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TL2101
professor emeritus


Reged: 09/17/10

Loc: Concord, Ca
CGEM Polar alignment Procedure?
      #5234724 - 05/22/12 07:51 PM

Currently I do a 2 star alignment then the HC polar alignment, undo sync and return and redo the 2 alignment stars by inputting the SOA #s and then add 4 calibration stars by entering in the 4 SAO #s of my own choosing.

I read where some owners just turn the mount off then on again after doing the polar alignment through the HC and then starting the 2 + 4 alignment procedure from scratch. This allows you to use the stars suggested by the HC using common names instead of SAO #'s.

My question is after I turn the mount off and then loosening the RA & DEC clutches returning the mount back to the index marks have any effect on the tracking accuracy of the polar alignment? Turning the mount off then back on again seems easier than punching in all the SAO #s. The CGEM instruction manual doesnt mention this restart procedure?

I am only looking to simplify the procedure I am not having any alignment or goto errors the mount has performed flawlessly.

Thanks

Edited by TL2101 (05/22/12 08:14 PM)


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oldstargazer
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Reged: 08/03/11

Loc: Western Oklahoma
Re: CGEM Polar alignment Procedure? new [Re: TL2101]
      #5234762 - 05/22/12 08:16 PM

That is how I do my CG5 mount. I have the mount setup balanced and roughly polar aligned then I do an alignment and then the polar align using the HC and then power off, reindex, realign and cal. stars, otherwise after doing polar align the gotos are usually off a little. After doing it this way the gotos are dead center and when do imaging that sure helps.

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Stew57
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM Polar alignment Procedure? new [Re: oldstargazer]
      #5234849 - 05/22/12 09:04 PM

My procedure.

After set up of mount, move mount to the home position (index marks) I have my mount positioned close to polar aligned via laser mounted on the finder.

I do a 2+4 alignment. Then the ASPA. After the aspa I send the mount to the home position. Do not use clutches! power off and redo a 2+4 alignment. I sometimes check my polar alignment via the display.

The pointing accuracy of this mount is great and the ASPA routine gets it real close.

mark


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mclewis1
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Reged: 02/25/06

Loc: New Brunswick, Canada
Re: CGEM Polar alignment Procedure? new [Re: oldstargazer]
      #5234860 - 05/22/12 09:09 PM

Celestron's instructions are indeed to not power off but to re do the initial alignment by un sync'ing/replacing the initial alignment stars after a polar alignment. This is obviously a bit easier to do if you are choosing named stars from lists rather than inputing SAO #s.

Most folks however seem to follow the initial align, polar align, power cycle, initial align methodology.

Edited by mclewis1 (05/25/12 08:43 PM)


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TL2101
professor emeritus


Reged: 09/17/10

Loc: Concord, Ca
Re: CGEM Polar alignment Procedure? new [Re: mclewis1]
      #5236212 - 05/23/12 06:30 PM

Thanks for sharing your alignment procedures. I went out this afternoon and played around with HC. I am such a dummy I was hitting the Star button and entering the SAO #'s and didn't think about hitting the list button for named stars after doing the ASPA.

I also had not tried the home position entry under utilities. This mount is new to me and am still learning. I may switch back and forth between the Star button and the list named stars button when selecting my calibration stars depending on where the star I want falls on the list. Practice makes perfect.

Thanks for your help.


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clchildress
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Reged: 04/22/10

Loc: NW Georgia, USA
Re: CGEM Polar alignment Procedure? new [Re: TL2101]
      #5238167 - 05/24/12 10:54 PM

OK, here is my $0.02. I don't power off. In fact, I don't even re-enter my go-to alignment most nights. The newest firmware for the CGEM eliminates many of the extra steps we used to go through. Please, make sure you have the latest firmware for your mount. Here is how I do it:

Preliminary: eyeball Polaris through the polar alignment scope, or at least the bore hole if you don't have the scope. Get Polaris in the center. Heck, if you have a polar alignment scope and it is properly collimated go ahead and use it as directed. This takes quite a bit of "slop" out of the initial setup. Just getting Polaris in the general neighborhood removes a good deal of guesswork and such.

1) Perform 2-star alignment, and add 3-4 calibration stars. By the end of this process, calibration stars should be very close to the center of the crosshairs. Don't agonize over this, but please take the time to get it close. Try not to go back and forth on the directional buttons when you're centering the star as this will introduce error.
2) I slew the telescope to a star fairly close to the meridian, not terribly bright and somewhat to the south. Then, initialize the ASPA sequence.
3) After synchronizing the star and manually adjusting the mount to polar align it, end the sequence in the hand controller. The new firmware automatically removes the earlier synchronization with the chosen star and updates the sky model.
4) Slew to the first of the two alignment stars. Is it dead on? Great! If not, then use the hand controller to update the alignment of the two alignment stars, then repeat the ASPA process. Chances are good you won't need to go through this more than twice if you started off with a rough polar alignment.

Usually, I get PA accurate enough for 2+ minute exposures with one try. Sometimes I do have to repeat the ASPA process to get accurate enough for acceptable AP. However, if you're just visually observing you should never need more than a "decent" polar alignment so just do it once and get to the fun stuff (observing).

I hope this helps, as opposed to confuse.


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TL2101
professor emeritus


Reged: 09/17/10

Loc: Concord, Ca
Re: CGEM Polar alignment Procedure? new [Re: clchildress]
      #5239603 - 05/25/12 07:46 PM

Thanks I will try doing ASPA twice next time I use my camera.

I have the following Firmware, HC: GEM 4.20 and MC:6.08 6.08.

Celestron's website shows:
http://www.celestron.com/c3/downloads.php?Products%5B%5D=548&Submit=Search

Hand Control Firmware Updates v.1.1.14?
Motor Control Firmware Updates v.2.2.5?





Edited by TL2101 (05/25/12 08:14 PM)


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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM Polar alignment Procedure? new [Re: TL2101]
      #5239641 - 05/25/12 08:08 PM

The ASPA depends on your alignment being accurate. Be very careful and precise in doing your 2+4 alignment and the ASPA will be more accurate.

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mclewis1
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Reged: 02/25/06

Loc: New Brunswick, Canada
Re: CGEM Polar alignment Procedure? new [Re: Stew57]
      #5239698 - 05/25/12 08:41 PM

Joe,

What you've picked up from the Celestron website are the firmware updater apps themselves. You run those apps (one for the hand controller, the other for the motor controllers) and then get to pick the appropriate firmware levels you want.

Have a look a Mike Swanson's www.nexstarsite.com for more info on the firmware and the updater applications.


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TL2101
professor emeritus


Reged: 09/17/10

Loc: Concord, Ca
Re: CGEM Polar alignment Procedure? new [Re: mclewis1]
      #5239912 - 05/25/12 10:53 PM

Mark,

Thanks for the heads up on the updater apps and link.


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PatHolland
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Reged: 10/11/09

Loc: Clever, Missouri
Re: CGEM Polar alignment Procedure? new [Re: clchildress]
      #5241525 - 05/26/12 11:43 PM

Thanks for the info Chris. I just updated my CGEM with both the hand controller and motor firmware updates and will try the ASPA next night out to see how well it works. Thanks for the heads up.

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vikan
member
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Reged: 10/05/10

Re: CGEM Polar alignment Procedure? new [Re: PatHolland]
      #5420232 - 09/13/12 11:39 PM

I got a new to me CGEM recently. I appreciate this thread very much. I have a follow up question.
I am doing 2+4 , polar alignment, 2+4 as suggested. It tracks fine.
Should "display align" read 0,0 after this procedure? For me it says 0,0 after polar alignment but not after the second 2+4 procedure.


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Stew57
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM Polar alignment Procedure? new [Re: vikan]
      #5420532 - 09/14/12 08:13 AM

The mount assumes perfect alignment and perfect polar alignment and gives you 0,0. Of course this is impossible because of flex, being a arc second off on an alignment star, etc.. The second is the amount the mount figures you are off with the new alignment. You can do a second aspa if you need to get really close but after 2 you are about as close as aspa can get you. Aspa is only as accutate as your original 2+4 alignment.

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SergeC
scholastic sledgehammer
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Reged: 03/23/12

Loc: Gainesville, FL
Re: CGEM Polar alignment Procedure? new [Re: Stew57]
      #5420596 - 09/14/12 09:08 AM Attachment (18 downloads)

I asked something along these lines regarding me CG5, and the consensus was:

1. Set up with a rough polar alignment by sighting through the bore or with a polar scope.
2. Do the 2+4 alignment routine, using a reticle EP to get precise alignment. You must do the 4 calibration stars to get an accurate measurement of cone error.
3. Do the ASPA routine on a star just west of the meridian and near the equator, if possible.
4. Slew to your alignment stars or use different stars if more convenient (using the named star list) and use the Align routine to replace their respective alignments. There is no need to re-sync the calibration stars if you haven't moved the OTA in the saddle.
5. Repeat the ASPA routine.
6. Re-sync the alignment stars.
7. Enjoy the fruits of your labor.

I tried this night before last in my backyard (suburban orange zone, average seeing and transparency), and here's a single test sub of M17 I took at 2min unguided, ISO1600 Pentax K100D, Meade 8" SCT with a f/6.3 reducer. Lots of noise (it was quite warm that night), but the stars aren't too bad, only a bit out of round, and that likely due to PE since I hadn't turned on PECTools. I think the engineers did a pretty darned fine job with the ASPA routine.


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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM Polar alignment Procedure? new [Re: SergeC]
      #5420672 - 09/14/12 10:04 AM

Taken from conversation with Celestron engineer;

"Errors in the mount model data degrade pointing accuracy. Errors in Polar Misalignment affect tracking accuracy.
Most of the time, I am much more concerned about tracking accuracy than pointing accuracy.

Question is a realignment needed after the use of ASPA or was my admittedly unscientific test an anomaly?

After you do your first 2+4, you can read the polar misalignment by ALIGN/Polar Align/Display Align. Write that down, I will come back to it. If you display align again after ASPA it will show your polar misalignment to be exactly zero (or sometimes just a few arc seconds off). How can it be EXACTLY zero? Surely there must be some error. The software assumes that all parts of the ASPA went perfectly. That you synced perfectly. That the pointing was perfect while adjusting the ALT and AZM axes. That your adjustments in ALT didn't tweak your RA axis a little bit in the AZ direction. That there was zero mount flexure on your ASPA star. These and many other assumptions are forced onto the software because it has no way to quantitatively measure those errors and account for them. Because none of these assumptions is true, the mount model now has data that is not correct. In particular, the mount models assessment of your polar misalignment has an unspecified amount of error. Errors in tracking have been improved, but the mount model (pointing) was degraded (slightly). If you redo your sky align, the mount model will once again have an accurate measure of your polar misalignment. It is my belief that if you had checked your polar misalignment after your second 2+4 alignment, you would have found that it was not exactly zero, but rather on the order of a few arcminutes. This was the error you introduced into your pointing model on objects near the pole.

A reasonable question would be "How big are the errors introduced into my mount model as a result of ASPA". The math is complex, and involves many mount/OTA errors that most people don't know and can't measure . But using strictly empirical data derived from re-checking my polar misalignment after re-doing my 2+4, I would say that ASPA will usually get you 90% closer to the pole than you were when you started the ASPA. That is, it converges on the pole with repeated execution until you get within the pointing accuracy of the mount. Despite being only 90% closer, the mount model will now report you are EXACTLY on the pole. (BTW it doesn't always miss by falling short, it could overshoot by 10% just as easily and there could be some mingling of errors between axes).

Finally, consider this, the measurement of polar misalignment is not going to be more accurate than the pointing accuracy of your mount model. For your own mount/OTA this seems to be a few arcminutes. Once you get within that limit, or even close to that limit, continued application of ASPA is not going to cause further convergence. In my experience, if the polar misalignment is less than 10 arcMinutes, then I will see less than one minute of Declination drift in 100 minute exposure set, and for what I do that is usually good enough. If my measured polar misalignment is 15 arcminutes or higher, or if I need to do several 100 minute sets, then I will usually do one more ASPA. As I mentioned, pointing accuracy is not critical for me. But if I redo my sky alignment anyway, I can usually measure a polar misalignment of less than 4 arcminutes on both axes, at least half of of which I would attribute to my mount model not knowing exactly where the pole is. If this level of polar alignment was not enough, then I would do drift alignment.

For my particular application, I do not need ultra precise pointing. As long as I start within 1.5 degrees of NCP, I do not feel compelled to redo my sky alignment after ASPA. I know my polar alignment is "close enough" and the errors I have put in my pointing model (if any) will not affect my work. Sometimes I do it anyway though, just to satisfy my curiosity. Your mileage may vary."


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: CGEM Polar alignment Procedure? new [Re: Stew57]
      #5421012 - 09/14/12 01:59 PM

I've never measured accuracy of ASPA but from experience I can say it's precise enough to do 30 min subs @765mm, at least.
Recent examples, each image 6x30min subs:





Knez

Edited by orion69 (09/14/12 02:00 PM)


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vikan
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Reged: 10/05/10

Re: CGEM Polar alignment Procedure? new [Re: SergeC]
      #5421065 - 09/14/12 02:40 PM

Quote:

I asked something along these lines regarding me CG5, and the consensus was:

1. Set up with a rough polar alignment by sighting through the bore or with a polar scope.
2. Do the 2+4 alignment routine, using a reticle EP to get precise alignment. You must do the 4 calibration stars to get an accurate measurement of cone error.
3. Do the ASPA routine on a star just west of the meridian and near the equator, if possible.
4. Slew to your alignment stars or use different stars if more convenient (using the named star list) and use the Align routine to replace their respective alignments. There is no need to re-sync the calibration stars if you haven't moved the OTA in the saddle.
5. Repeat the ASPA routine.
6. Re-sync the alignment stars.
7. Enjoy the fruits of your labor.





Thanks Serge. What kind of alignment error numbers do you get after the whole procedure?


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vikan
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Reged: 10/05/10

Re: CGEM Polar alignment Procedure? new [Re: Stew57]
      #5421076 - 09/14/12 02:50 PM

Mark,

Thanks for the excellent write up. Things are a lot more clear now. I got about 6' error last night. I will try to do better tonight. I cannot see polaris from my small back yard, so i'll probably have to do multiple ASAPs.


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Stew57
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM Polar alignment Procedure? new [Re: vikan]
      #5421094 - 09/14/12 03:07 PM

I use cross box on my camera for alignment. try that or a real high power eyepiece with cross hairs for the 2+4. Once you find a good polar alignment set some bricks under the grass so you can set the same way each evening. I have small painted dots on my back patio so I can at least start close.

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SergeC
scholastic sledgehammer
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Reged: 03/23/12

Loc: Gainesville, FL
Re: CGEM Polar alignment Procedure? new [Re: Stew57]
      #5421443 - 09/14/12 07:25 PM

Vikan

When I check what the HC reports after the first ASPA iteration it's being 00 00 10 +/- 3 on each access, and less than 00 00 05 after the second iteration. It's reported straight zeros a couple of times. I haven't seen any improvement with more iterations. The first few times I checked the drift after the 2 ASPA and couldn't detect any in 5 min, so now I just do the 2 iterations and let my autoguider take care of anything for exposures longer than a minute.


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Stew57
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM Polar alignment Procedure? new [Re: SergeC]
      #5421502 - 09/14/12 08:02 PM

If you have trouble guiding in dec you may want to be off a little with your polar alignment on purpose. Some cgems have a cogging problems with the servos. There are work arounds if you have problems. Celestron is working on a firmware fix for the problem also.

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vikan
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Reged: 10/05/10

Re: CGEM Polar alignment Procedure? new [Re: Stew57]
      #5421779 - 09/14/12 11:53 PM

Tried it now.

results: azm , alt
start : 33' , 8'
1st iter : 6' , 3'
2nd iter : 0'30", 3'16"
3rd iter : 1'57", 1'27"
4th iter : 0'19", 3'09"

Looks like after about 3 min error it is up and down.


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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM Polar alignment Procedure? new [Re: vikan]
      #5422108 - 09/15/12 10:22 AM

You may be a victum of the cogging effect. It generally shows as a saw tooth pattern on a graph of a guided dec axis.

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