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Equipment Discussions >> Mounts

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dawziecat
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Loc: Rural Nova Scotia
G11 Star Elongation: ONLY NEAR THE MERIDIAN
      #5432986 - 09/21/12 03:39 PM

I get good results with exposures up to 30 minutes with my G11 as long as I don't image within 15 degrees or so of the meridian.
Then things go to Hades and I get E-W elongation.

I have done my darndest to eliminate any possibility of flex and AM USING VERY SHORT FLs (caps intended). I get this elongation even at 180mm FL. Balance does NOT seem to be the problem. I've tried both equi-balance and east-heavy to VERY east heavy with no apparent difference. Imaging load is light . . . less than 15 pounds with a 180mm camera lens.

I guide with the STi guiding kit and PHD.

I've tried all I can think of to fix this frustrating problem.

The crux of the matter is (again, caps intended), IT ONLY HAPPENS WHEN IMAGING NEAR THE MERIDIAN!

Thirty minute subs look great well off the meridian. Within 15 degrees I have to cut back to 3 minutes or so.


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Mantis707
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Reged: 08/14/12

Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5433066 - 09/21/12 04:21 PM

Has it been like this since you got the mount or is this something that just started happening....(Changes in equipment payload, service done to the mount etc?)

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Stew57
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Mantis707]
      #5433102 - 09/21/12 04:46 PM

If your balanced east heavy the amount will change depending on the angle of the ra axis.

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David Ault
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Reged: 09/25/10

Loc: Georgetown, TX
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Stew57]
      #5433238 - 09/21/12 06:26 PM

Terry,

Have you noticed any cable bunching, anything restricting the movement of the RA axis? What does your guiding graph look like when approaching the meridian? Is it sending really large pulses E/W to try to correct, or is it behaving normally (which indicates some form of flexure)?

What kind of power supply are you using? If your powering off a battery or 12V supply, when the load shifts and it tries to draw more current the voltage may sag causing erratic behavior. I run mine with a custom 20A 15V power supply when I've got A/C power. Otherwise, I run a car battery through a power inverter and then to the power supply to guarantee proper voltage delivery (I know this is inefficient, but I was lazy and didn't want to build my own DC/DC step up power converter for running off the battery).

In my case when running the G11 off a 12V power supply or battery I had all sorts of speed control issues and strange random slews, etc. When I moved to the 15V 20A supply all those disappeared.

Regards,
David


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JoseBorrero
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: David Ault]
      #5433461 - 09/21/12 09:07 PM

That problem happen to me when I tried autoguide with my AT6RC and imaging with ED80. My solution was using a side by side with Orion st 80 as a guide scope to my ED80. It seem flexure was my problem.

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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: JoseBorrero]
      #5433938 - 09/22/12 07:23 AM

Thanks for the replies so far but nothing falls into place.

1/ I am unsure if this was always a problem or a recent occurrence. I always had some poor subs but only recently sleuthed it out to being only when very near the meridian.

2/ Power? Not likely. Using AC.

3/ Symptoms point to flexure but I can't see how. I am away from the gear now and can't post pics. I've tried several configurations. Latest is a Casady plate bolted directly to the G11. Camera (ST8300M) is bolted directly to a Losmandy D style dovetail through the 1/4”X20 tripod mounting screw. Lens is mounted to camera and is firmly supported under its objective end. STi guide scope mounted securely “Joey” style to dovetail. Everything is TIGHT!!!! I've used other configs too that were perhaps more prone to flex. My problem led me to this which I just can't see flexing unless the whole D plate is drooping or twisting. Seems inconceivable.

Flexure only at the merdidan and with such short Fls seems highly unlikely with this configuration. Weight is absolutely trivial too considering what this mount should be able to carry with ease.

4/Guiding seems normal. Nothing I can see unusual going on in the magnitude or size of the corrections . . . again pointing to flex I suppose. But only very near to the meridian??? And at very short FLs???


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mclewis1
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5434121 - 09/22/12 10:02 AM

Terry,

One thing that does change as you approach and drop away from the meridian is the amount of torque required by your motors ... the amount depends on how much out of balance your setup is.

I'd vary your balance a bit and see if you can either make the situation worse of better.

Have you ever manually checked the worm/ring gear spacing or tightness around the whole circumference of the ring gear? You're looking for any tightness or binding areas as you slowly drive the worm (without any load on the ring ... and then with your normal load).


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David Ault
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Reged: 09/25/10

Loc: Georgetown, TX
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: mclewis1]
      #5434231 - 09/22/12 11:14 AM

Terry,

If you aren't seeing any changes in guiding, it does seem less likely to be power, gear binding or a balance shift. Power could still be an issue though if your AC/DC converter is a 12V lower amperage model. The only reason I push this point is because your issues seem eerily similar to mine when I was running with a 12V power supply. It could supply 30A, so power delivery was not an issue. This told me the G11 really needs 15V+ (and probably over 10A, at least when slewing) to run correctly.

Does the imaging lens have moving elements? I've never really though about this before, but it does seem like most camera lenses are designed to be held horizontally, not vertically, so I just wonder if something is shifting around in there.

Do you know anyone with an off-axis guider setup? It might be difficult to get the off-axis guider between your camera and lens, but you could stick their whole scope/guider/camera setup on your mount and at least rule that out.

Regards,
David


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Hilmi
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: David Ault]
      #5434343 - 09/22/12 12:18 PM

I can think of two possibilities.

1) Lens barrel creep. One of my lenses has this nasty habit.
2) Your mount needs to be tightened up. I had all kinds of imaging nightmares until one day I said I have had enough and I just pulled the entire mount apart and rebuilt it. After tightening everything up, my problems went away.


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Hilmi]
      #5434862 - 09/22/12 06:21 PM

Again thanks for the replies.

1/ I am on the road and sort of unprepared to answer some specific questions . . . like what PS I use. It will be a week or so before I get home and have the gear available.

I have only camera lenses. I can't rule out movement of some internal elements but focus seems to hold true. Also, if this were so, a sub or two would be ruined and then the offending element should settle in and subs would be good again. Doesn't seem the case. All subs very near the meridian are poor. They become poor and they stay poor until I shift away from the meridian.

2/ Unfortunately, off axis guiding seems just not possible with EOS lenses period.

3/ I am reluctant to pull the mount entirely apart as Hilmi suggests. I have inspected the worm-gear spacing on the RA
axis and all seems well there . . . but my examination was very inexpert.

4/ I have nobody with whom I could swap gear to rule out the mount. So far I have ascertained the problem is definitely occurring with both the EF180mm f/3.5L and EF135mm f/2.0L EOS lenses. I have used other EOS lenses but have not specifically looked for this problem at the meridian yet. I will do so after the current imaging project with the EF135mm f2.0L is complete. They are of longer FL and the problem, if it occurs with them too, will be proportionately worse.

The problem I am encountering seems similar to what Steve Cannistra reported on this web page, i.e., inconsistent tracking near the meridian with his G11. His "fix" was a Temma 2.

I'm hoping for a . . . er . . . less drastic solution.


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Peter in Reno
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5434901 - 09/22/12 06:45 PM

Every mount will always need maintenance. This including gear meshing. I don't know how difficult to adjust the mesh for G11. It seems like your mount may be most sensitive near the Meridian. The worm could be floating above the gear and if the mesh is out of adjustment or got a little loose due to wear and tear since you bought the mount, then the gear mesh may need to be tweaked.

I've owned my A-P Mach1GTO for about a year and it is just about to need gear mesh adjustment because I can feel a very slight free play in RA axis. When I first purchased it brand new, the RA axis free play was tight.

Maybe your G11 just completed a break in period and needs RA mesh adjustment. From then on, it may not need adjustment for a while.

Does it matter which part of the sky when your mount is near the Meridian?

Peter


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zytrahus
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Reged: 06/16/09

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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5434943 - 09/22/12 07:05 PM

I'd rule out gear adjustment problem because it always happen near the meridian... well unless you always park your mount to CWD using HC or software in which case you are always working on the same teeth of the gear. So if you are always parking back to CWD everyday: try turning the mount on during the day for few hours just to use another section of the gear next time.
Worm adjustment rules itself out (because of the 4mn period).


I've seen balancing having some minor effect on star elongation. But if I understand correctly you can guide perfectly with 30 mn exposure until you are too close to meridian at which point 3mn are the max you can do. That is not a minor effect to me... It has to be something else. I mean, if I can 30mn perfect exposures it means that my balance really is good enough.

I always do a little bit of east heavy balance and more often that not I would image 10 degrees past the meridian without noticing any image degration in 10mn exposures. So I don't think it is a balance issue.


I am really thinking flexure also. That's pretty much the only thing left. Have you noticed if DEC was affecting the angle at which it starts messing up? I am not sure how your lenses are attached to your mount but I can imagine problems happening when the lenses are pointing straight up if they're only held by 1 screw.


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Peter in Reno
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: zytrahus]
      #5434967 - 09/22/12 07:25 PM

Can you post a picture of your setup including the short 180mm focal length scope mounted on G11. Make sure you show everything including counterweight, guide scope, camera, etc. We will have a better understanding since a picture contains a thousand words.

Thanks,
Peter


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dawziecat
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Reged: 10/20/10

Loc: Rural Nova Scotia
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5434998 - 09/22/12 07:48 PM

I will post pics of the setup when I get back home. On the road right now. It will be a week to 10 days.

In the interim, thanks all for the replies.


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Peter in Reno
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5435000 - 09/22/12 07:49 PM

Also, can you show pictures of elongated stars as well? Maybe just a single sub is enough.

Thanks,
Peter


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Hilmi
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5435353 - 09/23/12 01:06 AM

Just take a set of LN keys and tighten down everything. And by the way, the G11 is so easy to strip to little bits that you would be amazed at the simplicity of it all. Only annoying bit is getting the worm block back on. I always found that part hard.

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Nebhunter
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5438271 - 09/24/12 03:11 PM

For what it's worth - have you drift aligned the mount and for how long?

The meridian presents a different set of variables and this became evident with my older ST-4 guider. It has a calibration routine - and at the meridian being east heavy or otherwise has very little effect due to the position of everything. The counter weights are almost hanging straight down so how do you effect east heavy?

I found that loading the Dec axis east heavy did the trick. I'm a side by side set up and hung a large C clamp on the outside edge of the Dec plate. This caused the RA to become east heavy - improved tracking, but also loaded the Dec axis to one side. Otherwise with a neutral balance you are subject to the play in the worm/pinion. The guider will command a movement but the slack in the worm prevents any movement. Next cycle the guider says - wow way off - and does another bigger move. Finally the slack catches up - and it's way overshoot. Next cycle - guider tries to bring it back, but the slack in Dec won't take up.

I hope this is not confusing. I also solved the problem by making the worm mesh in Dec very snug to where the hand controller would message Dec lag off and on. The tighter mesh would then respond quickly to guider commands. But you need to constantly adjust mesh - and release it before each slew. So the C clamp became the answer for me. It's a 4" clamp. Can't hurt by trying it. It's offsetting the weight away from the counter weight shaft.


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Nebhunter]
      #5475283 - 10/17/12 03:12 PM

Hi to All:

I said I would post images of the gear when I got back so here they are.

The problem persists. I have taken the G11 apart pretty much. Adjusted the RA backlash until it is so small I can no longer measure it by any deflection of the CW shaft. I have not touched dec backlash but it seems pretty minimal. Both axes rotate smoothly with no binding in evidence. Balance seems fine but I've played with it too, to no avail.

I don't see how alignment can be an issue. Twenty minute subs were fine off the meridian last night, just as they always are. As I entered within 10 degrees of the meridian, they fell off a cliff, just as they always do, and I had to cut back to 5 minutes.

Despite all my Herculean efforts to eliminate flex and my adjusting the RA backlash, quite successfully I think too, nothing has made any difference.

Guiding seems fine with nothing untoward showing up on the PHD graphs.
Len retains sharp focus which, in my mind, rules out any "barrel creep" in the lens. It is a pretty light lens in any event. It happens with other lenses too. Likely all of them but I have exhaustively determined it to be so with at least two.

I just can not image near the meridian with this gear and remain perplexed at which component is causing the problem.


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orion69
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Reged: 05/09/10

Loc: Croatia
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5475303 - 10/17/12 03:22 PM

There is at least one thing left... Try OAG.

Knez


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korborh
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Reged: 01/29/11

Loc: Arizona
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: orion69]
      #5475439 - 10/17/12 04:45 PM

Terry, is the issue in tracking or guiding?
With only tracking, the elongation is in RA only or is it RA/DEC? Since in tracking only RA axis is moving, if elongation is in RA or other direction, then that would be good data point to investigate further.


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5476778 - 10/18/12 11:31 AM Attachment (27 downloads)

Hi Parijat and Knez:

First off, OAG is not an option. No OAG solution is available for EOS lenses due lack of back focus.

The direction of elongation appears to involve both RA and dec axes. I attach an image showing a portion of a ruined sub along with superimposed test trails.

Edited by dawziecat (10/18/12 11:32 AM)


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Footbag
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5476856 - 10/18/12 12:22 PM

This is interesting. To get that much drift with a camera lens you would expect whatever is flexing to be obvious. I'd be looking at the guidecamera/lens for the flexure. Is it possible that dovetail is flexing? The cameras are at opposite ends on opposite sides. This would likely cause flexure only at the meridian.

An OAG shouldn't be necessary at this FL.

Do you have any telescopes or just lenses? As a test, you could guide through the longest FL scope you can find and do your imaging piggyback. I'd try and rule out any lens creep.

Edited by Footbag (10/18/12 12:24 PM)


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Russ Hunter
newbie


Reged: 09/24/11

Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Footbag]
      #5476961 - 10/18/12 01:24 PM

I suspect you are getting some movement along the camera lens centerline. The only things restricting motion in that direction at the meridian are friction from the front band/wood block attachment and the attachment at the camera with the 1/4" bolt.

You may try shimming between the band and the lens to get more clamping force and washers under the heads of the wood screws to give the screws a better grip on the band. See if any of that changes things and if so work towards some improvements based on the results.

Longer term, I would suggest changing the wood block to an aluminum bar and replacing the washer shims with a bar also to get more contact area between the camera and the dovetail.

Russ


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korborh
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Russ Hunter]
      #5477963 - 10/18/12 11:17 PM

OK the pictures show the movement in both RA and DEC. I assume you are autoguiding, and this looks to me like flexure.
If you are only tracking (not autoguiding), then it has to be flexure since (1) only happens around meridian - so not polar alignment issue (2) tracking only moves RA and flex will affect both - this is what you are seeing.

One thing you can try is to stack your images *without alignment* and see if the stars gradually move from frame to frame. This is indicative of flexure if the stars move in some semi-random way from frame to frame. You can make these stacks and compare star movement for frames taken around meridian vs. before/after it.

Looking at your setup, it seems like flexure is your issue. You can try making the connections more solid as others have suggested.


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pfile
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Reged: 06/14/09

Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5478074 - 10/19/12 12:37 AM

right - to check flexure just:

1) turn off dithering
2) take maybe 5-8 guided subs
3) load them into DSS
4) set the first image as the reference
5) align only
and 6) compute offsets.

if you see a steady X/Y drift, you're looking at flexure.

for the life of me i could not eliminate flexure on my G11/AT6RC/finderscope guider combo. i had to move to an OAG and now... round stars. but it was a big hassle and as you say, it's impossible with camera lenses.

my flexure problems definitely changed depending on the orientation of the telescope. when the telescope was more horizontal, the problem was worse. in your case it sounds like you have the opposite problem; flexure when the tube is pointing up.

i agree it seems insane that you'd have problems with DF at such a short focal length, but other than maybe your clutches slipping, i can't see what else would cause the problem.

what happens if you take unguided images?


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TxStars
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5478447 - 10/19/12 09:33 AM

*How much (end play) if any is there in your R.A. worm?
(This can be checked by having clutch locked and moving RA by hand while watching the RA worm)


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Peter in Reno
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: TxStars]
      #5478662 - 10/19/12 11:40 AM

The picture of your setup not only shows the scope is pointing to the Meridian but also at Zenith. Do you still get elongated stars at the Meridian at different parts of the sky other than Zenith? Does it matter which part of the sky at the Meiridian or only at near Zenith and Meridian?

Peter


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Peter in Reno]
      #5479079 - 10/19/12 04:06 PM

Thanks again for the replies.

There is NO play end-to-end in my RA worm gear. Not detectable anyway as I was aware of that issue when I adjusted the RA backlash.

Problem seems most acute near the zenith and less so at different elevations on the meridian. I can not image objects much less than 65 degrees elevation on the meridian for reasons of trees. So this has not been thoroughly checked.

I tried the test both Parijat and pfile suggest some time ago. Took the results to the Yahoo PHD group and there was differing opinion as to whether it was flex or no.

I really believe the lens is VERY securely held, as is the camera body. In reality, the ugly pipe clamp was a desperation measure only. The wood block is actually wedged under the lens . . . not just sitting there. The pipe clamp is tight to the lens barrel in any event.

Yesterday I noticed that the lens barrel assembly of the STi guiding package was not so tightly screwed to the guide camera as perhaps it should have been. I tightened it up. I really didn't think I'd found the reason for the problem. I took a 20 minute image on the meridian at 135mm. It seemed OK. This is a first!

Later in the night as my target moved from well east of the meridian to 11 degrees west of it, I took a series of 10 minute RGB subs.

They seem fine.

It's too early for "Hallelujahs," but just maybe . . .

I'm hopeful but I need another try with 20 minute NB subs to confirm I am "out of the woods." I've been chasing this demon all summer. It's just not as if the guide scope was obviously loose and able to flop about. Just not really "tight." I would have thought too, that if this was the cause, it would have ruined a sub or two and then settled in but that is not what I have been seeing. So I remain "cautiously optimistic."

Thanks all for the suggestions. There have been times I have just given up on this issue in exasperation.


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Footbag
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5479304 - 10/19/12 06:55 PM

That sounds like good news.

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TxStars
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5479466 - 10/19/12 08:38 PM

Sometimes it is the little things (which are hard to see) which are the biggest problems.

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pfile
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: TxStars]
      #5481252 - 10/21/12 12:33 AM

thats great! i guess even at these image scales just a handful of microns of motion can ruin your images.

personally i have been battling flexure forever and despite all kinds of upgrades, just was never able to get round stars without an OAG. it leaves me wondering how people can guide 1000-2000mm scopes with a finder, and just what the heck is wrong with my setup.


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korborh
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: pfile]
      #5481909 - 10/21/12 12:58 PM

I too struggled with flexure - and wasted a lot of time and clear skies. Went with OAG and no more flexure. Flexure is especially evident in mirror based scopes, much less in refractors.
I have yet to see tightly guided long FL (2000mm+) images using a finder guider.


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pfile
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5481970 - 10/21/12 01:41 PM

if feynman can't do it then i don't feel so bad

i was under the impression that mirror movement/flexure was more of a problem in SCTs, or is that not true on a newer SCT with mirror locking? i am using RC telescopes...


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: pfile]
      #5482115 - 10/21/12 03:18 PM

I'm a little troubled at pfile's post here.
First of all, I remain "cautiously optimistic" my problem is fixed. Tonight should be clear and I'll know.

But I intend on purchasing an AT8RC in the near future. That's allowing they ever really do make any more of the silly things, which I'm increasingly coming to doubt!
The plan is to mount the STi guider on the top Vixen rail.

The alternative is a $995 SBIG OAG-8300. I had hoped I would be spared the extra Grand.

Not to mention the hassle of finding guide stars.

Edited by dawziecat (10/21/12 03:21 PM)


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korborh
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5482514 - 10/21/12 07:01 PM

Dawziecat, I was first introduced to flexure with my AT8RC scope. The mirror is supposedly 'fixed' , but one only need microns of flex to see a difference in long exposures. The AT RC scopes may be optically very good but mechanically not so much especially for the mechanical demands of the RC design. The rear focuser assembly is attached to the primary mirror !!! Not only does that increase flexure but also collimation change based on where you are pointing. The difficulty in collimating the AT RC should not be taken lightly. It is very easy to screw up and extremely hard to get right afterward.

You will be better off buying the EdgeHD 8" with an OAG. Large flat field, very easy to collimate.


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pfile
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5482960 - 10/22/12 12:44 AM

interestingly, even with the OAG i am seeing a small amount of flexure, and this is on an AT6RC with a TRF-2008 (fl = 1100mm) riding on a G11/Gemini-2, with a moonlite focuser.

with a lodestar as the guide camera it's not been too hard to find guidestars. i think the ST-i is of comparable sensitivity.

at 1100mm i'm able to get mostly-round stars at up to 20 minutes, though recently i have been doing 7 minute exposures. the last 2 nights i managed a total of 42 7-minute subs without having to throw one away, which is some kind of miracle.

i have an AT10RC sitting here in the box which i tried to use a couple of times with a 12x80 finder (~330mm fl) and a meade DSI. guider scale ~4.7arcsec/px. was never able to escape flexure. have not tried the OAG on the AT10 yet but i feel that i will probably have some elongation at the 7min subexposure length.


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: pfile]
      #5483211 - 10/22/12 08:14 AM

This thread has taken an unexpected, but illuminating and somewhat alarming turn.

First off, last night was not a success. I was wrong. The problem is still with me. It does appear that, once in a while anyway, I do get acceptable subs on the merdian. But far more often I do not. Again, this is at a mere 135mm FL for heaven's sake!

I certainly never expected to see a recommendation of an HD Celestron over an AT8RC for AP!

The 2030mm FL and f/10.0 FR seems the last thing I need too.
I see Optec are now offering .62 compressors for the HD line.

Still flex with an OAG on an AT6RC? How can that be unless the guider is not doing its job?

I am so gun-shy now I am reluctant to "throw good money after bad" until I can ascertain what is going on to cause this tracking at the meridian problem.

It's time to take the STi guide scope off and give it a good shake to see if there is a loose element rattling about in there.

I've been battling this problem for months and have determined precisely nothing about what is causing it.

It appears I may just have to buy a small imaging refractor and try it out. Not really how I wanted to spend my money.

Edited by dawziecat (10/22/12 08:15 AM)


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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5483380 - 10/22/12 10:38 AM

May I suggest something to help you trouble shoot. Try the 135 mm lens without guiding. That should tell you if the guide scope is the problem. I image at 360mm without guiding on my G11 and I have routeanly managed 5 or more minutes unguided

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lawrie
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5483405 - 10/22/12 10:52 AM

Hi Terry,
I have the same issue, my set up is a bit heavier than yours.
I have a 9.25 edge/hyperstar using a st8300c, I have a 66mm piggybacked on this using a DSI.
I was thinking more along the balance lines, specifially from the cables, everything else seems tight, but that amount of elongation is from some very tiny movement or restriction.


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pfile
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5483554 - 10/22/12 12:21 PM

Quote:


Still flex with an OAG on an AT6RC? How can that be unless the guider is not doing its job?





sorry for the threadjack... all i know is that there is a tiny amount of drift in both RA and DEC when guiding that setup. it's not really terribly significant, something like a 4.7um pixel in 20 minutes. but it's there. after all, the OAG/guide camera could sag slightly on it's stalk.


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blueman
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5483741 - 10/22/12 02:13 PM

Hi, I too had an AT8RC and I used it with a G-11. I fought flexure from day one for about a year and never really was able to eliminate it. Finally I sold the AT8RC and went back to refractors and there was no flexure, even though I am using the same guide system. I sold the G-11 and now use an AP900, but still use the same guide setup.

I tried OAG and it did help, but I did not like using it, I prefered the guide scope.

It could be that you have a balance issue that come in at the meridian due to the shift from East to West. But I think you are dealing with flexure in the AT8RC. As the other fellow said, the mechanics are just not good enough for a heavier camera and there appears to be a lack of stiffness in the rear end of the scope. It is either that or the mirror is moving internally.
Blueman
Quote:

This thread has taken an unexpected, but illuminating and somewhat alarming turn.

First off, last night was not a success. I was wrong. The problem is still with me. It does appear that, once in a while anyway, I do get acceptable subs on the merdian. But far more often I do not. Again, this is at a mere 135mm FL for heaven's sake!

I certainly never expected to see a recommendation of an HD Celestron over an AT8RC for AP!

The 2030mm FL and f/10.0 FR seems the last thing I need too.
I see Optec are now offering .62 compressors for the HD line.

Still flex with an OAG on an AT6RC? How can that be unless the guider is not doing its job?

I am so gun-shy now I am reluctant to "throw good money after bad" until I can ascertain what is going on to cause this tracking at the meridian problem.

It's time to take the STi guide scope off and give it a good shake to see if there is a loose element rattling about in there.

I've been battling this problem for months and have determined precisely nothing about what is causing it.

It appears I may just have to buy a small imaging refractor and try it out. Not really how I wanted to spend my money.




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pfile
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: blueman]
      #5484141 - 10/22/12 06:43 PM

blueman, his current setup is an EOS lens attached to a CCD camera. the focal length is pretty short which means if it is flexure it's got to be really, really bad due to the large image scale. that's the puzzlement.

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blueman
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: pfile]
      #5484204 - 10/22/12 07:26 PM

Hi,
Well, the fact that it has this at Meridian is a real clue to me. This has to have something to do with balance. This is the point where the weight shifts from East to West. If the weight is balanced, well then this is a very big problem transition.
Blueman


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Hilmi]
      #5484223 - 10/22/12 07:34 PM

Quote:

May I suggest something to help you trouble shoot. Try the 135 mm lens without guiding. That should tell you if the guide scope is the problem. I image at 360mm without guiding on my G11 and I have routeanly managed 5 or more minutes unguided




Hi Hilmi:

Not sure what this really would prove other than my alignment is so-so?

I've had "accidental" unguided subs. The results are not pretty when that happens.

I've bolted pieces of metal to the dovetail and relocated the STi and guide scope so that they are now alongside, and very close to the taking lens. So, I need another test.

There does not appear to be anything obviously loose in the guide camera or guide scope assembly. No rattles.


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pfile
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5484471 - 10/22/12 10:56 PM

if polar alignment is nailed, and there is no drift during unguided exposures, then drift during guided exposures = differential flexure.

if you have the exact same elongation with guiding off, then there's something else wrong.


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gdd
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: blueman]
      #5484527 - 10/22/12 11:36 PM

Quote:

Well, the fact that it has this at Meridian is a real clue to me. This has to have something to do with balance. This is the point where the weight shifts from East to West. If the weight is balanced, well then this is a very big problem transition.





If it has to do with balance near the meridian, Rainier has some posts on the Yahoo Losmandy_Users forum about using a cord and a small weight to ensure the balance remains east heavy when tracking or guiding past the meridian. I recall he also posted some pictures either in the photos or the files section.

Gale

(changed "small wait" to "small weight")

Edited by gdd (10/23/12 12:47 AM)


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gdd
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5484532 - 10/22/12 11:41 PM

Quote:

It appears I may just have to buy a small imaging refractor and try it out. Not really how I wanted to spend my money.





Or maybe just a cheap fast refractor like the ST80 or a finder scope with a standard focuser. If you have no problem with this, thenn perhaps the problem is internal shifting of components within the telephoto lens.

Gale


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blueman
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: gdd]
      #5484578 - 10/23/12 12:20 AM

I made one of the string weights and it worked quite well. It lets you just balance the scope and then use it for the East bias.
Blueman
Quote:

Quote:

Well, the fact that it has this at Meridian is a real clue to me. This has to have something to do with balance. This is the point where the weight shifts from East to West. If the weight is balanced, well then this is a very big problem transition.





If it has to do with balance near the meridian, Rainier has some posts on the Yahoo Losmandy_Users forum about using a cord and a small wait to ensure the balance remains east heavy when tracking or guiding past the meridian. I recall he also posted some pictures either in the photos or the files section.

Gale




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SMigol
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: blueman]
      #5484692 - 10/23/12 01:31 AM

Here's a different way of thinking...

Since the subject is short focal length with very long subs (mention of 30 minutes for example) with a small guide scope, I began thinking of field rotation around a guide star.

I have a spreadsheet that I use to help me understand if my polar alignment is good enough to eliminate field rotation. Based on some assumptions (Kodak 8300 chip, 135mm lens, Crescent Nebula for a target, guide target in the middle of the field of view, and 30 minute sub duration) I see that the calculations to ensure no visible field rotation across an 8 degree wide view requires polar alignment to be within 10 arc minutes of true north.

If polar alignment is off, guiding will introduce field rotation, but this would be seen as a rotation on the whole field. So far, we've seen ruined subs where there is some diagonal movement, but is this movement the same length across the whole frame? Guiding at a point significantly far from the center of the main imaging scope could set up what appears as drifting across the whole frame.

What I'm not sure about would be how this might appear at different placements away from the meridian.


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korborh
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: SMigol]
      #5484948 - 10/23/12 09:43 AM

Quote:

What I'm not sure about would be how this might appear at different placements away from the meridian.




Yes....field rotation would not explain that. I can only think of flexure or something not tight.


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SMigol
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5485050 - 10/23/12 10:59 AM

Or lens elements shifting...

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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: SMigol]
      #5485313 - 10/23/12 01:42 PM

Again, thanks all for the ideas. After battling this for months I need fresh viewpoints.

I went from initially thinking the lenses and taking camera were indeed flexing and needed better support. I feel I have done that in spades!

Then I jumped on blaming the mount. Something wrong, loose or dirty gears or something off-centered causing binding or whatever. Was always doubtful about it though as the guider seems not to see it. Nothing much ever shows in the guiding curves. I have not successfully analyzed the guiding logs in detail but a cursory examination of them shows nothing to my eye either. I have diassambled the mount, re lubed much of it and checked gear meshing. All seems well with it.

My Alignment is not something I'd want to be judged on.
I have been more meticulous in the past and drift aligned but never with a anything more powerful than a camera lens. I use the Losmandy polar alignment scope but it has been adjusted as well as it can be. I have verified its accuracy to be within 20 arc minutes. I feel pretty sure my alignment is no more than that off and likely it is less.
It is good enough for good 30 minute subs at FLs up to 300mm most of the time and to 600mm at 15 minutes. So, I don't think this is field rotation. In any event, why would field rotation manifest itself to a more objectionable degree as the meridian or, I am starting to feel more precisely, the zenith is approached?

This problem, localized to the zenith area seems unrelated to alignment or consequent field rotation.

There has been a recent development as stated in my last post. I relocated the STi Guide camera and guide scope on the dovetail. They are no longer up at the tip of the dovetail as in these pictures. I cobbled together a bracket that would make Rube Goldberg cringe in embarrassment. They are now located down by the camera lens and CCD camera on short pieces of 1/4 inch steel bolted to the dovetail.

I tried this out last night and got all good 10-minute subs in a four hour run from 45 degrees east of the meridian to 15 degrees west of it. This includes about a half dozen subs through the "danger zone" and all of them were good.

I hope to verify this success tonight and will try longer NB subs.

Again thanks to all for their ideas. I needed the input.


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gdd
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5485545 - 10/23/12 04:02 PM

Quote:

It is good enough for good 30 minute subs at FLs up to 300mm most of the time and to 600mm at 15 minutes. So, I don't think this is field rotation. In any event, why would field rotation manifest itself to a more objectionable degree as the meridian or, I am starting to feel more precisely, the zenith is approached




Field rotation does not depend on focal length. Given the same polar misalignment and exposure time, an image taken at 18mm will show the same field rotation as ones taken at 135mm and 600mm.

Gale


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SMigol
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: gdd]
      #5485559 - 10/23/12 04:09 PM


Quote:


Field rotation does not depend on focal length. Given the same polar misalignment and exposure time, an image taken at 18mm will show the same field rotation as ones taken at 135mm and 600mm.

Gale




True. Still, the imaging resolution (in arcseconds/pixel) determines if this error is visible.


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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: SMigol]
      #5485704 - 10/23/12 05:21 PM

good news. so it was DF then, caused by a long lever arm for the guider?

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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: pfile]
      #5486351 - 10/24/12 01:11 AM

If I was making a movie about a mad scientist working out of home, I'd use your setup as a background prop. I think you need to talk to ADM to get you a nice sleek looking set up that is probably much stiffer than you'r current arrangement.

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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: pfile]
      #5487171 - 10/24/12 03:26 PM

Pretty long post. Summary: Better but still a problem.

Results from last night:
First up was a short test run imaging IC1396 across the meridian with 15 min NB subs. The very first was was bad. Not so bad as previously. Definitely better but not a keeper. The next three, as it crossed the meridian were good.

Well three out of four ain't bad. Far better than what I have been getting.

The definitive test followed. A run of fifteen 20-minute subs across an arc of about 75 degrees of sky culminating 15 degrees west of the meridian.

The last good exposure ended with the target still 3 degrees from the meridian. The next, encompassing the arc from 3 degrees west of the meridian to 2 degrees east, i.e., actual meridian crossing, was bad. Then a good one followed by two more bad and then another good one.

While I am happy that things do indeed seem far better than they were, I am more perplexed than ever at this seeming new variability.

Likely this behaviour would go entirely unnoticed with sub lengths of five minutes. Perhaps even 10 minutes.

Hilmi:
Throwing money at custom hardware is way premature without knowing where this problem is occurring.
I would like to try a set of non-adjustable rings with flat tops and mount the guider to the tops as is often done. Far easier with 'scopes of uniform tube diameter than irregularly shaped camera lenses though. That would be an expensive custom job with each lens requiring two entirely different ring sizes. I don't think adjustable rings would be wise either.

I think an FSQ might be cheaper than having such custom hardware manufactured for my several lenses, a couple of them very large and heavy!

Don't be fooled by the "inelegance." The whole front-of-lens, pipe clamp, wood block appears now to be entirely unnecessary. They were added in desperation to try and localize the problem. Others image successfully with the EF200mm f/2.8L, a slightly heavier and longer lens, with no front support whatsoever.

There have been suggestions of lens elements shifting. Anyone who has focused one of these short FL camera lenses on the night sky appreciates just how fine an adjustment must be made to get them in focus. There is just no way in hades an element could slip without focus going out the window! The lenses, this same thing happens with more than one and perhaps all my lenses, show no focus shift at the meridian when "the wheels fall off."

These tests have convinced me it is indeed flex I am dealing with though. Well, pretty much anyway. I am still unsure of precisely what is flexing, why it ONLY happens so precisely at the zenith and still flabbergasted I am having all this difficulty at such silly-short focal lengths!

And, again, only, and almost precisely, at the meridian/zenith . . . no where else. Perfectly good 20-minute subs all night long elsewhere. It's as if something goes "sproing" at the zenith. . . not a continuous, smooth flexing at all. It really still doesn't make sense to me. The only thing I have not thoroughly checked is a dec imbalance as the lens goes vertical. But the guiding curve should show that. It doesn't.

Aside on the EF135mm f/2.0L:
My run on Simeis 147 is done. It's time to go back to a lens that has higher image quality and better focusing adjustment than this EF135mm f/2.0L. This lens has a poor reputation amongst the small cadre of APers using EOS lenses. It is well deserved in my experience. I needed the FOV for Simeis 147 on the ST-8300M but don't recommend this lens to anyone for AP unless you are prepared to stop it well down. How much depends on your sensor size and how important the corners may be to your framing.
Even at the small sensor size of the ST-8300M, image quality in the corners at f/2.8 is just absolutely terrible! But I needed the speed for Simeis 147. I can't image how bad this lens would appear in the corners on a full frame sensor. And at f/2.0? As they are purported to say in NYC . . . "Fuhgeddaboudit!"


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Footbag
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5487486 - 10/24/12 06:57 PM

How about your cables, particularly on the guider? Are they fastened down? What is the barrel I see? Could that be shifting? If the back of the guide camera gets pulled one way or another it can also introduce flexure.

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pfile
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Footbag]
      #5487715 - 10/24/12 09:50 PM

i still think you should try to really dial in the polar alignment and do some unguided exposures through the trouble area to see what happens. this should help prove that it is flexure and not something weird with the mount. at a short FL it should not be super hard to get the alignment to be good enough.

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Dan Finnerty
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5487838 - 10/24/12 11:26 PM

I just browsed through these posts, so perhaps I missed something, but I don't recall anyone questioning the EOS lens bayonet interface to the camera. I know even on my 5D2 and T3i, there is some movement if I try flexing the lens-camera junction. I can imagine the lens rocking from one side to the other as it crosses through zenith. If it is pointed north or south of zenith, perhaps it is already stably tilted in a downward direction, but only shifts when crossing through zenith.

I know you are using an ST-8300, but it must have a bayonet adapter for the EOS lens mount, and perhaps there is some flexure there. If it seems even slightly loose, perhaps try putting three pieces of removable (not permanent) tape evenly spaced around the edge of the adapter to tighten up the mounting (avoiding the electrical contacts obviously).

No idea if this helps, but so far nothing suggested has nailed the problem. Good luck!


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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Dan Finnerty]
      #5488567 - 10/25/12 12:35 PM

this is true, many of the canon bayonet adapters on my T-rings fit way too loosely. i had to shim the front of the camera with scotch tape as you suggest.

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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Dan Finnerty]
      #5488608 - 10/25/12 01:06 PM

Again, thanks for the replies.

My ultra-wide field projects being done for the time being, I have dismounted the EF135mm f/2.0L EOS lens and remounted the EF300mm f/2.8L. Those familiar with the Canon EOS lens series will recognize these two lenses are entirely different in size and weight!

I immediately tried out IC1396 as it tracked across the meridian. A series of 9 fifteen minute Ha subs. All were fine.

There is a big but though . . .

I then imaged Sh2-205, well off the meridian with the 300mm and . . . problems.

So, this is a mess IMO. On the one hand I can not fathom how the small lens was flexing or why relocating the guider seemed to fix the problem with it completely. That seems illogical on the face of it.

Dan mentions the EOS adapter. Now this is a source of concern when imaging with these lenses. However, the manner in which the small lens was mounted, with support for both the camera and the lens, thoroughly immobilized that adapter joint. So much was this the case, that I could not readily release the lens from the camera without undoing the bolts holding both camera and lens to the dovetail beforehand. That wooden block, meant to stop any possible downward sag of the front of the lens, actually is a tight fit. It, ever so slightly, wedges the lens up against the adapter joint, making that joint very tight.

So, problems only at the meridian with two small lenses and problems only off the meridian with a much larger one. The method of mounting the large and the short FL lenses is entirely different.
Sure I have been imaging with these lenses for 18 months or so and had some pretty good results. Much of that was with a DSLR rather than a heavy mono CCD with 8 position filter wheel stressing a third-party EOS adapter joint. And I have been doing far more in the way of NB too. So my exposures have increasingly been longer of late, making this problem far more evident.

I see no realistic way out of this. There is no off-the-shelf mounting hardware to rigidly support these lenses and, simultaneously, support the camera as well. Unless both are supported, that EOS adapter joint will always be a concern, as Dan points out.

So, I am at an impasse. The lenses are good, but doing long subs with them demands a complex mounting system that is just not available and I fear would be far too costly to have a machinist custom fabricate from scratch.

It seems I will have to be satisfied with either losing a lot of long subs to flexure or limiting my exposure duration and relying on more numerous but shorter subs.

I'd sure like to hear from other EOS imagers and if any have successfully and consistently achieved good 15 to 30 minute subs with EOS lenses ranging from 135mm to 600mm in FL.

I have done it . . . but certainly not consistently.

While I see no solution, I do feel I now know where the problem is and think both my G11 and STi guide kit have a clean bill of health. I could put my lenses on an AP3600 and have the exact same problem!

Again thanks to all for their suggestions. It's been a long, drawn out, soap opera-like saga.

PS: I'll try the tape shim in that joint with the 300mm. Great suggestion!

Edited by dawziecat (10/25/12 01:13 PM)


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korborh
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5488617 - 10/25/12 01:14 PM

Your solution is OAG.

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psu_13
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5488656 - 10/25/12 01:42 PM

I don't mean to be combative, but how exactly would you hook up an OAG to an SLR camera lens?

Anyway, it does seem like lighter cameras or shorter exposures are things to investigate here. These are obviously tradeoffs that you might not want to make.


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5488770 - 10/25/12 02:44 PM

Quote:

Your solution is OAG.




I'm afraid the short available back focus of these camera lenses precludes OAG.

Case in point:

SBIG make an EOS adapter specifically for their 8300 series cameras.
They also make the OAG8300.
But there is insufficient back focus to use both of these simultaneously.

So, not only do SBIG not do it, I presume the reason they don't is because it simply it can't be done.

Pity. .


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terry59
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5488858 - 10/25/12 03:44 PM

Quote:

Sure I have been imaging with these lenses for 18 months or so and had some pretty good results. Much of that was with a DSLR rather than a heavy mono CCD with 8 position filter wheel stressing a third-party EOS adapter joint.




Terry - Have you cosidered doing without the filter wheel and using single filters? I plan to do that with my Atik 314L+, Geoptik adapter and Nikkor glass.


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: terry59]
      #5488894 - 10/25/12 04:13 PM

Quote:


Terry - Have you cosidered doing without the filter wheel and using single filters? I plan to do that with my Atik 314L+, Geoptik adapter and Nikkor glass.




Hi Terry:
No, I have not considered that. I would have to get an additional spacer from SBIG to do so. Not sure how I would use the filters then though. Think that spacer is really just meant for using their OSC ST-8300 cameras with EOS lenses.
It's not really an attractive option, but thanks.


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blueman
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5489118 - 10/25/12 07:18 PM

Here is how I look at these problems.
The G-11 has a 4 minute worm period. So, if it can do 4 minutes with good results, then it is not likely the worm that is the issue, especially if you can do 8-10 minutes. Then the worm is just not going to be the problem.

So, if the worm is not the issue then the imaging train or the balance is the issue. Of course polar alignment is a problem, but not just at the Meridian.

This leaves flexure or balance. I think you have seen that it is likely flexure and I would agree that is probably what it is too.

The problem is, the flexure only has to be .0015" to give a problem. This is HARD to measure to be honest, but it can be there when things seem solid.

The best way to mount the guide scope is to have it SOLID to the dovetail with a dovetail clamp and use solid clamshell rings to hold the guide scope. Then the guide camera should be screwed to the scope, even the focuser or compression ring can have small flexure that is hard to measure.

Finally, the image scope and camera (lens and camera) must also be mounted to the dovetail SOLIDLY with no flexible materials or clamps.

I found flexure for 3 years and I did not get away from it until I did what I just said. Everything screws together in my system, no compression rings. Clamshell rings are used and tightly clamped and those are bolted directly to the dovetail or dovetail clamp.

It can be very frustrating trying to find flexure.
Blueman


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gdd
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: blueman]
      #5489452 - 10/25/12 11:10 PM

Quote:

The best way to mount the guide scope is to have it SOLID to the dovetail with a dovetail clamp and use solid clamshell rings to hold the guide scope.




A pair of 3-point rings not solid enough? It does seem there could be some flexing or slipping on the screws.

Gale


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blueman
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: gdd]
      #5489497 - 10/25/12 11:36 PM

No, I tried 3 point rings and there was flexure. But, if you are using a very small guide scope, 50-60mm and a light camera, maybe you could use 3 point rings.
Blueman
Quote:

Quote:

The best way to mount the guide scope is to have it SOLID to the dovetail with a dovetail clamp and use solid clamshell rings to hold the guide scope.




A pair of 3-point rings not solid enough? It does seem there could be some flexing or slipping on the screws.

Gale




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Dan Finnerty
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: dawziecat]
      #5489621 - 10/26/12 01:26 AM

What a frustrating problem. Makes me want to tear my hair out just reading about your efforts!

You mentioned the filter wheel which got me thinking. How firmly are the filters mounted in the wheel? In theory, if they shift side to side there would be no visible effect, but what if some axial tilt were involved? That could cause a small image shift as you cross the meridian...


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dawziecat
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Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: Dan Finnerty]
      #5489707 - 10/26/12 05:18 AM

Quote:

What a frustrating problem. Makes me want to tear my hair out just reading about your efforts!





With the problem having moved to off the meridian with the EF300mm f/2.8L IS, it's starting to feel like playing "Whac-A-Mole!"

There was a day when I thought it might be the filters in the wheel. I could hear a clicking sound as I tilted the lens/camera assembly back and forth and it seemed to be coming from the filter wheel. It was actually the element in a UV "protective" filter on the Canon EF135mm f/2.0L though. The glass filter is pretty loose in its ring. The motion of the glass was strictly parallel to the lens front element though. Unlikely a factor. I removed it anyway. Made no difference.

Now that I have changed lenses, and the problem occurs not near the zenith, but off the meridian entirely, pretty much exonerates the camera and wheel assembly.

I am going to try to mount the small STi guider assembly directly onto the taking Canon lens. The 600mm f/2.8 lens is big enough that I should be able to do that. I'll try the 300 f/2.8 first. Not sure I'll be able to do this successfully but it's worth a try. I may just introduce even more flex. But if I am able to verify attaching the guider directly, piggyback-style, onto the large lenses actually works, even as a test with stuff cobbled together, I might be able to get some hardware together to do it properly. It won't do for small lenses but if I can image better with the big 300mm and 600mm lenses I'll be happy.

I again imaged IC1396 across the meridian this evening with the 300mm. Quite successful with 15-minute subs. But I cut back to 5 minutes on The Jellyfish off the meridian.

Five minute Ha subs are certainly not the theoretical ideal. Each sub looks pretty noisy. But, stack three times as many frames up as you get with 15-minute subs and they look pretty good. For the moment I have to console myself with that. It works well at f/2.8. Don't think I'd want to try it at f/7 or f/8 though.

Thanks for the suggestion about the filter wheel but I don't think that's it.


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dawziecat
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Meaning of this drift during a run? new [Re: dawziecat]
      #5490042 - 10/26/12 11:20 AM Attachment (14 downloads)

Last night I imaged The Jellyfish for 5 hours straight.
Attached is a jpg showing the first and the last subs taken during this run superimposed on each other in PS with opacity at 50%.

Could someone interpret the meaning of this drift over 5 hours for me.


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Peter in Reno
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Re: Meaning of this drift during a run? new [Re: dawziecat]
      #5490087 - 10/26/12 11:52 AM

Can you stack all subs WITHOUT alignment? This will tell you whether it's flexure or bad polar alignment. If the star trails are arced (curved), that means bad polar alignment. If the star trails are straight, wiggly or zig-zag, then it means flexure.

I am assuming you didn't dither the images during capture.

From your image, it looks a bit arced to me meaning it might be mis-polar alignment.

Peter


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gdd
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Re: Meaning of this drift during a run? new [Re: dawziecat]
      #5490178 - 10/26/12 12:52 PM

Quote:

Last night I imaged The Jellyfish for 5 hours straight.





Isn't 5 hours a long time to expect the mount to track perfectly? Even if you drift align you are ensuring no star trails for only 5-20 minutes or so. Michael Covington in "Astrophotograpy for the Amateur" explains that a misalignment can result in an fast/slow tracking rate and/or field rotation depending on the direction of the misalignment. However guiding should have corrected the fast/slow tracking rate problem.

Gale


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blueman
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Re: Meaning of this drift during a run? new [Re: gdd]
      #5490248 - 10/26/12 01:30 PM

I can image for 9 huors and other than the movement caused by dithering, there is no movement between the subs, even after the meridian flip.
So, you can track for many hours without drift.
Blueman
Quote:

Quote:

Last night I imaged The Jellyfish for 5 hours straight.





Isn't 5 hours a long time to expect the mount to track perfectly? Even if you drift align you are ensuring no star trails for only 5-20 minutes or so. Michael Covington in "Astrophotograpy for the Amateur" explains that a misalignment can result in an fast/slow tracking rate and/or field rotation depending on the direction of the misalignment. However guiding should have corrected the fast/slow tracking rate problem.

Gale




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gdd
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Re: Meaning of this drift during a run? new [Re: blueman]
      #5490268 - 10/26/12 01:43 PM

Quote:

I can image for 9 huors and other than the movement caused by dithering, there is no movement between the subs, even after the meridian flip.
So, you can track for many hours without drift.





What is "dithering"?

So 9 hours of tracking (without guiding)can be done with a careful drift alignment.

Gale


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Peter in Reno
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Re: Meaning of this drift during a run? new [Re: gdd]
      #5490281 - 10/26/12 01:49 PM

Please look at this for dithering:

http://www.hiddenloft.com/notes/dithering.htm

Peter


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Peter in Reno
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Re: Meaning of this drift during a run? new [Re: gdd]
      #5490304 - 10/26/12 02:03 PM

Quote:

So 9 hours of tracking (without guiding)can be done with a careful drift alignment.




I am not sure if this is possible without modeling. The stars move at different rates at different parts of the sky due to refraction. Stars track at sideral rate at the Meridian. So without modeling the sky with modeling software like The Sky, I am not sure it is possible to track perfectly even with perfect polar alignment.

For example in my case. One night before the sky got dark, I polar aligned my mount and slew the mount to a bright star near target DSO for imaging. I centered the star with my imaging camera and the sky was still not dark. I went inside the house to watch TV until the sky got dark. 30 minutes later I went outside to check if the star is still centered of my camera. It drifted in RA direction. I wondered why it drifted in RA and later I found out about refraction.

I believe autoguiding is necessary even with perfect polar alignment and without modeling.

Peter


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dawziecat
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Re: Meaning of this drift during a run? new [Re: Peter in Reno]
      #5490310 - 10/26/12 02:06 PM Attachment (11 downloads)

Quote:

Can you stack all subs WITHOUT alignment?

Peter




Attached, stacked without alignment. No dithering.


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Peter in Reno
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Re: Meaning of this drift during a run? new [Re: dawziecat]
      #5490334 - 10/26/12 02:24 PM

Thanks for creating a new image. The star trails look straight and it looks like flexure to me.

This setup might be more stable:

http://www.teleskop-express.de/shop/product_info.php/info/p2836_Geoptik-T2-Adaptor-for-CANON-EOS-Lens-with-1-4--Phototripod.html

Peter


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SMigol
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Re: Meaning of this drift during a run? new [Re: dawziecat]
      #5490406 - 10/26/12 03:08 PM

Very interesting that the lower left has nearly no movement but the rest of the frame features arcs around that lower point. Was your guide star in the lower left? If so, I'm still thinking field rotation which is polar alignment.

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Peter in Reno
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Re: Meaning of this drift during a run? new [Re: SMigol]
      #5490472 - 10/26/12 03:56 PM

Yeah, you are correct. I had a hard time seeing the image since it's highly compressed. Now you mention it, I do notice stars are longer and slightly curved relative to lower left. It does look like field rotation.

Try align guide scope with main scope so the same guide star is at the center in both guide and main scopes. Assuming your mount is well polar aligned, choosing a guide star away from target DSO can artificially mimic bad polar alignment. Once both guide and main scopes are aligned, pick a guide star right at the center.

Peter


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dawziecat
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Re: Meaning of this drift during a run? new [Re: Peter in Reno]
      #5490560 - 10/26/12 05:08 PM

I have mounted the guider onto the Canon taking lens. I hope rigidly enough to preclude flex but it's hard to feel certain about that. If I get good 15 minute subs over a wide swathe of sky, it will be absolute proof of flex to my mind.

It it doesn't, it might just mean I was not able to attach the guider to the lens with sufficient rigidity, in which case, nothing will be proved.

I will also check the alignment by turning off dec guiding and seeing what PHD indicates. I have done that in the past and there was no dec deviation in a 10 minute interval. That was some time ago though and the mount head has been removed and replaced since although the tripod was not moved.


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Peter in Reno
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Re: Meaning of this drift during a run? new [Re: dawziecat]
      #5490582 - 10/26/12 05:21 PM

If you are responding to my latest post, I was not implying flexure. I was incorrect the first time I saw your latest image without alignment. Your polar alignment may be fine but if the guide star is quite a bit away from the center of your main camera, it could artificially mimic bad polar alignment. I mentioned to align (or sync) your guide scope with main scope and try to find a guide star at dead center of guide scope. This same guide star should also be dead center of main scope as well.

Peter


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blueman
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Re: Meaning of this drift during a run? new [Re: Peter in Reno]
      #5490627 - 10/26/12 05:54 PM

Oh, maybe I misunderstood the question?
Yes, I guide, but I thought it was being stated that with guiding you will still get drift.
Blueman
Quote:

Quote:

So 9 hours of tracking (without guiding)can be done with a careful drift alignment.




I am not sure if this is possible without modeling. The stars move at different rates at different parts of the sky due to refraction. Stars track at sideral rate at the Meridian. So without modeling the sky with modeling software like The Sky, I am not sure it is possible to track perfectly even with perfect polar alignment.

For example in my case. One night before the sky got dark, I polar aligned my mount and slew the mount to a bright star near target DSO for imaging. I centered the star with my imaging camera and the sky was still not dark. I went inside the house to watch TV until the sky got dark. 30 minutes later I went outside to check if the star is still centered of my camera. It drifted in RA direction. I wondered why it drifted in RA and later I found out about refraction.

I believe autoguiding is necessary even with perfect polar alignment and without modeling.

Peter




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Peter in Reno
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Re: Meaning of this drift during a run? new [Re: blueman]
      #5490654 - 10/26/12 06:10 PM

Yes, but it's not yet clear what caused the drift during guiding. That's what this thread is about.

Other stated that with perfect polar alignment you can image for 9 hours unguided and I assumed that modeling of the sky was not included so I don't think it's possible due to refraction in the sky.

Peter


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blueman
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Re: Meaning of this drift during a run? new [Re: Peter in Reno]
      #5490903 - 10/26/12 09:33 PM

I do not use a model, I polar align and synch to one star and the AP-900 is ready to take 9 hours of images with no decernable drift.
Blueman
Quote:

Yes, but it's not yet clear what caused the drift during guiding. That's what this thread is about.

Other stated that with perfect polar alignment you can image for 9 hours unguided and I assumed that modeling of the sky was not included so I don't think it's possible due to refraction in the sky.

Peter




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Peter in Reno
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Re: Meaning of this drift during a run? new [Re: blueman]
      #5490939 - 10/26/12 10:03 PM

That's probably because you always use autoguiding to correct any refraction. I also have A-P mount and do the same thing you do. Without modeling and perfect polar alignment, I don't think I can trust unguided imaging for 9 hours unsupervised.

Peter


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blueman
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Re: Meaning of this drift during a run? new [Re: Peter in Reno]
      #5491000 - 10/26/12 10:51 PM

As I stated twice, I do not do unguided 9 hour images. I was under the impression that the question was, can you guide for 9 hours without drift.
Blueman
Quote:

That's probably because you always use autoguiding to correct any refraction. I also have A-P mount and do the same thing you do. Without modeling and perfect polar alignment, I don't think I can trust unguided imaging for 9 hours unsupervised.

Peter




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