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Equipment Discussions >> Cats & Casses

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pfile
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Reged: 06/14/09

AT6RC collimation under different loads
      #5510913 - 11/09/12 12:58 AM

well, i kind of posted this over in the CCD section, but maybe that was the wrong place.

the short question: is it normal to have collimation results that are wildly different when switching camera payloads on an AT RC telescope?

when using my DSLR+FR my collimation is spot-on and my field is flat.

when i switched to a heavier camera (STT-8300M), FHWM analysis across the field shows that the optical center is nowhere near the center of the CCD. the flattener has left the train due to plumbing issues. so i don't expect the field to be flat, but i expected it to be centered on the CCD.

is it normal to have to recollimate in this situation, or do i have some other problem?


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WadeH237
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5511313 - 11/09/12 09:45 AM

It sounds like your focuser might be sagging a bit.

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pfile
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Re: AT6RC collimation under different loads new [Re: WadeH237]
      #5511391 - 11/09/12 10:37 AM

I thought of that - but the focuser is a brand-new moonlite. also I see the problem if the telescope is pointed straight up...

could it be that the moonlite is not up to the task? the weight adjustment is tightened down as far as I could with my fingers.


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Peter in Reno
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5511463 - 11/09/12 11:27 AM

I believe the focuser tube (not external focuser) inside OTA is connected to the primary mirror which is not a good design and causes collimation changes from point to point if there is a somewhat heavy imaging train. So when the scope is pointing to Zenith, that probably creates maximum stress to primary mirror/focuser tube.

Peter

Edited by Peter in Reno (11/09/12 11:29 AM)


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pfile
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Re: AT6RC collimation under different loads new [Re: Peter in Reno]
      #5511531 - 11/09/12 12:19 PM

yeah, actually i should have linked my post in the CCD forum over here. one of my possible conclusions is that it's a 'cheap' scope and the way the primary is connected to the focuser tube is causing the problem.

the fwhm analysis shows what looks like sag, so i guessed that when the weight of the camera is pulling straight down on the mirror, that the tilt would be gone. but you are right that this situation represents the maximum force on the focuser tube and perhaps just the slight angle (i can't point exactly straight up due to the tripod) is enough to cause the tilt.

anyway, is it a dumb idea to recollimate the scope under this load?


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Peter in Reno
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5511549 - 11/09/12 12:33 PM

If you collimate at Zenith, the mirror/focuser tube may still shift elsewhere in the sky.

Peter


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pfile
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Re: AT6RC collimation under different loads new [Re: Peter in Reno]
      #5511968 - 11/09/12 04:47 PM

i guess i'm boned then i hope the AT10RC is more tolerant of this heavier load. that's my final destination here...

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Peter in Reno
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5511975 - 11/09/12 04:58 PM

Not necessarily. How far off is the collimation under heavy load? I have seen many fantastic images taken with AT6RC scopes. The focal length may be short enough not to notice collimation shifts at prime focus or with focal reducer.

You should double check with the manufacturer about their design in AT10RC. I believe AT8RC has same design as AT6RC.

Peter


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jjongmans
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Reged: 02/11/12

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Re: AT6RC collimation under different loads new [Re: Peter in Reno]
      #5511997 - 11/09/12 05:17 PM

All the GSO RC's (6", 8", 10" and 12") have the same design: the focuser is connected to the primary.

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pfile
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Re: AT6RC collimation under different loads new [Re: jjongmans]
      #5512025 - 11/09/12 05:33 PM

GAH maybe it's a tad stronger, we shall see. lots of people use these scopes successfully so in theory it should be possible.

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korborh
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Re: AT6RC collimation under different loads new [Re: jjongmans]
      #5512029 - 11/09/12 05:36 PM

pfile, are you planning to get the AT10RC scope? That is a much more expensive scope and it also has the focuser connected to primary mirror design flaw. So you may want to take this into consideration before buying.
These scopes are very hard to collimate correctly, so I think you should not try to tweak it for different parts of the sky. Apart from wearing out the mechanics of adjustments, it will wear you out also


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jjongmans
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5512031 - 11/09/12 05:37 PM

Is the miscollimation only noticeable with software, or can you easily see it?

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pfile
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Re: AT6RC collimation under different loads new [Re: Peter in Reno]
      #5512032 - 11/09/12 05:37 PM

Quote:

Not necessarily. How far off is the collimation under heavy load? I have seen many fantastic images taken with AT6RC scopes. The focal length may be short enough not to notice collimation shifts at prime focus or with focal reducer.

You should double check with the manufacturer about their design in AT10RC. I believe AT8RC has same design as AT6RC.

Peter




it's way off, check this out:



the stars in the corner look like seagulls.


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pfile
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5512040 - 11/09/12 05:42 PM Attachment (41 downloads)

here's a screenshot of the upper left corner and the lower right corner...

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pfile
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5512041 - 11/09/12 05:42 PM Attachment (37 downloads)

and the lower right

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Peter in Reno
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5512045 - 11/09/12 05:46 PM

Not good.

Maybe if you collimate at Zenith, it might minimize collimation shift elsewhere in the sky.

Peter


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pfile
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Re: AT6RC collimation under different loads new [Re: korborh]
      #5512049 - 11/09/12 05:47 PM

Quote:

pfile, are you planning to get the AT10RC scope? That is a much more expensive scope and it also has the focuser connected to primary mirror design flaw. So you may want to take this into consideration before buying.
These scopes are very hard to collimate correctly, so I think you should not try to tweak it for different parts of the sky. Apart from wearing out the mechanics of adjustments, it will wear you out also




well, i already have it, i have not deployed it for real yet. basically i have been working my way up to it, trying to get my mount tuned up and understanding how to use OAG with the AT6.

i freely admit that i did not think all of this out very well. the AT6 was a bit of an impulse buy since they were so steeply discounted. and i was pretty successful with it with a light load (DSLR + TRF-2008). so basically i never saw the problems that the mirror/focustube design can cause.

that experience made me think i could probably be successful with the AT10RC. i may be in for a surprise, but i will at least try, since i already have the scope. at least i bought it used


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jjongmans
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5512051 - 11/09/12 05:48 PM

Did you adjust the primary already? If so, maybe you should try to tighten the collimation bolts.

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pfile
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Re: AT6RC collimation under different loads new [Re: jjongmans]
      #5512066 - 11/09/12 05:58 PM

Quote:

Did you adjust the primary already? If so, maybe you should try to tighten the collimation bolts.




i have not adjusted this telescope at all, ever, mainly because of what korborh said above... the collimation looked really good with the DSLR/TRF so i never had to mess with it.

i am willing to try collimating it, because i've come to realize that if you own one of these telescopes, you must learn how to do it. having said that, i had the AT10 collimated professionally. of course, once there's a camera attached to it, it may be out of collimation again just as the AT6 seems to be.


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jjongmans
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5512073 - 11/09/12 06:01 PM

How heavy is your ccd including filterwheel, oag, etc?

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pfile
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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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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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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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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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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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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
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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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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
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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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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
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Re: AT6RC collimation under different loads new [Re: jjongmans]
      #5515718 - 11/12/12 10:22 AM Attachment (26 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
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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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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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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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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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Re: AT6RC collimation under different loads new [Re: pfile]
      #5516721 - 11/12/12 08:51 PM Attachment (30 downloads)

well, this is interesting: on the AT6, when i attach the tak collimation scope, the donut on the secondary is definitely not centered up. furthermore if i rotate the focuser, the light area around the dot moves as i rotate the focuser, as though it's orbiting a point.

i *think* this means that the focuser first needs collimation/tilt adjustment as per John D's instructions above. then perhaps the scope needs collimation.

strange as CCDInspector says everything's mostly okay with the reducer/flattener in place. can an FR/FF really cover up a collimation problem?


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Re: AT6RC collimation under different loads new [Re: pfile]
      #5517184 - 11/13/12 01:39 AM

well i fixed up the AT6 but then did not test it, since the AT10 is on the G11 right now. i messed with the AT6 primary, which perhaps i should not have done. when i got the outer annulus of the primary to be concentric, the shadow of the baffle was way off... so i put it back.

i put the tak scope on the AT10 and it looked mostly okay, but i tried to center up everything a little better just by adjusting the secondary a tad.

whaddya know? round stars with no coma. CCDInspector says it's off by 3" now. seems like i can tweak it a little more but i was worried that i'd mess it up.

i think the seeing is not that good tonight as even though my B-mask says focus is spot on, the stars look a little fat.

i'm not sure i'm out of the woods yet, but thanks to everyone for their insights and ideas. i probably will upgrade the focuser and then i'll have to go thru this all again...


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Re: AT6RC collimation under different loads new [Re: pfile]
      #5518971 - 11/14/12 12:58 AM

Quote:

well, this is interesting: on the AT6, when i attach the tak collimation scope, the donut on the secondary is definitely not centered up. furthermore if i rotate the focuser, the light area around the dot moves as i rotate the focuser, as though it's orbiting a point.

i *think* this means that the focuser first needs collimation/tilt adjustment as per John D's instructions above. then perhaps the scope needs collimation.

strange as CCDInspector says everything's mostly okay with the reducer/flattener in place. can an FR/FF really cover up a collimation problem?




If you do get a adjustable focus ring, I think Jared's suggestion of leaving the focuser put is a good one. I actually went the the trouble of getting the laser to agree with the centered chesire view and a good star test -i.e. with donut centered in the chesire and a good star test, I can spin the focuser with the laser and the it remains pointed at the center spot. It took me forever to get it there thru much trial and error (I can't even give you a good way to do it) and there is really no need to be able to spin the focuser. Also, bear in mind that you may loose the a slight bit of symmetry on the outer ring looking thru a chesire if you tilt the focuser since you may end up slightly out of parellel with the baffle tube. I assume the tak scope does not see these anyway... (I use a "poor mans tak scope," small set of binocs thru a tecton chesire.)

FYI, I took a purely mechanical approach to my collimation due to my chip size (ST2000 is about 2/3 the FOV of your chip). There is hardly any dicernible coma "doppler effect" in the corners of my images in focus unless the collimation is out of wack. I can detect the "fleeing astig" in the corners of defocused images however. I don't need a field flattner with my camera.

Good luck,

JD


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Re: AT6RC collimation under different loads new [Re: jaddbd]
      #5519352 - 11/14/12 11:01 AM

you can actually see all the rings in the tak scope, including the outer edge of the mirror. what's weird is that on the 6 i cant simultaneously get the baffles centered up on the secondary and also see the entire outer edge of the primary.

supposedly the moonlites are collimated at the factory, and i have not messed with the focuser tilt, so i'm not sure why i'd be off axis like that. maybe the scope is mechanically messed up.

yeah - i don't have any intention of rotating the focuser, i'm going to leave it put.


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Re: AT6RC collimation under different loads new [Re: pfile]
      #5521227 - 11/15/12 01:25 PM

random updates: on the AT6RC, the reason i could not get the image of the primary mirror centered up is that the Tak scope was crooked (!!). it has a very deep groove and unless the focuser's brass ring is down in this groove, the scope will be crooked. the scope's flange ends up not flush with the focuser when it's installed properly.

on the AT10RC, i tried CCDInspector's defocused star collimation tool (using Altair) and ended up worse off than what i had with the Tak. that's kind of disappointing. but what's interesting is my original star shapes were kind of triangular/boomerang shaped. i think the secondary mirror must have been pinched somehow since no matter how far off the collimation is now, i never see star shapes like that. maybe just loosening/tightening one of the secondary collimation screws relieved the pressure.

looks like 2 weeks of clouds and rain now, so testing is probably over. what's bothering me about the AT10 is that no matter how close i've been to proper secondary alignment, i've never seen a FWHM less than about 4 at the optical center. it could be that it would dramatically improve once collimation is spot on...?


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JoseBorrero
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5522963 - 11/16/12 01:57 PM

best collimator is the chessire eyepiece from scopestuff. http://www.scopestuff.com/ss_ec13.htm I used for my AT6RC with no problem, follow the instruction in the scope manual.


consider a Moonlite focuser as it has more torque.


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Re: AT6RC collimation under different loads new [Re: JoseBorrero]
      #5522971 - 11/16/12 02:04 PM

the tak scope is much the same and is also good for collimating RC telescopes. i do already have a moonlite focuser on the AT6RC. this is why i was surprised that it was apparently sagging. but i don't think it is sagging.

at this point i believe the AT6RC is collimated properly, but i have not been able to do the star test due to the weather.


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jaddbd
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5523424 - 11/16/12 08:23 PM Attachment (27 downloads)

I downloaded a copy of ccdinspector to see how the collimation matched. I am feeling better about my methods. Cool program. I put about a 30th of screw turn to get the collimation error from hovering around 3" to get it to hover around 1". Seeing is less than average tonight.

I have not seen the AT6, but I still think the whole primary assembly might not be slipping. You problably already have, but I would make sure the primary lock screws are tight, and the small primary collimation screws are at least firm to the allen wrench... also check the screws that hold the rear plate to the tube (on the side of the tube).

John D


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Re: AT6RC collimation under different loads new [Re: jaddbd]
      #5523561 - 11/16/12 10:30 PM

that curvature map looks great. nice work.

good point about the side screws. i'll check those. i had to remove/replace the top 2 screws to mount a vixen rail on top, but i had done that long before i tried the STT-8300.

of course now i have adjusted the primary in the AT6 so yeah, before i take it outside again i'll make sure the collimation lock screws are tight. now i'm not certain that the secondary was so far out of whack due to my problem aligning the tak scope, but at this point that's all behind me - the mirrors have been moved!

on the AT10 the secondary collimation screws are pretty tight - i'm not sure i could move them 1/30th of a turn! but i can see that such tiny adjustments are needed once you get close.


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jaddbd
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5523651 - 11/16/12 11:34 PM

Quote:

that curvature map looks great. nice work.

good point about the side screws. i'll check those. i had to remove/replace the top 2 screws to mount a vixen rail on top, but i had done that long before i tried the STT-8300.

of course now i have adjusted the primary in the AT6 so yeah, before i take it outside again i'll make sure the collimation lock screws are tight. now i'm not certain that the secondary was so far out of whack due to my problem aligning the tak scope, but at this point that's all behind me - the mirrors have been moved!

on the AT10 the secondary collimation screws are pretty tight - i'm not sure i could move them 1/30th of a turn! but i can see that such tiny adjustments are needed once you get close.




If 6 has middle screws under the rails like the 10, they also hold the tube to the back plate.

Also, the secondary collimation screws push/pull against the center screw. So you can loosen all a bit 3 equa-distant to give them more play if they are to tight. 1/30 - think of 2 minutes on the clock - a tiny tweak...

Hope to hear good things about the STT - I have that on the radar right now...


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CCDMan
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5718501 - 03/07/13 01:56 PM

I have owned two RCOS scopes and an OGS and have a AT10RCF on order (my 14.5 RCOS is too heavy for my bad back). I have to wonder what imbecile decided to mount the focuser the way AT did?! It really could not have saved that much money. Fortunately, I have a big lathe and if push comes to shove I plan to machine a completely new rear cell with the focuser mounted the way it should have been in the first place, on the back plate with the primary cell attached the back plate by the collimation screws. What the heck, just some aluminum and some time. The aluminum is cheap and my time as a retired guy is worth nothing at all!

Does anyone have pictures of how the rear cell is attached to the carbon tube? Not very good pics on the web. It appears that there are (3?) screws thru the carbon fiber that hold the rear cell to the tube. Hopefully they did not glue it as well. Looks like one would just disassemble, remove the primary, cut off the back of the tube where the focuser attaches now, and build a ring that attaches to the tube and a flat rear cell plate which the primary collimation mechanism and the focuser attach to. In other words how everyone BUT the Chinese do it! As long as you keep it close to centered (and that can be adjusted with the secondary anyway to a degree) and maintain the correct mirror spacing, you would wind up with a much better system.

An alternative method, and much simpler if the die cast rear cell is strong enough, would be to machine two large rings that would go both behind and in front of the back wall of the present die cast rear cell just around the light path opening. Then drill 3-4 symmetric holes, say 1/4-3/8 diameter and tap the inner ring to receive the bolts. The outer ring would sit out on standoffs just far enough out to clear the convex curvature of the backside of the rear cell. The focuser of choice could mount to the plate. The standoffs could be adjusted to make for a perfectly aligned focuser and any light gap could be sealed with a large o-ring.

Trust me, a properly made RC is not at all hard to collimate, I can do my RCOS 14.5 in about 15 minutes and nail it every time to the point where a star tweak is only needed about 20% of the time. The problem is that this thing does not seem to fit the definition of "properly made". So properly remake it. <g>

One thing I should point out is that you really do not want to use the Tak scope just in the focuser. The focuser should be off the scope. Use a machined, threaded adapter and some machined, threaded extension tubes that will put the Tak scope back where the focuser would be. The better RC scopes use 2.7 AP extension tubes threaded together, threaded into a plate machined to attach to the rear cell. That plus a custom AP to Tak scope adapter for the back end of the AP tubes. I have a AT10RC rear thread to AP thread adapter ordered from Precise Parts to get me there for this scope (did not feel like making one myself <g>). If you use the Tak just in your focuser, you are just introducing another variable that has to be sorted out. Note that you can still use the Tak scope on the back of the focuser and if the two methods do not agree, your focuser can be identified as at least part of the problem.

Edited by CCDMan (03/07/13 07:33 PM)


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Re: AT6RC collimation under different loads new [Re: CCDMan]
      #5719539 - 03/07/13 10:29 PM

well that would be an awesome mod, that's for sure. i have been imaging with the at10rc and have not returned to the at6rc. it still needs to be star tested.

i don't have carbon tube ATRCs but i do believe the metal one are held together as you describe.

are you using the deep sky instruments method of collimating with the camera attached?

Edited by pfile (03/07/13 10:30 PM)


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CCDMan
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5720289 - 03/08/13 11:02 AM

Quote:

are you using the deep sky instruments method of collimating with the camera attached?




No, never. I have tried that and it is a major hassle. If done right, and I just did it pre-sale on my 14.5 RCOS last night, it is quicker and just as accurate with a good eyepiece (after doing daytime collimation with a laser and Tak scope). It does not entirely apply to the ATRC scopes but Ken Crawford did a great video on RC collimation that is on the RCOS site.

Having said that, I acquired sanity last night after thinking thru this purchase and plan to call Anacortes when they open and cancel the ATRC order and order a Tak TOA 130 instead. Shorter F.L but WAY easier to deal with. I am getting too old for this S***.....


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saadabbasi
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Re: AT6RC collimation under different loads new [Re: CCDMan]
      #5720293 - 03/08/13 11:05 AM

Quote:

Quote:

are you using the deep sky instruments method of collimating with the camera attached?




No, never. I have tried that and it is a major hassle. If done right, and I just did it pre-sale on my 14.5 RCOS last night, it is quicker and just as accurate with a good eyepiece (after doing daytime collimation with a laser and Tak scope). It does not entirely apply to the ATRC scopes but Ken Crawford did a great video on RC collimation that is on the RCOS site.

Having said that, I acquired sanity last night after thinking thru this purchase and plan to call Anacortes when they open and cancel the ATRC order and order a Tak TOA 130 instead. Shorter F.L but WAY easier to deal with. I am getting too old for this S***.....




Good choice on the Tak. I find the collimation very difficult but then again I am rather inexperienced.


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Re: AT6RC collimation under different loads new [Re: CCDMan]
      #5721213 - 03/08/13 09:04 PM

Quote:


Having said that, I acquired sanity last night after thinking thru this purchase and plan to call Anacortes when they open and cancel the ATRC order and order a Tak TOA 130 instead. Shorter F.L but WAY easier to deal with. I am getting too old for this S***.....




i hear that... can't go wrong with takahashi.


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CCDMan
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5750766 - 03/22/13 10:03 PM

UPDATE:

I indeed did get the TOA-130 but decided to also get the smaller AT8RCF. Dirt cheap after my credits at Anacortes
on the cameras and such so out of pocket was about 1,100. I figure for that price I can afford to play with a better
rear cell. It also helps that I picked up a used TOA-130 with 4 inch flattener for less than half retail for the new
one!

Will let the forum know how it goes and take pictures of any modifications. I will do this step-wise from standard
configuration, getting however radical is needed to deal with any issues. This will be fun. AT8 is due Monday, picked
up TOA today.


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Re: AT6RC collimation under different loads new [Re: CCDMan]
      #5751143 - 03/23/13 02:43 AM

excellent, yes, please keep us posted.

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CCDMan
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Re: AT6RC collimation under different loads new [Re: pfile]
      #5826368 - 04/27/13 01:46 PM

I have finally found time what with all the upgrades to my system (mainly refractors as they are my main "quality" scopes at this time) to begin messing with my "throw-away" AT8RC. I have not started testing yet but may begin as early as tonite.

I have made up a page where I will post my impressions and results as this proceeds. Since it must be combined with my real imaging on the refractors, it will be somewhat hit and miss and mainly done during full moon. Once I get to modifications (assuming they are required), I will also need to find the time to do the machining. The bottom line is this will be catch as catch can and may take much of the summer.

ATRC Testing and Modification

Take Care


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Re: AT6RC collimation under different loads new [Re: CCDMan]
      #5827413 - 04/27/13 10:20 PM

thanks i'll keep an eye on that page.

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frolinmod
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Reged: 08/06/10

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Re: AT6RC collimation under different loads new [Re: CCDMan]
      #5827769 - 04/28/13 05:26 AM

According to your web page:

Quote:

Stay tuned to this page for testing results, updates, and modifications to the scope as they progress thru summer of 2103.




Gee, will it take you that long?


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CCDMan
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Re: AT6RC collimation under different loads new [Re: frolinmod]
      #5829318 - 04/28/13 11:06 PM

Quote:

Gee, will it take you that long?




Probably. It is only one of four scopes and not my primary imaging scope. For me the RC is in the category of "when I have time and feel like it". More of a fun experiment that I can do when I feel like messing with testing and machining and that is only once in a while. I try to keep it a hobby!

Take Care

Edited by CCDMan (04/28/13 11:09 PM)


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BlueGrass
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Re: AT6RC collimation under different loads new [Re: CCDMan]
      #5829371 - 04/28/13 11:54 PM

I think you'll find the FL and overall capability of these RC scopes, should be given a more significant look. Sal's, Warren's and Dan's work and results are exemplary. For the cost, when properly collimated and used, they can deliver outstanding results. I've had just a few nights for testing and imaging with my RC6 since acquiring it early last year, but have spent the last 4 months, when time has allowed, working on getting an OAG setup working and dialing in the collimation. I too have used APOs almost exclusively for the past few years but given the 1300+Fl and advantages of a fixed primary design, the RC is a good compromise between traditional SCTs and APOs. We'll be interested in your progress and results, particularly in regards to flexure and guiding setup used.

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Re: AT6RC collimation under different loads new [Re: BlueGrass]
      #5829503 - 04/29/13 02:37 AM

OAG is key, i never had round stars on the AT6 or the AT10RC until i started using an OAG...

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CCDMan
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Loc: Oregon, USA
Re: AT6RC collimation under different loads new [Re: pfile]
      #5830633 - 04/29/13 04:17 PM

Quote:

OAG is key, i never had round stars on the AT6 or the AT10RC until i started using an OAG...




The only way to guide a mirror scope! Guide scopes are almost entirely useless with mirror scopes, too much chance of flexure. They can work but even when they do the results are hit and miss. Trust me, tried that 15 years ago and it just is not reliable. Either one of the old self-guide SBIG cameras, one of the newer SBIG cameras with the OAG built in, or a separate guide cam with an OAG are the only reliable methods. I prefer the SBIG solution since there is less hardware to deal with and less weight.

Now with completely refractor systems, guide scopes can be a nice way to go, but even that assumes one's mounting solution for both the imaging scope and guide scope are rigidly and carefully designed.

Edited by CCDMan (04/29/13 04:21 PM)


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