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CGEM alignment question for imaging

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#1 zavisa

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Posted 26 January 2011 - 11:03 PM

Hi all -

Received my CGEM last week (unfortunately, missing counterweight bar) - still waiting.

My question: I either setup in my driveway which is west viewing or my backyard which is east viewing. Is it sufficient to do a 2SA + 4 calibration stars and then polar align for longer exposure imaging when I can only align with stars from either east or west? I can see Polaris from both.

Also, I'm assuming (hoping more like it) that I will be able to hook up my new oag and star shooter autoguider to track accurately after performing the alignment steps above. To date I've only been able to take many shorts so I don't know what exposure time it will take to get my histogram correct for the targets I've tried (Orion and Andromeda), but obviously I want to be able to accurately track long enough for better exposure. Was only able to get 10 to 20 seconds with my nexstar mount.

Thoughts?

#2 snommisbor

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Posted 27 January 2011 - 01:04 AM

For me most of the stars I use are more in the SW and SE. I cant even see Polaris from my backyard which is where I set up in the shadow of my house when the neighbor has his light on, plus no street lights either. So I am sure you will be fine in either location. Now that I am imaging I do 2 star align then 1 calib star then polar align the mount, then go back and realign the 2 star align and then I go back and add 4 calib stars.

#3 freestar8n

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Posted 27 January 2011 - 07:30 AM

Hi-

You need to use alignment and calibration stars from opposite sides of the meridian. Ideally the stars should be spread across the sky, but even if the only ones you can see are just across the meridian, that should work. If the sky is blocked by trees then you may be able to see bright stars through the branches - especially in winter. Once you add one calibration star, the goto accuracy may be good enough that you can see others through the trees easily and just center and add them as calibration stars.

Once you do the 2+4 you can then do the polar alignment using a star low in the south.

Frank

#4 bbbri

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Posted 27 January 2011 - 11:00 AM

I can use all eastern stars to align from my yard. I have to choose one that's nearly overhead, and the second about 45 degrees above the horizon. Choose up to 4 calibration stars close to your intended targets and you will get accurate gotos. The controller is picky about which ones to use for polar alignment, and it will let you know if it doesn't like a star. I haven't used my CGEM lately or I'd be able to tell you precisely which stars to choose. Capella and Betelgeuse might work as eastern alignment stars in the evening.

#5 freestar8n

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Posted 27 January 2011 - 11:17 AM

I'm not sure what you're saying. The alignment stars need to be on one side of the meridian, and the calibration stars need to be on the other. I'm not sure the cgem will even allow you to have them all on the eastern side of the meridian. Is that what you are doing somehow?

Frank

#6 zavisa

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Posted 27 January 2011 - 11:48 AM

Frank - haven't done it yet, still waiting for missing parts. In a nutshell, yes that is what I am asking. Can I align and calibrate using only stars from one side of the meridian.

Since I plan on guiding, is is necessary to go through a full 2asa + 4 calibration and then polar align? I would assume (we all know the problem with that) that with a good polar alignment, I could guide accurately enough for imaging as long as my goto got me to my target.

#7 Peter in Reno

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Posted 27 January 2011 - 11:55 AM

The purpose of calibration stars at opposite side of Meridian is to compensate for any orthogonal errors between OTA and mount. If you do Meridian flipping (that's how GEM works), the OTA might be slightly not parallel to the saddle plate on CGEM and calibration stars helps compensate so the GoTo will be accurate.

Fork mounts like CPC series scopes do not require calibration stars because the OTA is always orthogonal with the mount and there is no Meridian flipping in fork mounts.

Peter

#8 freestar8n

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Posted 27 January 2011 - 12:26 PM

Hi-

I'm pretty sure the cgem won't let you mix alignment stars on one side with calibration stars on the same side - so I wasn't sure what Brian was saying. Even if it did let you, the calibration wouldn't be as good.

No - you don't need accurate goto's for imaging as long as the target is on the chip, but you do need accurate goto's in order for the all star polar alignment to be accurate.

So - if you drift align or something to do your polar alignment, and if you can put the object on your ccd, then you don't need to use calibration stars at all.

Frank

#9 snommisbor

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Posted 27 January 2011 - 12:56 PM

The CGEM will only name stars that would work for 2 star align as an option which is always in the West, then when you add Calib it will only give stars you can use in the East, so if you scroll to a star in the list it will be good based upon time and location you entered. For instance Deneb is always a Calib star in the summer but now it shows up as a 2 star align option. Also when you ASPA it will tell you if it doesnt like a certain star i.e. the other night it didnt like Betleguiese but it did like Rigel.

#10 freestar8n

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Posted 27 January 2011 - 01:06 PM

Hi-

You can use alignment stars in the east during 2-star alignment by holding down the Menu button for a second or two. It will switch from offering alignment stars in the west to stars in the east. Once you finish the two alignment stars, it will offer calibration stars from the opposite side.

Frank

#11 Peter in Reno

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Posted 27 January 2011 - 02:08 PM

So - if you drift align or something to do your polar alignment, and if you can put the object on your ccd, then you don't need to use calibration stars at all.


What if the OTA is not perfectly orthogonal with the mount, would calibration stars help compensate for that? It should help increase GoTo accuracy for both sides of the Meridian.

Peter

#12 freestar8n

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Posted 27 January 2011 - 02:34 PM

I was responding to his remark that he doesn't really need goto accuracy as long as the object is on the ccd. If he wants to use all star polar alignment, then he will need goto accuracy for the polar alignment to be good - and the calibration stars will correct for non-orthogonality, but an overlooked part is the declination offset of zero for the mount.

As long as the mount is polar aligned - it could be bent all kinds of ways and it will track just fine once the object is on the chip.

For me, I want good goto accuracy for imaging because it saves a lot of time - and I just want to know that when I go somewhere, that's where I'll be. I also do synchs using plate solves in maxim.

Frank

#13 bdawg6381

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Posted 27 January 2011 - 03:48 PM

Zavisa, check your PMs... I've got an extra CGEM counterweight bar you can borrow till yours arrives if you want (and I live in TN as well).

#14 Kenny2004

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Posted 05 February 2011 - 09:43 AM

Is the All Star Polar Alignment really that accurate and good enough for long exposure imaging without the need of a Polar scope?

#15 jrcrilly

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Posted 05 February 2011 - 10:03 AM

Is the All Star Polar Alignment really that accurate and good enough for long exposure imaging without the need of a Polar scope?


It's at least as accurate as a polar scope. Whether either is sufficiently accurate for imaging depends on the extent to which you are willing to rely on guiding.

#16 dale c

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Posted 05 February 2011 - 12:14 PM

If you are planning to image with the mount, you will have to align the mount. A polar scope will help with the initial setup but you can guess it pretty well. To get the most out of the mount, it will have to be drift aligned. You don't want any computer interferrance with the drive system when drift aligning so I would reccomend not doing any computer alignment. If the drive requires a computer alignment do a one star. If you have to do a two star accept the second star without making any correction. Any computer corrections to the drive system will screw up the drift alignment.. If the mount isn't drift aligned you will have field rotation to some extent.

#17 Goran Strand

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Posted 06 February 2011 - 03:24 AM

How accurate do I need to polar align my mount for guided photography? I use a 2 SA and one calibration star, then I do a polar alignment from the menu. After that I start shooting and guiding with my 80mm/f5 guidescope. Do you think that's enough?

#18 mclewis1

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Posted 06 February 2011 - 08:43 AM

Goran,

I would use 2 stars for alignment and 3 or 4 calibration stars. By the third or forth calibration star the mount should be putting it very close to the center of the fov. I usually just keep adding calibration stars until I don't have to move the mount at all to center the star (usually 4 stars).

I've found that using 4 calibration stars really improves both my gotos and the accuracy of the polar alignment done afterwards. The extra calibration stars really helps reduce cone error.






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