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Equipment Discussions >> Mounts

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pfile
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Reged: 06/14/09

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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dawziecat
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Reged: 10/20/10

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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
scholastic sledgehammer
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Reged: 01/29/11

Loc: Arizona
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
sage


Reged: 05/30/10

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
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
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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Reged: 07/18/11

Loc: Colorado, USA
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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Reged: 10/20/10

Loc: Rural Nova Scotia
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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Reged: 07/20/07

Loc: California
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
Pooh-Bah


Reged: 11/23/05

Loc: N Seattle suburb, WA
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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Reged: 07/20/07

Loc: California
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
sage


Reged: 09/11/11

Loc: Pasadena, CA
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
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
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
Carpal Tunnel
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Reged: 10/20/10

Loc: Rural Nova Scotia
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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Reged: 07/15/08

Loc: Reno, NV
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
Pooh-Bah


Reged: 11/23/05

Loc: N Seattle suburb, WA
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
Photon Catcher
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Reged: 07/20/07

Loc: California
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
Pooh-Bah


Reged: 11/23/05

Loc: N Seattle suburb, WA
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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Reged: 07/15/08

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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
Carpal Tunnel
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Reged: 10/20/10

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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 (12 downloads)

Quote:

Can you stack all subs WITHOUT alignment?

Peter




Attached, stacked without alignment. No dithering.


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