every time you re-point somewhere far from where you did the initial calibration.

Also.. when doing your star alignment when the CGEM DX booted up, did you always end the alignment on each star using only the UP and RIGHT buttons?

oh and Mark, my CGEM is gone. had to eat a huge loss selling it, but can't really say I miss it. I only reply to these threads out of force of habit.


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Cliff Hipsher
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Reged: 12/31/08

Loc: North Chesterfield, VA
Re: CGEM-DX will not guide in dec new [Re: Stargazer78]
      #6057319 - 09/01/13 09:44 AM

Why not do a drift alignment? As I understand it, if your polar alignment is spot on, you do not to guide in DEC...

One other thing.. Do the stars trail in the same direction in both the guided and unguided images?


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Raginar
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: Polar Alignment new [Re: orlyandico]
      #6057333 - 09/01/13 09:56 AM

Orly,

Glad you got rid of your CGEM. Stargazer, let's settle down and make sure we have everything configured before jumping to conclusions. I imaged with an AT8IN on a regular CGEM without much difficulty. The biggest problem I had was differential flexure (the two tubes were flexing as it moved around, causing guiding issues).

So, first off you need to read this:
http://www.cloudynights.com/item.php?item_id=2755

Second, IF you're using ASPA, you'll find that you are Polar Aligned well enough to guide. Your shorttube 80, how is it mounted to the AT8IN? Are you using clam shells? The stock Orion rings? You'll find many people complain about the stock orion rings. I would look for a more secure connection. My personal solution was an ADM MDS system that I bolted to the top of my AT8IN. The ADM rings are much more 'solid' and the MDS gives you a firm connection to the imaging scope. So, there is a possibility that this is the source of your problem. But, lets continue.

Third, you're going to have to change your settings. I usually ended up around 80 on RA aggressiveness. Change your min motion to .2. And, your max movement for RA/Declination to around 1500 for each (probably high but OK for starting). Increase your calibration steps to 2000-4000ms depending on your setup. I use a a mini-guider and I need 4000ms to see adequate movement with my setup. Yours will be somewhere in between. You'll see in the article that you want somewhere between 8 to 20 steps for RA and declination.

One thing on declination calibration. Are you using the relays or ASCOM? I've found on many of the mounts I have owned that declination guiding through ASCOM results in 'backlash'. I don't know why, but I find if I hit the 'up' button a few times when PHD says 'Clearing Backlash...' It will help it figure out a correct calibration. I find if the 'Clearing Backlash..' stays up for more than 2-5 seconds, that's when to do this. This is cited in an article on the Stark Labs Yahoo! Group as well.

Finally, Make sure your guide scope is FIRMLY attached to your AT8IN. Make sure the camera is FIRMLY attached to the extension/focuser/whatever. These are often frustrating sources of guiding problems.

The key to successful imaging is taking a balanced approach. Change one variable at a time and see what it does. Don't make huge changes to your system as you're going to find that introduces more problems that you won't be able to fix once its dark out.

Finally, TRUST ASPA. It works, period dot to an accuracy enough that you will be able to perform guided AP with your AT8IN. Don't mess with your polar alignment past doing one iteration. Just make sure you are doing the 'other gotchas' such as 'up and right' to each star. That you are doing a full 2 + 4 prior to syncing on an appropriate ASPA star. An appropriate ASAP star is one that is near the meridian, but also roughly at your latitude (for me, I look for a star at around 45 degrees off the horizon).

Check your settings in your mount. What is your guide rate at? Make sure it's at around 50%. Turn off Backlash Compensation (aka set to 0). Your mount needs to be balanced 'perfectly'. Great article here:

http://starizona.com/acb/basics/using_balancing_gem.aspx

If you're not balanced in the declination axis (a common problem with newts and SCTs), you'll see some pretty crazy stuff with PHD too.

So, come back with more info and let us know. This will sound harsh, but it's probably not your equipment but user error that is generating your problems. I went through the exact same problems when I started. Now, I have it figured out (some times) .

Good luck,

Edited by Raginar (09/01/13 10:01 AM)


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Polar Alignment new [Re: Raginar]
      #6057437 - 09/01/13 11:07 AM

I think Min Motion of 0.2 is excessive. We're talking about an 800mm main scope and 400mm guide scope. So a min motion of 0.5 should be good enough.

Didn't know you need 8-20 steps for calibration.. I get by with 3 steps. Maybe that's why my guiding is always jumpy.


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Raginar
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: Polar Alignment new [Re: orlyandico]
      #6057490 - 09/01/13 11:32 AM

Orly,

That's from the paper in the Yahoo! Group and personal experience. Anything more than that is... excessive. Anything less, and I get poor guiding performance.

.2 is excessive huh? I never had much success with numbers higher than that. It's based on your image scale correct?


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Polar Alignment new [Re: Raginar]
      #6057530 - 09/01/13 11:53 AM

The Min Motion is for the subpixel guiding. Most people need 0.1 or 0.2 because they're using very short guide scopes, like 50mm finders or the SBIG eFinder.

But if using a long guide scope, a larger figure is more appropriate. I'm using 0.3, my guide scope is 560mm focal length and my main scope is 600mm. On an OAG I have used Min Motion = 1.


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oo_void
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Reged: 11/13/09

Loc: San Francisco, CA
Re: Polar Alignment new [Re: orlyandico]
      #6057653 - 09/01/13 01:04 PM

IMHO, DEC drift after a good polar alignment is almost always a balance issue (an AT8IN with an ST80 is a lot of gear with weird centers of gravity). Orlyandico's recommendations will help compensate for that in PHD.

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Stargazer78
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Reged: 03/05/13

Loc: Susanville California
Re: Polar Alignment new [Re: oo_void]
      #6057673 - 09/01/13 01:21 PM

Ill give it another go tonight. I have never heard of using the up and right buttons only. Ill change my settings in phd too. My exposures for phd has been 2 seconds. Is it possible I am chasing the seein?

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TimP
member


Reged: 02/24/09

Re: Polar Alignment new [Re: Stargazer78]
      #6057701 - 09/01/13 01:39 PM

Quote:

Lol. That's my issue I cannot guide at all. That's why I believe I need the beta fix.




I am still working on my knowledge base at the school of hard knocks on my CGEM DX. I can tell you this though. I had the same problem as you. I hyper tuned the mount which really helped on balancing but I still had problems with not being able to guide. I thought it was my Orion guider so I bought a Starlight Xpress Superstar auto guider. Guiding did not work. Hmmmm !! I downloaded the Beta CGEM software. The slews are now a little jumpy from time to time but I am now guiding. I still had a little problem with making my Orion 80mm guide scope solid so I'm going with an Off Axis Guider. Heck! Celestron even replaced my mount. You live and you learn. If I had the money I would have went with a more high end mount but I don't , So I didn't.


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Stargazer78
professor emeritus
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Reged: 03/05/13

Loc: Susanville California
Re: Polar Alignment new [Re: TimP]
      #6057741 - 09/01/13 02:03 PM

PHD GRAPH from last night.



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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: Polar Alignment new [Re: Stargazer78]
      #6057746 - 09/01/13 02:09 PM

Looks as though (1) lots of dec drift (polar alignment issue) and (2) dec guiding isn't enabled.

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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Polar Alignment new [Re: jrcrilly]
      #6057754 - 09/01/13 02:14 PM

DEC guiding is enabled "Auto" but Mx Dec is 100.

This is not enough for the DEC guiding to chase the declination drift. Which means your polar alignment is way off. I somehow mysteriously got a fantastic polar alignment, <2 arc-minutes off the pole. I only need a DEC correction perhaps once in 10-15 seconds, and it's only 100-150 ms.

In your case your Mx Dec is 100ms and its drifting away like crazy.

I suspect your ASPA is incorrect because you didn't end all alignment movements with UP and RIGHT.


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Stargazer78
professor emeritus
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Reged: 03/05/13

Loc: Susanville California
Re: Polar Alignment new [Re: orlyandico]
      #6057774 - 09/01/13 02:29 PM

Ok so I used nunki to do my aspa. I use my autoguider and phd. I center it using the bullseye. Once its centered I wait for 15 mins before I confirm the polar alignment routine. Could all of my issues be poor polar alignment? My display reads
AZM 00 00 00
ALT 00 00 00
That means I am aligned correctly right?


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