Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Mounts

Pages: 1 | 2 | 3 | 4 | 5 | (show all)
dawziecat
Carpal Tunnel
*****

Reged: 10/20/10

Loc: Rural Nova Scotia
Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN new [Re: korborh]
      #5476778 - 10/18/12 11:31 AM Attachment (26 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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Footbag
Post Laureate
*****

Reged: 04/13/09

Loc: Scranton, PA
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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
korborh
professor emeritus
*****

Reged: 01/29/11

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pfile
Post Laureate


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?


Post Extras: Print Post   Remind Me!   Notify Moderator  
TxStars
Pooh-Bah
*****

Reged: 10/01/05

Loc: Lost In Space
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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Peter in Reno
Postmaster
*****

Reged: 07/15/08

Loc: Reno, NV
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


Post Extras: Print Post   Remind Me!   Notify Moderator  
dawziecat
Carpal Tunnel
*****

Reged: 10/20/10

Loc: Rural Nova Scotia
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Footbag
Post Laureate
*****

Reged: 04/13/09

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

That sounds like good news.

Post Extras: Print Post   Remind Me!   Notify Moderator  
TxStars
Pooh-Bah
*****

Reged: 10/01/05

Loc: Lost In Space
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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
pfile
Post Laureate


Reged: 06/14/09

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
korborh
professor emeritus
*****

Reged: 01/29/11

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pfile
Post Laureate


Reged: 06/14/09

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...


Post Extras: Print Post   Remind Me!   Notify Moderator  
dawziecat
Carpal Tunnel
*****

Reged: 10/20/10

Loc: Rural Nova Scotia
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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
korborh
professor emeritus
*****

Reged: 01/29/11

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pfile
Post Laureate


Reged: 06/14/09

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
dawziecat
Carpal Tunnel
*****

Reged: 10/20/10

Loc: Rural Nova Scotia
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)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Hilmi
Post Laureate
*****

Reged: 03/07/10

Loc: Muscat, Sultanate of Oman
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

Post Extras: Print Post   Remind Me!   Notify Moderator  
lawrie
Carpal Tunnel
*****

Reged: 01/31/06

Loc: Okanagan Valley
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
pfile
Post Laureate


Reged: 06/14/09

Re: G11 Star Elongation: ONLY NEAR THE MERIDIAN [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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | (show all)


Extra information
16 registered and 32 anonymous users are browsing this forum.

Moderator:  Dave M, richard7, bilgebay, iceblaze 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 4708

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics