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Equipment Discussions >> Mounts

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blueman
Photon Catcher
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Reged: 07/20/07

Loc: California
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
Post Laureate


Reged: 06/14/09

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
Photon Catcher
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Reged: 07/20/07

Loc: California
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
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
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
Post Laureate


Reged: 06/14/09

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
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
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
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
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
Photon Catcher
*****

Reged: 07/20/07

Loc: California
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
sage


Reged: 07/30/10

Loc: California, USA
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
professor emeritus
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Reged: 01/29/11

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
sage


Reged: 07/30/10

Loc: California, USA
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
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
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
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
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
sage


Reged: 07/30/10

Loc: California, USA
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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pfile
Post Laureate


Reged: 06/14/09

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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Hilmi
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Reged: 03/07/10

Loc: Muscat, Sultanate of Oman
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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dawziecat
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
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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Reged: 04/13/09

Loc: Scranton, PA
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
Post Laureate


Reged: 06/14/09

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
sage


Reged: 09/11/11

Loc: Pasadena, CA
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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