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Equipment Discussions >> Cats & Casses

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pfile
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Reged: 06/14/09

Re: AT6RC collimation under different loads new [Re: jjongmans]
      #5512104 - 11/09/12 06:21 PM

the camera is 1.8 pounds and the filter wheel is 2.5 pounds, for a total of 4.3 pounds (about 2kg). it probably weighs a tad bit more because there are 6 filters in the wheel.

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Jeff in Austin
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5512269 - 11/09/12 08:28 PM

I'm using an AT12RC and had an AT8RC and didn't have any of the problems described with a 6+ lb load.

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pfile
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Reged: 06/14/09

Re: AT6RC collimation under different loads new [Re: Jeff in Austin]
      #5512271 - 11/09/12 08:29 PM

cool, well the weather is bad but maybe i will put the camera on the AT10 indoors and see what a flat looks like. did you upgrade the focuser on the AT8 or the AT12?

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korborh
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5512365 - 11/09/12 09:51 PM

I had the AT8RC and also attempted to tweak collimation. What you need to know is there are many degrees of freedom for collimation adjustment ( or screw-up) on this scope (1) secondary mirror tilt (2) primary mirror tilt (3) Axial alignment of primary/secondary (4) distance between primary and secondary.

So one can appreciate how hard it will be to re-collimate if it gets messed up. In fact I tried for a week and then gave up, got it professionally collimated and sold it.

Warning - do not make large collimation adjustments to the primary as it can potentially mess up 3 of the 4 alignment requirements above.
So that is why I do not recommend tweaking collimation for the flexure issues or if you are getting good images. These scopes probably need a complex bench setup to bring them back to collimation if gone too far.


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pfile
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Reged: 06/14/09

Re: AT6RC collimation under different loads new [Re: korborh]
      #5512422 - 11/09/12 10:53 PM

in fact i think astro-tech/gso initially recommended to people that they should not try to adjust the primary mirror, ever. but since the problem here seems to be shifting of the primary, it seems like that would be necessary.

i fear that my experience would mirror yours (ha ha) if i tried to collimate the scope, and unfortunately there's no way to find out without trying...


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pfile
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Reged: 06/14/09

Re: AT6RC collimation under different loads new [Re: pfile]
      #5512588 - 11/10/12 01:13 AM

well, i can't say for sure my focus is correct, but the flats coming out of the AT10 look a lot more reasonable with the same camera. tomorrow night it might be clear so i'll try it out under the stars.

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jaddbd
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5513480 - 11/10/12 05:36 PM

I just took my 10CF apart and re-assembled as to clean the mirrors so I thought I would chime in.

As mentioned earlier make sure the primary collimation bolts are tight. They hold the entire mirror/baffle assembly to the back plate.

I don’t have any problems with sag with 6+lbs attached to a feather touch and 4 inches of extenders.

Having the focuser coupled with the baffle and primary mirror cell does make collimation a royal pain. The directions that come with the scope (simply lining everything up with the chesire) assume that your scope is “mechanically perfect” (fat chance). After making my chesire view like just like the instructions, when I did a star test it was less than concentric. If you get a good star test after following the instructions you are good to go. Here is what I finally came up with to get things in order:

1. I got a focus pitch adapter and used a laser to insure that the focal plane at the end of the focuser is parallel to the primary mirror. I did this by carefully centering the laser by making sure it did not “draw a circle” on the secondary when you spin the focuser. Then I played with it until it agreed with the chesire view – pointing center and staying there when focuser is rotated... (lots of trail and error – once and you are done though). If you are lucky enough to get one that is sufficiently square, you don’t need to do this. I ended up pitching the focuser about 1/3/ to ½ a millimeter to get it good.

2. Center the chesire view and adjust the secondary as per the instructions. I ignore the outer rings.

3. Star test to look to see if you have concentric rings, if not adjust the primary to get it as concentric as possible (I use a planetary web cam for this). Be careful adjusting the bolts, they don’t have a great deal of play in or out (thread exposure) –don’t make large adjustments on any one screw. You have to loosen the big one first to tighten the little one and visa versa.

4. go back repeat step 2 center the secondary, then star test again, adjust primary. Repeat steps 2 and 3 until they chesire is centered and star test is good. They should get closer each time.

I spent a ridiculous amount of time went I first got the scope trying to get the scope to where I was happy with it. I was dreading taking the mirror out for cleaning as to avoid the nightmare again. But just a couple a quick repeats of steps 2 and 3 and I was back in business after a half hour or so.

John D
Maryland

Edited by jaddbd (11/10/12 05:45 PM)


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Jared
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Re: AT6RC collimation under different loads new [Re: jaddbd]
      #5513517 - 11/10/12 06:25 PM

I had very minor problems with sag on my AT10RCF with a fairly heavy camera and the FeatherTouch 3" focuser. Pretty minor, though--nothing like you are experiencing--and that was with a 35mm chip in an SBIG STL camera with an Astrodon Monster MOAG. Sounds like something isn't tightened down on your sample.

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pfile
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Reged: 06/14/09

Re: AT6RC collimation under different loads new [Re: Jared]
      #5513549 - 11/10/12 06:56 PM

well it's a long thread now but the key thing is that with this train:

AT6RC - 1" extension - moonlite focuser - trf-2008 - canon50d

the collimation is spot on per CCDinspector.

simply going to

AT6RC - 1" ext. - 2" ext - moonlite - FW8G-STT - STT-8300M

yields the plots shown above.

so i suppose something *could* be loose but the canon setup is probably only 2 pounds lighter. no matter the telescope orientation, my FWHM across the field is pretty constant.

anyway the weather looks halfway reasonable so i'll do some tests with the AT10RC tonight.

John D, thanks for the post. as it turns out, Jared actually collimated my 10 and it's pretty close, if not spot on, at least with the 50D hanging off of the focuser. we'll see if that holds with the heavier SBIG package tonight. i own the takahashi collimation scope, but at this point it should be pretty close, so the final tweak would have to be done with a star test or CCDInspector. I'd prefer to use CCDInspector (or a Pixinsight script i discovered) to collimate with the camera in place.

as you say, the bummer is that final tweaks still require adjusting the primary, which then requires the secondary adjustment, etc. etc. it's kind of a mess.


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pfile
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Reged: 06/14/09

Re: AT6RC collimation under different loads new [Re: pfile]
      #5514981 - 11/11/12 07:37 PM

gah, i think the AT10RC is out of collimation with the load as well. when it was collimated, it did not have the camera package on it and in order to get the STT-8300M on there i had to remove the focuser in order to add another extension ring. the focuser is the stock focuser, which is probably not helping anything.

so at this point i guess i have to try collimating, probably with CCDInspector, so i can do it with the camera attached. up for grabs is whether or not i should upgrade the focuser first.

do the shapes of the stars mean anything with respect to how the scope is miscollimated? at this point i'm wondering if i can get away with adjusting the secondary only.


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jaddbd
scholastic sledgehammer


Reged: 05/18/05

Loc: Woodbine, MD
Re: AT6RC collimation under different loads new [Re: pfile]
      #5515125 - 11/11/12 09:35 PM

If the primary is pitched relitive to the secondary (not parallel) you will most likely see an astigmatism in the defocused images... If the dot is centered in the chesire and the defocused image is not symetric - you will probably want to adjust the primary.

Reviews of the stock focuser have been less than favorable for carrying any kind load.... I have never seen one in person. Astronomics and others sell the 10s without a focuser (probably for a reason).

JD


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korborh
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Reged: 01/29/11

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Re: AT6RC collimation under different loads new [Re: jaddbd]
      #5515262 - 11/11/12 11:01 PM

I had the moonlite on my AT8RC and it made things worse due to the added weight and hence flexure/collimation issues. The primary mirror adjustments also become mushy with added weight induced torque.

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jaddbd
scholastic sledgehammer


Reged: 05/18/05

Loc: Woodbine, MD
Re: AT6RC collimation under different loads new [Re: korborh]
      #5515321 - 11/11/12 11:58 PM

Yeah, I agree, the primary adjustment leaves a bit to be desired "loosen one, tighten the other, see where it ends up." I don't really have to much of a sag problem though. My weakest point is where the filter wheel/camera hit the focuser. I might be being helped by the pitch adjustment ring which basically ends up ratcheted tightly to the back plate. The extenders and focusers go on the ring. The ring will not come off matter how hard you try to turn it unless you release the tension on the pitch adjustment. So that might be an added benifit. Also the CF tube might help handle the tork.

JD


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pfile
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Re: AT6RC collimation under different loads new [Re: jaddbd]
      #5515366 - 11/12/12 12:33 AM

I'm away from my computer but I will post an image of the stars in the center of the field with the AT10. i suppose it stands to reason that the primary is now tipped due to the torque of the camera/filters.

at this point I'm wondering if I should throw more money at this (focuser) or give up. seems a bit premature to give up since I have yet to adjust anything...


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jjongmans
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Reged: 02/11/12

Loc: The Netherlands
Re: AT6RC collimation under different loads new [Re: pfile]
      #5515476 - 11/12/12 03:39 AM

I use this ring for adjusting the focuser.
http://www.teleskop-express.de/shop/product_info.php/info/p4043_TS-focuser-co...


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jaddbd
scholastic sledgehammer


Reged: 05/18/05

Loc: Woodbine, MD
Re: AT6RC collimation under different loads new [Re: jjongmans]
      #5515718 - 11/12/12 10:22 AM Attachment (27 downloads)

I have the same deal.

Here is a photo of mine. My original concern was that it would introduce flexure, but it might have the opposite effect. Here is a picture. You can see how it might be creating a brace against the back of the OTA. The blue tape marker is a reference marking the direction to "shim" the focuser to get it square (only took a fraction of a mm).

JD


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jaddbd
scholastic sledgehammer


Reged: 05/18/05

Loc: Woodbine, MD
Re: AT6RC collimation under different loads new [Re: pfile]
      #5515905 - 11/12/12 12:17 PM

Quote:

I'm away from my computer but I will post an image of the stars in the center of the field with the AT10. i suppose it stands to reason that the primary is now tipped due to the torque of the camera/filters.

at this point I'm wondering if I should throw more money at this (focuser) or give up. seems a bit premature to give up since I have yet to adjust anything...




The trade off is the alternative RCs cost more 2x the coin (even after you add a permium focuser), so to a certain extent you get what you pay for. It IS difficult to set up if you are picky about collimation, but I was able to get mine to "good enough".

My only other bone to pick with the design is the inability to easily adjust the spacing (you have to push/pull the collimating/center screws on the secondary).

Good news for me is that once I do get it to a satisfactory point, it does not drift much over time. I had not touched mine for more than a very minor tweak in a year before taking it apart the other day.

The glass in mine is decent, shows a little over-correction in the star test (I lowered the secondary a bit to compensate), but I am able to get round stars and FWHM values corner to corner in the low 2s minimum/mid 2s average when the seeing is "good" (not a consistant event in the mid-atlantic) with 15min subs (granted my chip is only 1200 x 1600). When the seeing is not so good, FWHM comes in around 3 to low 3s. So despite it's ergonomic shortcomings, I still think its good bang for the buck, but you might have to get your hands dirty.

I got this scope to for a desired image scale and to get away from all the headaches that imaging with a SCT presented; and although not perfect, it has done that.

My experience and MHO, your milage might vary.

JD


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pfile
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Reged: 06/14/09

Re: AT6RC collimation under different loads new [Re: jaddbd]
      #5516015 - 11/12/12 01:29 PM

so let me take a step back and understand. in the case of the AT6, i've got the optical center way off in the upper left hand corner. stars look pretty tight there. so are you guys saying essentially that just tipping the focuser to bring the optical center to the center of the chip should solve the problem? the moonlite does have collimation adjustments, so i wonder if i don't need the focuser tip ring that has been mentioned. anyway, that ring is for the 10 and would not fit the 6; not sure if there is one for the 6 or not.

if this is true, it seems like the "right" thing to do, since the scope is essentially in good collimation when i use a lighter imaging package. of course the load could still be too great and the collimation would shift as the OTA angle changes, but it's at least worth trying.

for the AT10 (and sorry the thread has drifted to both OTAs) i see some coma shapes in the stars, so i think that it's not just a matter of focuser tip/tilt. however, tilting the focuser could still be a necessary component of the solution.

i am still working on the screenshots from the 10.


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Jared
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5516158 - 11/12/12 02:53 PM

Quote:

gah, i think the AT10RC is out of collimation with the load as well. when it was collimated, it did not have the camera package on it and in order to get the STT-8300M on there i had to remove the focuser in order to add another extension ring. the focuser is the stock focuser, which is probably not helping anything.

so at this point i guess i have to try collimating, probably with CCDInspector, so i can do it with the camera attached. up for grabs is whether or not i should upgrade the focuser first.

do the shapes of the stars mean anything with respect to how the scope is miscollimated? at this point i'm wondering if i can get away with adjusting the secondary only.




Speaking generally for a Ritchey Chretien... The shape of the stars in the middle of the field will tell you how the secondary is adjusted. The shapes of stars in the corners will tell you how the primary is adjusted.

If out-of-focus stars in the middle of the field show concentric rings, then the center of the optical axis of the secondary is correctly pointed at the center of the optical axis of the primary.

In the corners, the stars will likely show radial astigmatism. There is some astigmatism inherent in the RC design as well as some field curvature. If you have the primary adjusted properly, stars in all four corners will show the exact same astigmatism (same shape "football" with the point of the football pointed towards the middle of the field).

Collimating the scope would probably be really easy if it weren't for the fact that you have a third complication--focuser tilt. If the focuser is not perpendicular to the optical axis, the amount of curvature will vary from one corner to another. It can be hard to determine whether there is a problem with the primary, or whether the focuser is slightly out of whack. Since the included focuser doesn't include any adjustment for the focuser tilt, I would just assume it's good and collimate using secondary and primary alone.

As you yourself mentioned, adjusting the primary will require that you re-adjust the secondary, so it may take several iterations. Just be patient and you'll get there. Keep in mind that as you get close, variations from frame to frame in CCD Inspector may be larger than the actual miscollimation, so you may want to average several frames together--CCD Inspector can do this.

One other "hint" I will offer... Most scopes in this price range come with rotating focusers including the AT10 (whether it's the FeatherTouch or the stock focuser). Using this feature--the rotation--will cause the focuser to become either more or less square to the optical axis. It isn't a big shift, but it's enough to be visible in your results. If you collimate with the focuser in a given position, DON'T MOVE IT from that position.

As far as the AT6 goes... I'm not certain why you are seeing such different results depending on what camera is attached... Sounds like something isn't square, though. I doubt it is a focuser "sag" issue, though, or you'd be seeing it with both cameras. I ran into this once with an FLI camera, and it turned out to be the filter wheel. One of the cover screws for the filter wheel was in a hole that hadn't been countersunk quite as far as the rest, and so that screw was about 1/5th of a millimeter from being flush to the cover. That was enough to keep the camera itself from mounting flush to the filter wheel. I ended up with a very slightly tilted camera. An amazingly small tilt was very obvious in the corners of the field of view. I'm not saying that's what's wrong with your SBIG, but it could be something along those lines. Make sure the filter wheel is perfectly square to the camera, and that it is perfectly square to the tube extensions and/or focuser.


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pfile
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Re: AT6RC collimation under different loads new [Re: Jared]
      #5516235 - 11/12/12 03:45 PM

thanks Jared. yes, i have started to wonder if the filter wheel might be a problem. one experiment i wanted to try was to remove the filter wheel and see if that changes anything. of course, that also changes the weight as well.

wrt focuser rotation - yeah, understood. in this situation i ended up having to remove the focuser from the OTA in order to put the extension rings in. this may have resulted in a different focuser tilt when i reinstalled it, not sure.

i need to do the high-power test to see the rings. as i've mentioned though i worry that if i were to collimate "perfectly" with the star test, i'll be back out of collimation after i install the camera.

i see that CCDInspector has a 'defocused star mode' for collimation with the camera in-place. do you think that this is only a substitute for the initial collimation and not for the high-power star test?


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