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Equipment Discussions >> Cats & Casses

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KerryR
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Reged: 12/05/07

Loc: SW Michigan
Re: A Simple SCT Question new [Re: T1R2]
      #6041524 - 08/23/13 09:52 AM

I read somewhere that astigmatism that disappears at about the point where 4 or 5 rings are visible, as you move away from focus, will have very, very little impact on the image at focus. No idea how accurate a statement that is. Moot anyway, because it looks to me that this scope is worse than that.

The ovals are not smooth ovals, which suggests something more than just 'simple' astigmatism.

I get the feeling this is in the glass. But, if it were me, and the scope couldn't be returned for a refund or different ota, I'd take it apart and look for things that might be pressing too tightly on the mirror(s), such as the rear plate that holds the primary mirror on the carrier, the shims that absorb the space between the bore in the mirror and the carrier, or perhaps a deformed gasket between the front of the mirror and the carrier, or etc.

You have to be prepared for the possibility that you might damage the scope and be left with nothing. Personally, I enjoy this sort of trouble shooting and disassembly, it's part of the hobby for me.


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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: KerryR]
      #6041658 - 08/23/13 11:02 AM

Thanks again everyone.

The scope was unused but not from Meade and Meade's warranties are non-transferable. I tried! I also did not use the scope for a few weeks after I got it and it took several months to convince myself that I had a problem. The vendor has a 30 day return policy of course.

I have done the front view thing. It does really look like there is a problem with everything lining up. I have also peered up the baffle many times in different light and am pretty convinced that the secondary is not quite centered with the baffle aperture. I'm not really sure any of this means anything but I guess it all points towards a tilted baffle tube.

Since it really does seem to be reasonably bad astigmatism (bright stars are never nice little points like in my C9.25) I'm going to try to get the mirror out again. Maybe I just wasn't aggressive enough with it. Also, I still remarkably get not terrible views with the scope so I have hopes for great things if it survives my surgery.

One more question before the anesthesia ... are ACFs normally more difficult to collimate then non-ACF or Edge SCTs? I seem to get slightly different results with different amounts of intra- and extra- focus and I usually just have to pick some compromise whereas it's pretty easy to home in on collimation with my C9.25.


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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: A Simple SCT Question new [Re: MrJones]
      #6041950 - 08/23/13 02:12 PM

Quote:

Thanks again everyone.

The scope was unused but not from Meade and Meade's warranties are non-transferable. I tried! I also did not use the scope for a few weeks after I got it and it took several months to convince myself that I had a problem. The vendor has a 30 day return policy of course.

I have done the front view thing. It does really look like there is a problem with everything lining up. I have also peered up the baffle many times in different light and am pretty convinced that the secondary is not quite centered with the baffle aperture. I'm not really sure any of this means anything but I guess it all points towards a tilted baffle tube.

Since it really does seem to be reasonably bad astigmatism (bright stars are never nice little points like in my C9.25) I'm going to try to get the mirror out again. Maybe I just wasn't aggressive enough with it. Also, I still remarkably get not terrible views with the scope so I have hopes for great things if it survives my surgery.

One more question before the anesthesia ... are ACFs normally more difficult to collimate then non-ACF or Edge SCTs? I seem to get slightly different results with different amounts of intra- and extra- focus and I usually just have to pick some compromise whereas it's pretty easy to home in on collimation with my C9.25.




In an SCT with a spherical secondary, astigmatism is usually from a mis-centered corrector. (A mis-centered secondary can be adjusted for simply by tilting it till collimated.) However, you seem to have an ACF so its secondary is aspherical, IIRC. This means it has to be dead center with the primary, in addition to the corrector being centered as well. Unless there is some stress on the primary which is causing the astigmatism or the baffle tube is out of whack, I would start with the corrector/secondary.

There have been methods published by folks on how to center correctors and/or secondaries on the web. Unfortunately, I can't remember any off-hand right now. A simple method to check for the corrector's centering would be use a sight tube (the kind used for Newt collimation). Alternately, you can pull the secondary and stick a collimation laser in the visual back and then check if the spot comes out centered.

In any two-mirror optical system, the collimation steps are to center the secondary, tilt the secondary and then tilt the primary, with perhaps another iteration or two of this process. With SCTs there's an added complication in the form of the corrector and the primary being fixed in tilt but the basic principle still applies.

Finally, I should say that it's very unlikely that the astigmatism is ground into the glass due to the mass production methods used so you have hope of getting it out of the system. I recently bought a 4" JSO SCT that also showed astigmatism (stars were crosses at focus). A little measurement showed that the corrector wasn't centered. Fixing it and recollimating removed the astigmatism entirely.

Tanveer.


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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: TG]
      #6042201 - 08/23/13 04:45 PM

Thanks, more to think about. I have centered the corrector + secondary with calibers as close as I could and it didn't seem to be off much if any and didn't affect the astigmatism. The Meade is different than my C9.25 in that the secondary holder fits very snugly in the corrector hole - so no play to adjust the secondary relative to the corrector. I didn't mention it but I have also done the artificial star tests previously, rotating the corrector and secondary separately to see if that helped and also removing the cork spacers and trying the corrector in NSEW type positions. I got a little variety in results but the factory marked positions seemed as good as anything else so I kept them there. I also have spent a lot of time looking up the baffle tube with sight tube and my laser. It does seem like the secondary is not quite centered but there is not enough play in the corrector even after removing the cork spacers to get it to a centered position through the sight tube, and I have no idea if this is the correct way to assess the secondary position.

It's a little embarrassing to admit, but I tried the mirror again since I have the day off and it slid right out one I perfectly aligned it with the slots as per KerryR and the 14" disassembly guide. In fact this scope is pretty much the same as that 14" minus the mirror lock. Thumbs up to Meade engineering for the construction at least - very solid.

Without the mirror I now feel safe to stick the calipers down there and it sure does look like the baffle tube is tilted about 0.5 mm as I measure a 1 mm difference between 2 opposite sides.

So it's down to just the main tube and baffle tube and I apparently have to bake this to loosen the thread lock and be able to unscrew the baffle tube. I'll probably go for it, center the tube somehow if I can remove it, reassemble and test before I fool around with the mirror assembly.


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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: A Simple SCT Question new [Re: MrJones]
      #6042943 - 08/24/13 04:12 AM

I agree it looks more like the secondary is a bit off center

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KerryR
Post Laureate
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Reged: 12/05/07

Loc: SW Michigan
Re: A Simple SCT Question new [Re: orion61]
      #6043659 - 08/24/13 01:50 PM

I'll mention it again: The ovals don't look smooth, they look lumpy to me, which might suggest a rough surface, and/or primary mirror mounting issue.

I'd remove the primary from the carrier, now that you have it out anyway, and investigate issues on at the carrier/primary interface. Mark the back of the primary relative to the focus arm, and the focus arm relative to the carrier so that you can return it all to the original rotational alignment. Look at how tight the mirror fits on the carrier, how tight the focus arm is pressing on the back, and examine the gasket material on the carrier that contacts the surface of the mirror, look for lumps in it.

By the way, should you ever need to clean a Meade SCT primary (not likely), it's way easier to do so with the mirror removed from the carrier... Celestron's, because they're glued on to the carrier, are far harder to get a clean rinse...


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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: KerryR]
      #6043980 - 08/24/13 05:39 PM

I may have found at least part of the problem. After baking the scope tube + baffle and trying to unscrew the baffle tube I noticed the big blue main tube had slipped part way out of the bottom black part that the baffle is screwed into. This set off a light bulb - maybe it had never been properly set in the first place which would tilt the secondary and corrector vs. baffle and primary as a tilted blue tube would also tilt the upper parts. Sure enough after I put the scope on end, facing down, and tapped on the bottom with a rubber mallet until everything seemed fully seated (while still hot), the baffle tube now measures as perfectly centered with the calipers. I can see paint lines on the blue tube from the previous positioning so it's definitely different. I put it back together and quick artificial star tests look better but still not perfect and I'm going to try it outside tonight.

I wasn't sure what to do with the mirror so didn't touch it. The mirror is big and thick, at least 1/2", compared to the tiny carrier in this 6". The carrier only covers the inner half of the mirror and it's pretty thin so it doesn't seem like it could stress the mirror easily.


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T1R2
Carpal Tunnel


Reged: 06/11/13

Loc: NeverWhere, 35*N
Re: A Simple SCT Question new [Re: MrJones]
      #6044638 - 08/25/13 03:18 AM

The first thing I would check is to remove the retaining rind around the corrector and see if all the little cork or cardboard spacers all even, no need to worry about were their supposed to be, just make sure their all evenly spaced, and when putting the screws back make sure you do like lug nuts on a tire, and DO NOT over tighten, you could crack the corrector, I take mine off whenever needed and its really no big deal, just use a little caution, get you workspace uncluttered, in the articles section on CN, there is a indepth description called Cleaning and Modifying the C11 Beast, even though its for a celestron scope its basically the same, I would read "removing the corrector plate" section first.

Edit: This is by far the easiest adj. to look for and correct, if its missing the spacers on one side, this will more than likely be the reason for a off-center secondary. If its not the cause, then focus on the primary mirror.

Edited by T1R2 (08/25/13 03:31 AM)


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Ed Holland
Carpal Tunnel


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: A Simple SCT Question new [Re: T1R2]
      #6044648 - 08/25/13 03:37 AM

Mr Jones - earlier I was part way though posting with a suggestion to check primary alignment, but was interrupted. Your comment about the rear cell and tube being properly seated together is important, and I urge you to make further tests now that you think alignment is improved. This is very similar to the situation discovered with my C8, and in that case, careful re alignment of the primary and re collimation took away all doubts I'd had about the optics.

Ed


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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: Ed Holland]
      #6045140 - 08/25/13 12:59 PM Attachment (33 downloads)

And thanks again everyone.

It was not any better under the stars last night. Unfortunately the rear cell, main tube, front cell alignment is another variable that I hadn't considered or even read about before though.

I decided to take KerryR's advice and disassemble the mirror carrier and know what he's talking about now. Certainly the oval and lumpy diffraction patterns are not good. This photo shows the back of the mirror and the carrier part that goes through the mirror and slides up and down the baffle. It had some scraggly masking tape on it to probably make it fit snugger. I took it off and the tube is still snug enough in the mirror without it. So maybe ... I'll try again tonight.

T1R2 - I have spent a couple afternoons fooling with moving the corrector around and recollimating on an ariticial star without seeing much change. I am limited to the thickness of the cork pads for movement since the corrector holder is pretty snug in the corrector hole. If I can get rid of a big chunk of the astigmatism I will try this again though.


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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: MrJones]
      #6045144 - 08/25/13 01:00 PM Attachment (29 downloads)

Also, the back of the mirror and carrier.

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KerryR
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Reged: 12/05/07

Loc: SW Michigan
Re: A Simple SCT Question new [Re: MrJones]
      #6045430 - 08/25/13 03:48 PM

When you put the primary and carrier back together, just snug the rear threaded mounting plate-- no need to go super tight. Be sure there's nothing between the mirror face and carrier, or between the rear mounting plate back of the mirror that could apply funky stress to the glass. Most of all, though, don't go crazy tightening it. You don't want the mirror to be able to spin while in transport, but you don't want it so tight that a lack of parallelness between the front of the mirror and the back could distort the glass when the plate is tightened down.

The tape is used to suck up the difference in diameter between the carrier and hole in the mirror-- it's what holds the primary centered on the baffle. If the stock tape was too thick, you could try a thinner tape, like scotch tape. The mirror should not be able to move laterally on the carrier at all, but it shouldn't be wedged on very tightly either.

While you've got the thing apart, I'd remove the secondary from the secondary housing-- be sure the collimating screws aren't too long and are cantacting the back of the secondary mirror. If the secondary pivots on a central point, be sure the screws aren't pulling the plate onto the pivot with so much force that the plate and glass are flexing. My LX90 does not use a pivot point, rather the secondary 'floats' on 3 screws and 3 springs with no central pivot, but maybe yours is different.

Hopefully you find the culprit soon!


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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: KerryR]
      #6046529 - 08/26/13 08:11 AM

Still not better! The mirror initially seemed ok other than the masking tape (nothing too tight) and it made no difference to redo it.

The secondary is the floating style. Are these easy to reset in their mounts?

Also the secondary is centered in the front cell (measured with calipers) but I still need a good way to confirm that it's aligned with the primary, baffle and rear cell so that's next. I still suspect a tube misalignment now that I know the cells could have moved.


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KerryR
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Reged: 12/05/07

Loc: SW Michigan
Re: A Simple SCT Question new [Re: MrJones]
      #6046659 - 08/26/13 10:04 AM

I had no issue re-attaching the my LX90 secondary, so, if yours is the same, it's pretty easy. Use powder-free exam gloves to handle the secondary-- prevents accidental finger prints on the surface.

Let us know what you find!


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Ed Holland
Carpal Tunnel


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: A Simple SCT Question new [Re: KerryR]
      #6047194 - 08/26/13 03:19 PM

With my Mak, it was obvious that the mirror was tilted on the carrier. I was able to place things on a flat surface, and measure the height of the edge of the mirror to determine the extent and direction of the tilt. If you can assemble so that the rear assembly and mirror are together, without the tube, then it might be possible to discern any misalignment, again using a flat surface, & measuring to the mirror's edge.

Just a thought.

Ed


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T1R2
Carpal Tunnel


Reged: 06/11/13

Loc: NeverWhere, 35*N
Re: A Simple SCT Question new [Re: KerryR]
      #6047209 - 08/26/13 03:28 PM

MrJones, sometimes you just get a lemon, if it were me I'd send it back and not worry. A new scope should be under the stars and only need a tweek in collimation, not disassembled on a work bench, after reading what has already been done and still no difference, it sounds like a bad mirror or secondary. Get you a new scope.

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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: T1R2]
      #6048549 - 08/27/13 10:34 AM Attachment (20 downloads)

Hey it's fun - I can strip a Meade ACF in 6 minutes now. That must be a useful talent somehow. I can't return it and I'm not going to sell it or use it much in this condition.

Anyway I did check the primary for tilt on the carrier and couldn't see any with the calipers.

I reset the secondary but it looked like it was probably ok and it made no difference. It had that black sticky foam on it that was a bear to get off and I replaced it with silicone and blackened the back of the mirror just to be sure as the silicone is clear. It is a pivot system after all.

From the front 10-15' away view something is still off (not a lot but it's just "mild" astigmatism) so I have stripped it down again and the baffle tube is still tilted relative to the main tube. This might be the last time but I'm going to heat it up again and see if I can get that as close as possible. It does behave like either the primary is tilted or the secondary is not centered with the diffraction pattern moving around as I rack the focus in and out and the oval shape.

Add: the pivot does not touch the mirror as it looks like it might!


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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: MrJones]
      #6050616 - 08/28/13 12:30 PM Attachment (22 downloads)

I finally made some progress. What worked is:

1) Heat the tube+cells with no glass and use a wood hoe shaft to tilt the baffle tube until it was as straight with calipers as I could make it relative to the main blue tube. The rear cell was creaking as it moved. I put a bead of superglue around the rear cell/main tube joint when it was cool. I also confirmed that the front cell was still flat relative to the rear cell with a bubble level. This made a difference right away with artificial star tests, with a more round diffraction pattern.

2) Then I rotated the secondary and recollimated until I got the least amount of astigmatism. This was a noticeable improvement also. I tried offsetting the secondary on it's holder plate and rotating that to get more offset but it wasn't better than centering the secondary on it's plate.

Here's an artificial star test. The scales are not quite the same but you can see it's more round. You can also see that I still have the problem of if I collimate on one side of focus, it's a little off on the other side.

It's better though - taking a break!


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Oscar Szentirmai
member


Reged: 12/28/06

Loc: London - UK
Re: A Simple SCT Question new [Re: MrJones]
      #6061098 - 09/03/13 01:57 PM

good luck!!
I hope u fix it...
I'll keep checking back..


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MrJones
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Reged: 09/15/10

Loc: Indiana
Re: A Simple SCT Question new [Re: Oscar Szentirmai]
      #6062390 - 09/04/13 09:58 AM Attachment (15 downloads)

Thanks! And so much for the subject line.

After more work on collimation it is better but still with some problems that must be mostly in the mirror figures at this point.

The primary mirror has little to no tilt as the collimation does not change with focus much if any now. I still need to average collimation with different amounts of intra- and extra- focus because there still seems to be no "best" collimation.

Here is one last artificial star photo at the worst cast which is pretty close to focus and on whatever side of focus is worse. It looks too much like M57. The other side of focus is better and more defocusing looks better until eventually with big diffraction circles you can barely tell that there's a problem.


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