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Atlas EQ-G, imaging C-11, 80mm ST, PHD Guiding

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#1 Peter in Reno

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Posted 19 November 2008 - 11:59 AM

I have the following equipment:

Atlas EQ-G mount
Celestron 11" OTA with ADM's Losmandy dovetail plate and saddle
Orion Short Tube 80mm for guiding with clam shell tube rings using ADM mini-dovetail mounted on C-11
Orion StarShoot Pro Color CCD camera
Orion SS Auto Guider
12V 75 Amp-Hour Marine Deep Cycle Battery

When I do three star alignment using 5mm illuminated reticle eyepiece, I use directional buttons of keypad. I noticed when I release the left directional button, the star does not immediately stop, it keeps going for a short time and then stop. Other three directional buttons stop immediately. Do I have a bad motor for the left directional button? The mount is about 4 months old. I know it's under warranty but I already did some modifications like 1" thick counter weight shafts and drilled/tapped three larger holes behind latitude scales so I can tighten latitude adjustment.

After three star alignment, the mount pointing accuracy is very good. It always find the object I want to image with C-11. PHD guider seems to work well. The guide star seems to be always centered when imaging. I make sure the mount is well balanced before starting three star alignment.

The real problem I am having is exposures over 10 minutes when imaging with C-11. The star trails (or elongated stars) get bigger with longer exposure. Is it because of heavy OTA plus guide scope or the left directional button as descibed in first paragraph? C-11 plus guide scope weighs 38 pounds. I made sure the equipment I have do not have flex issues especially on guide scope. I use clam shell tube rings instead of three screws tube rings for that reason. If PHD Guider keeps the guide star where it supposed to be, then why am I getting star trails (or elongated stars) longer than 10 minutes exposure with C-11?

Thanks,
Peter

#2 yg1968

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Posted 19 November 2008 - 12:26 PM

Have you tried updating the firmware on the Atlas? I know that there was an issue where the mount would keep slewing until you pressed on the direction button again. But I believe that the issue was fixed in one of the firmware revisions.

#3 Peter in Reno

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Posted 19 November 2008 - 01:28 PM

I already updated to latest version 3.23 about two months ago.

Peter

#4 nofxrx

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Posted 19 November 2008 - 06:48 PM

Are you Polar Aligning the mount(i.e. Drift Alignment,some form of a Polar alignment biult into the HC,not sure if the Atlas has this or not) ?
Even with a "close" PA you will see drift when imaging at that C11's extreme F/L. You need to do a VERY good Drift alignment,THEN do your GOTO alignment for accurate object placement.The mounts SW will automatically correct for an imperfect PA and will still produce very accurate GOTO's.In other words your mount does not need to be pointed at the NCP for good GOTO's.It does however NEED to be very accurately pointed at the NCP for imaging at anything over~1000mm.I can just use the PA scope on my CG5 when imaging with my ED80 and get no trailing or field rotation with exposures of 20minutes.When imaging with my C9.25 I make sure to do a drift alignment so that there is no drift visible for at least 1.5x the exporure length(i.e. if I plan on shooting 10 min subs,I make sure there is no drift for over 15 minutes).
If this is all stuff you know the disregard.But I have seen many imagers fed up with there system not producing good images when its just because they dont know or dont want to drift align.
My :penny::penny:

#5 rmollise

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Posted 19 November 2008 - 07:04 PM

The real problem I am having is exposures over 10 minutes when imaging with C-11. The star trails (or elongated stars) get bigger with longer exposure. Is it because of heavy OTA plus guide scope or the left directional button as descibed in first paragraph? C-11 plus guide scope weighs 38 pounds. I made sure the equipment I have do not have flex issues especially on guide scope. I use clam shell tube rings instead of three screws tube rings for that reason. If PHD Guider keeps the guide star where it supposed to be, then why am I getting star trails (or elongated stars) longer than 10 minutes exposure with C-11?

Thanks,
Peter


Are you imaging at f/10? If so _get the focal length down_ to at least f/6.3 with a reducer/corrector it will make imaging easier and FOVs wider.

You want your polar alignment to be purty good when you are at these integration times. Consider stacking 3 - 5 minute subs.

Make sure you are well balanced, meaning, just slightly east heavy.

:cool:

#6 Peter in Reno

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Posted 19 November 2008 - 07:09 PM

Because C-11 plus guide scope weighs 38 pounds, it's extremely difficult to do drift alignment. It's extremely difficult to turn the latitude adjustment bolts with 38 pounds on the mount. I am afraid of snapping the bolts or ruin the threads in the mount. I always check for polar alignment with polar scope WITHOUT the scopes mounted. Everytime I set up the mount and look through the polar scope, the latitude is always spot on and the only adjustment I make is azimuth. I figured this is as good polar alignment as I can get minus drift alignment.

Peter

#7 Peter in Reno

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Posted 19 November 2008 - 07:18 PM

Uncle Rod,

Yes, I am imaging at F/10. I already tried f/6.3 focal reducer but I get huge vignetting probably because I have to use about 4" extension in order to prevent from the camera to hit the JMI Motofocus. The diameter of StarShoot Pro CCD camera is huge. The reducer was connected to the rear cell first, then 4" extension and then the camera. I can't yet figured out how to mount the reducer between the 4" extension and the camera. Will it work better if the reducer is mounted right in front of the camera?

Peter

EDIT: Please define "East heavy". Does it mean when counter weight shaft is at horizontal and pointing east and counter weight end is heavier than the scopes?

#8 Peter in Reno

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Posted 19 November 2008 - 07:22 PM

M1 was taken last night. 15 frames of 10 minutes each at F/10. Stacked with Deep Sky Stacker. Pre and post processed with Nebulosity. I am a little surprised how well it worked.

Peter

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#9 Awesomelenny

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Posted 19 November 2008 - 07:48 PM

Hi Peter,

First off, you have an excellent image of M1! Better than any I have ever taken yet!

You mentioned something that homes right in on what the problem is.

You say that the guiding is dead on ... all the time ... but the image trails.

That is a classic example of flexure in your equipment. This is due to stresses between your guide scope, the brackets on it and the main scope that it is attached to.

It is imperative that every contact point is extremely tight. Thus the guide scope ring knobs must have delrin tips to prevent marring the finish on the guide scope. I happen to have Losmandy guide scope rings on a losmandy dovetail bar bolted onto the radius blocks that are bolted right onto the OTA ends. The Losmandy radius blocks are made to fit various manufacturer OTA's such as your C-11. This sort of rigid assembly prevents the minute amounts of flexure that ruins hours of good work in imaging.

Like Unc Rod mentions, taking shorter but more numerous subs will rid of this problem too. But if you want to add images like Hyd.-Alpha narrowband on the emission nebulae like M1 and many others to bring out the chaotic nature of these celestial events requires much longer exposures like 10 minutes or more at a time, you need rigidity with heavier duty support systems.

Fun, isn't it? :jump:

#10 Nate B.

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Posted 19 November 2008 - 08:06 PM

I'm no SCT expert but my imaging buddy just installed mirror locking bolts on his C11 because no matter how much flexure he eliminated, how tweaked his guiding was, etc. he couldn't overcome the slight movement of his mirror over a 10 minute exposure. He had oblong stars. If that's your problem, rather than double stars or something like that, I think it could very well be mirror shift.

#11 Peter in Reno

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Posted 19 November 2008 - 08:13 PM

Len,

I had ADM's three point guide scope rings with Delrin tips and I was getting minor star trails with PHD guiding. I later read a blog from Stark-labs about flexure between guide scope tube rings and main scope. So I replaced ADM's tube rings with clam shell type to maximize contact points but I am still seeing same amount of trails. If you look at my signature, I am using high end equipment to minimize flexure. I think my ADM's mini-dovetail system is very rigid and tight. My guide scope weighs only 2.5 pounds. I didn't want to use Losmandy dovetail system because it's quite heavy and weight is one of telescopes' enemy.

Peter

#12 Peter in Reno

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Posted 19 November 2008 - 08:17 PM

I'm no SCT expert but my imaging buddy just installed mirror locking bolts on his C11 because no matter how much flexure he eliminated, how tweaked his guiding was, etc. he couldn't overcome the slight movement of his mirror over a 10 minute exposure. He had oblong stars. If that's your problem, rather than double stars or something like that, I think it could very well be mirror shift.


Please tell me where he got the mirror locking bolts. Did he install a focuser at the rear cell like Crayford style?

Thanks,
Peter

#13 Nate B.

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Posted 19 November 2008 - 09:53 PM

He did the job with parts from local stores; nothing special. I will ask him what site he used as a guide but I'm about to leave the country for business so I won't be able to post an answer for a little while. I'd suggest you google C11 mirror locking bolts and you should find some good information.

#14 Peter in Reno

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Posted 19 November 2008 - 10:47 PM

That's why I asked you because I have tried Googling for it without much luck. I'll search deeper. Do you remember if your friend had to take apart the telescope to install locking bolt?

Thanks,
Peter

#15 John Miele

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Posted 19 November 2008 - 11:17 PM

Peter,

I have almost the exact same setup as you excpet I use a Meade DSI pro and DSI II for guiding and imaging. Right now, I'm using a 4" refractor with the ST-80 to get experience collecting data and learning to process. but eventually I want to use my C11 as you are doing. I also have the ADM mounting hardware and it seems very very rigid. I have no advice to give because I'm not at your point yet. But you M1 image is just beautiful and gives me inspiration for what I might be able to achieve. Thank you for sharing it and I'll look forward to hearing more from you. If mirror shift is indeed the problem, I'll probalby have to deal with it as well. Of course right now, I'm happy to go for 3 or 4 minutes with good stars. I have never even tried 10 minutes yet!

John

#16 X-Lurker

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Posted 19 November 2008 - 11:37 PM

Nice image Peter! Could wind be an issue? I'm imaging with a 10" newt and the very slightest breeze gets me football shaped stars...

#17 Peter in Reno

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Posted 19 November 2008 - 11:40 PM

John,

Thanks for your kind comment. My goal is to be able to expose images beyond 10 minutes. I have an 80mm EON scope piggybacked by 80mm short tube and I have not yet started imaging with it but I plan to soon.

Peter

#18 Peter in Reno

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Posted 19 November 2008 - 11:43 PM

Larry,

There was no wind the night M1 was imaged. It was one of the calmest night in a long time. Good thinking.

Peter

#19 Peter in Reno

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Posted 19 November 2008 - 11:47 PM

M57 was my first photo.

9 frames of 10 minutes each.

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#20 Peter in Reno

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Posted 19 November 2008 - 11:49 PM

M27 was my second photo:

16 frames of 10 minutes each.

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#21 AlexN

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Posted 20 November 2008 - 12:19 AM

I use the same setup, And at F/6.3 I dont have a problem running 20 ~ 25 min exposures... I sometimes spend up to 40 minutes tweaking the balance until it is perfect... with this much weight on the mount, your balance has to be SPOT ON!! DEC balance should be 100% perfect, I use a weight to unbalance it by 2 ounces in RA (heavy on the east) I do this using a small bag attached to the bottom of the counter weight shaft, with a 4x 0.5oz fishing sinkers in it. I balance it to be EXACTLY balanced in RA with the bag attached, then remove the bag before imaging.

Mirror shift could be a problem, but I've found its not for me, unless goes really close to the meridian..

Do you have your ST80 on top of your C11 or side by side? I use mine side by side, Doing this is good for a few reasons. 1. You keep the weight closer to the polar axis, thus putting less strain on the mount.
2. You have less chance of flexture (although, with the Losmandy bar and saddle, your flex there should be minimal.

If I think of anything else, I'll post back

You have a nice, and very capable setup! :) and once you iron out this little issue, you'll love it to bits (as I do)

Oh, Collimate the scope... I know people say its not overly important on the SCT scopes... It is... Check it, because the stars looking slightly out of shape could be collimation....

Cheers.
Alex.

#22 AlexN

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Posted 20 November 2008 - 01:22 AM

Oh thats it... Drift align the mount using the ST80 on its own, with a reticule ep and a 2x barlow, or using a camera with computer aided drift align routine (K3CCD tools has a good one, or a program like EQ Align or WCS) drift align it until you see no drift at all for 10mins at the ep, or untill your software program reports little to no drift, Park the mount, then set it up with the C11+ST80 etc... Thats what I do here, I drift align with just my 90mm refractor and my DBK31 to measure drift through K3CCD tools. Then swap out the 90mm for the side by side C11 + ST80, balance it up, do my star alignment, and away I go...

I strongly recommend using more than just the polar scope for imaging with long focal lengths (2.8m of the C11 is LOOOONG!)

#23 Peter in Reno

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Posted 20 November 2008 - 02:01 AM

That's a great idea about drift alignment using a smaller and lighter scope. I hope it's not too time consuming.

My guide scope is currently mounted on top of C-11 via ADM's mini-dovetail system.

Thanks,
Peter

#24 AlexN

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Posted 20 November 2008 - 02:36 AM

Once you get the hang of drift alignment, it only takes 25 ~ 30 mins to get it pretty good. if you use a your SSAG and K3CCD tools (or one of the other programs) it can take as little as 15 minutes between setup and perfect alignment.

#25 Strgazr27

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Posted 20 November 2008 - 03:54 AM

Keep in mind that doing a drift alignment with the 90 will get you close but when imaging through the 11, especially at it's native Fl will show errors in your PA that the 90 would not show. This will be less evident on shorter length groups of images but start stacking 3-4 hours of subs and it will show.

CS's






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