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AT6RC collimation under different loads

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#26 pfile

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Posted 10 November 2012 - 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.

#27 jaddbd

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Posted 10 November 2012 - 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

#28 Jared

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Posted 10 November 2012 - 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.

#29 pfile

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Posted 10 November 2012 - 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.

#30 pfile

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Posted 11 November 2012 - 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.

#31 jaddbd

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Posted 11 November 2012 - 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

#32 korborh

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Posted 11 November 2012 - 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.

#33 jaddbd

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Posted 11 November 2012 - 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

#34 pfile

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Posted 12 November 2012 - 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...

#35 jjongmans

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Posted 12 November 2012 - 03:39 AM

I use this ring for adjusting the focuser.
http://www.teleskop-...S-focuser-co...

#36 jaddbd

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Posted 12 November 2012 - 10:22 AM

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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#37 jaddbd

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Posted 12 November 2012 - 12:17 PM

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

#38 pfile

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Posted 12 November 2012 - 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.

#39 Jared

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Posted 12 November 2012 - 02:53 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.


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.

#40 pfile

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Posted 12 November 2012 - 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?

#41 pfile

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Posted 12 November 2012 - 08:51 PM

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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#42 pfile

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Posted 13 November 2012 - 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...

#43 jaddbd

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Posted 14 November 2012 - 12:58 AM

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

#44 pfile

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Posted 14 November 2012 - 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.

#45 pfile

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Posted 15 November 2012 - 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...?

#46 JoseBorrero

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Posted 16 November 2012 - 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. :waytogo:


consider a Moonlite focuser as it has more torque. :waytogo:

#47 pfile

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Posted 16 November 2012 - 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.

#48 jaddbd

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Posted 16 November 2012 - 08:23 PM

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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#49 pfile

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Posted 16 November 2012 - 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.

#50 jaddbd

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Posted 16 November 2012 - 11:34 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.


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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