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C14: How much mirror flop is too much?

catadioptric Celestron optics SCT collimation
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#1 ccwemyss

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Posted 07 August 2018 - 10:50 PM

I recently purchased a used C14. A black-tube model, from about 2000. When I got it onto my G11 mount, I was having trouble building a model, and then realized that the image was shifting as much as 25 minutes of arc as it was pointing in different directions. Collimation was also going significantly out in different parts of the sky.

 

After reading many how-to threads, I removed the corrector, built a custom pin-wrench, and tightened the retaining ring on the front of the mirror. It is now down to about 4.5 minutes of image shift. Focus shift movement alone is about 0.4 minutes. Seeing has (of course) been lousy since I did this, so it's hard to get a really good collimation, but there still appears to be a small change associated with the flop (in part of the sky the secondary shadow looks centered, in others it seems to be a tiny bit off, but there is also heavy distortion with heat waves from the ground). 

 

Is this amount of flop considered reasonable in a C14? Or should I try tightening it more? The ring is firmly hand-tight, done with the mirror face up, shipping locks disengaged, and focus varied between attempts to turn it. I don't want to pinch the mirror. 

 

That's the basic question of the post.

 

 

Back story for those who are interested and to anticipate some possible questions:

 

The previous owner replaced the tube with carbon fiber, and in the process put a chip in the edge of the corrector (only about 3/16" by 3/8" of it extends beyond the ring and is blacked out). I have been in touch with Celestron, and they diagnosed it as mirror flop, and said to send it in. But with a chip in the glass, that seems more risky than normal, and there is no original packaging (the OTA was deforked). So I hope to avoid that. 

 

In the process of getting to the diagnosis, I have checked the secondary to ensure that the collimation screws are all tight, the secondary is not loose on its plate, and the secondary housing isn't loose (it's an early Fastar model).

 

The measurement of flop is by carefully centering an object in the finder (I've used three different finders and mounts to ensure it's not finder flop), then comparing the position of the object in the eyepiece to the field of view. First an object near the meridian in the south is carefully centered in both, then the mount is flipped to the far side of the meridian, then the comparison is done. I flip back and forth at least 3 times to check repeatability. Centering the object in the eyepiece produces a clear displacement from the crosshairs in the finder, which eliminates a parallax effect with the hairs as a source. 

 

Initially, an object that was centered on one side would be just beyond the 42 minute FOV of a 41mm Panoptic on the flip side. After tightening, it is about 2/3 to 3/4 of the way out from the center of the 11 minute FOV of a 10mm Delos. 

 

I have also tried two visual backs and three diagonals to ensure that there isn't flexure or miscollimation in the tail end of the optical train, and I have tried flexing and wiggling the corrector to ensure that it isn't loose. Note that the method does maximize the potential for flop, as it puts the OTA almost horizontal on one side then almost horizontal on its opposite side. 

 

What remains seems to be residual mirror flop, with perhaps 10% attributable to focus shift. There is a small change in focus with the flip, and the image shifts less than half a minute when reversing focus direction, which I assume as a component contributed to the total by the slider tipping on the baffle tube. I haven't pulled the mirror and slider assembly to regrease it. All of the tightening was done with the mirror still in the OTA.

 

I'd like to know if this is still out of spec for a typical non-HD C14, or if it's now in the normal behavior range, and I should either stop worrying or look to an alternative such as flop stoppers and a Crayford focuser, rather than torquing more on the retaining ring. 

 

Thanks in advance for any guidance. 

 

Chip W. 



#2 jhayes_tucson

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Posted 07 August 2018 - 11:48 PM

Chip,

Let me see if I understand what you are doing correctly.   Are you saying that you are tightening the retaining ring that contacts the front of the mirror where the baffle passes through the mirror?  If so, STOP!  If the system is properly built, that ring should not have much (if any) effect on mirror flop.  It can, however, have a big effect on the shape of the primary mirror--depending on what you have.  On the older scopes, Celestron used to install a thin cork washer between the ring and the glass.  On some newer systems, the guys at Synta did away with all washers to simply allow the aluminum retaining ring to directly contact the glass (which is a bad idea.)  If your system does not have the cork washer, it may be possible to stress fracture the glass to produce a chip by over-tightening the ring.  Even with the washer, over-tightening the ring will probably distort the figure of the mirror.  The mirror should be firmly bonded to the baffle slider (and rear plate) with potting compound and that's mostly what constrains the mirror position.  That ring at the base of the baffle should be lightly snug and it's there mostly as a safety stop as well as to constrain the mirror while the potting compound sets.

 

Image shift is due primarily to mechanical slop between the baffle and the mirror slider.  A little bit of additional flop comes from mechanical flexure in the system at different pointing angles.  Tightening the ring at the base of the baffle will not have much-if any effect unless the mirror has become de-bonded from it's support structure.  Since you started with 25' of motion and your alignment is changing, your mirror may not be properly bonded and that's a real problem.  How much focus shift do you normally see?

 

John


Edited by jhayes_tucson, 07 August 2018 - 11:52 PM.

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#3 gnowellsct

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Posted 08 August 2018 - 12:22 AM

Well John tightening that ring on my 02 C14 was what made it usable.  A number of people on Yahoo C14 did it, I learned it from them.  It is a tried and true procedure.   I think back in 00 they were using the cork.   If it's ring on glass then I guess someone is going to have to come up with a solution for that because it sure has h*ll is no fun to use a scope that can't hold collimation.

 

I am unfamiliar with current practice regarding cork.  

 

I think however that if the ring tightening is hand-tight that one should not proceed further.  Nonetheless, once upon a time, the ring seemed tight but I still had the problem.   I think it was the C14...I've done this procedure fairly often.   I wondered whether the lock nut was seating or binding on the threads due to inexact machining.  I put a *tiny* amount of grease on the threads and it did another quarter turn without much effort; it went "through" the tight spot, was loose, then snugged.   And that's where it's been ever since.

 

If Celestron says they have a solution to collimation flop I say let them do it, there is a point beyond which further intervention is not good.

 

I am at a point where I don't really have issues (knock on wood) with collimation flop or mirror image shift.    

 

I'm going to offer two points of view on this.  One is what I've seen John write:  don't think baffle grease helps with image shift.  It can't help.  He has reasons why.  It's a fluid.  It flows.  It's not a spacer.

 

Out in the wild, people do pull the primaries out and regrease, and the results are often good.  After moving the mirror up and down be careful to wipe excess grease from the bottom.   I used Slick 50 wheel bearing grease.   I don't want to argue the point, John knows about 100x more than me, but  it is what it is.  

 

On my SCTs I have forgotten that image shift is an issue and only remember when I see it here.  My scope was unusable when delivered, tightening the lock nut made it into a performer.   I did the modification myself because my first C14 arrived as shattered glass and I didn't want to ship it anywhere.   I did all that 16 years ago.  

 

There are two ways to improve shipping safety of an SCT in my view.  One is, remove the fastar and pack the secondary separately.  It was the torque on the fastar that did in my first SCT.  The secondary fell off, shattered against the primary, and that was that.  So you get a high level of improved safety by packing the secondary separately.  The second thing to do is, remove the corrector plate and pack it in a separate box the way you would pack a dob mirror.  It has to be completely tight and cushioned in the first box and then surrounded by a second box with additional bubble wrap.  The primary, locked down with the shipping screws, is actually pretty secure.  Close the tube up with the dust cover.  

 

regards, 

Greg N


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#4 gnowellsct

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Posted 08 August 2018 - 12:32 AM

John might actually like this document....maybe he can figure out what those mysterious marks are in pictures 3 and 4.   This is the write up I did of my smashed c14.  --Greg



#5 gnowellsct

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Posted 08 August 2018 - 12:39 AM

A photographic essay on packing SCTs and refractors...

 

1 correct  with baggie.JPG

 

2 lens paper and bottom layer.JPG

 

3 top cover and letter.JPG

 

4 packing the tube.JPG

 

5 - c8 inner box in outer box.JPG



#6 gnowellsct

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Posted 08 August 2018 - 12:47 AM

I should mention that the hard plastic storage box in pics 1-3 was quite tough and got double boxed in the same manner as shown in 4 and 5.  So that c8 went out in two boxes, each box insured for full value, because if one box goes, the whole system goes.   

 

The corrector plate of a c14 is not that hard to pack for shipping but I do think that a C14 OTA would be daunting.  I would be tempted to dismantle it. 

 

some more pics:

 

 

6 top with packing foam.JPG

 

7 c8 ready to go.JPG

 

8 refractor box (676 x 507).jpg

 

9 padding inside inner box.JPG

 

10 lens cell in inner box.JPG

 



#7 gnowellsct

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Posted 08 August 2018 - 12:48 AM

Anyhow that's how I pack.  So far so good.  Also it turns out I got my C14 around June of 2001.  For some reason I had thought it was 2002.  It took about 6 months for the second one to come.   Greg N



#8 jhayes_tucson

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Posted 08 August 2018 - 12:51 AM

Greg,

Maybe tightening that ring helps but if so, it indicates that the potting compound was probably not properly applied in the first place (and that has happened) or that the mirror has somehow come loose from the potting compound.  Over-tightening the nut at the base of the baffle on the front of the mirror can definitely distort the mirror, which may cause poor imaging performance.  It takes almost nothing to distort a C14 mirror figure.  This seems completely counterintuitive since the retaining ring is at the center of the mirror so it doesn't seem like it should be able to change the figure by compressing the mirror at the center; but, I've seen it happen and it can be significant.  If you can tighten the ring and make things better without distorting the mirror, I'm good with that; but, you just have to recognize that:  

 

1) That retaining ring is not there to control mirror flop.  In fact, Celestron removed that threaded ring from later models and to my knowledge, none of the C14HD scopes have it.

 

2) It is very easy to inadvertently over-tighten the ring in a way that affects imaging performance.

 

Finally, with regard to the effect grease has on focusing, I've never seen anyone produce any actual data on this.  Sure a lot of folks add grease and they think that it helps, but I'd like to see someone do a before and after comparison like I did with the design that I did for Celestron (https://www.cloudyni...cuser/?hl= rasa).  That's the way to tell what it really does and how to demonstrate how long it lasts.

 

John


Edited by jhayes_tucson, 08 August 2018 - 11:56 AM.


#9 jhayes_tucson

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Posted 08 August 2018 - 12:58 AM

Anyhow that's how I pack.  So far so good.  Also it turns out I got my C14 around June of 2001.  For some reason I had thought it was 2002.  It took about 6 months for the second one to come.   Greg N

 

Greg,

Your packing looks very nice, but you lost me somewhere along the way.  How did OP's question about mirror flop morph into a description about how to pack a scope for shipping?  It's late and I'm tired.  Maybe I'm missing something...  tongue2.gif

 

John


Edited by jhayes_tucson, 08 August 2018 - 12:59 AM.

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#10 ccwemyss

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Posted 08 August 2018 - 12:39 PM

Thanks for the feedback!

 

The ring was definitely loose. It was easy to turn it about 120 degrees clockwise, until it was firmly snug. My wrench is a square piece of wood, only a bit bigger than the ring, with a hole to fit over the slider tube, and a pair of screws going though it that fit into the holes in the ring. I built it so that it wouldn't provide a significant lever arm. I tightened it with about the amount of force that I would use in snugging a large filter onto a telephoto lens.

 

The result was a substantial decrease in flop, similar to what was reported in other threads and the instructions that were originally posted on Yahoo. I haven't seen any distortion in the diffraction rings after doing this (as best I can tell with the poor seeing). 

 

But the question is still whether the remaining flop is out of spec for an older C14, or if people are used to pointing carefully with a finder at nearly 400X and not having objects always inside the inner 50% of the FOV in the eyepiece.

 

I recalculated the displacement after posting. At 3/4 out in an 11' field, it is more like 4.1'. Given that focus shift is about 0.4', which I assume is mostly the slider tipping on the baffle tube, that would leave 3.7' that is from flop of the mirror with respect to the slider.

 

I was just hoping to hear from enough people about what they see in their scopes to establish a range of what is normal in the field. I think that if we can gather that data, it will also be a useful benchmark for the community.

 

As to the suggestions for splitting it up and packing it to ship, that looks great for general shipping. But Celestron wants to do a full cleaning and collimation (including ensuring that the Fastar housing and corrector are properly aligned and centered). They don't want to then disassemble the scope and return it in pieces, and they only reuse the customer's packaging. So they want it packed as an assembled unit in a way that they can easily repack for the return trip. I could pay UPS to custom build a box, but I'm still nervous about the chip in the corrector becoming a crack when subjected to shock.

 

With regard to regreasing, I also have my doubts, as it seems like a temporary solution. Although a thicker grease like Slick 50 would probably extend the effectiveness for long enough to be quite useful. I've also seen John's thread where he and another member talk about drilling holes in the slider and inserting nylon screws to take up the slack, but I'm not yet ready to pull the mirror out and go to that length for what would be less than half a minute of movement.

 

With respect to the fact that newer units don't have retaining rings, I wonder if there might have been a reformulation of the RTV in the meantime that makes it more rigid. Is it possible that the older RTV had a tiny bit of flex and that the ring and cork washer were meant to supply a bit of compression? I've seen some of John's earlier posts where he talks about the effects of microns in movement being amplified by the secondary into very visible effects.

 

My main interest is being able to build a good pointing model, and retaining collimation. I wanted to step up in aperture over my C11 so that I could go after visual observations of fainter galaxies and to get better resolution on planets and globulars. That requires accurate gotos and good collimation. 

 

I will also say that, when the seeing was good (which was before tightening), when it was at an angle where collimation was good, the views of Jupiter and Saturn were excellent. So optically, it seems to be a quality unit.

 

Regards,

Chip W.


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#11 gnowellsct

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Posted 08 August 2018 - 03:10 PM

Greg,

Your packing looks very nice, but you lost me somewhere along the way.  How did OP's question about mirror flop morph into a description about how to pack a scope for shipping?  It's late and I'm tired.  Maybe I'm missing something...  tongue2.gif

 

John

He was talking about sending it into Celestron and had concerns about the risks.   Celestron said it could fix the side to side problem.  

 

That's about it.  

 

I agree that we lack controlled studies on this stuff but....that's the case for about 98% of the DIY stuff in these fora.  What the heck is potting compound?  Do you have a pic?

 

There is the additional issue of the placebo effect.  Suppose you do the lube and you think it worked really well.  And you think that for the next twenty years until you croak.  You're happy.  You have a C14 with no image shift.  And the problem with this is.....?

 

John don't forget to look at the C14 autopsy report link.  (posted above, post #4) I'd like your view on the weird markings on the secondary holder.  

 

thanks

Greg N


Edited by gnowellsct, 08 August 2018 - 03:11 PM.


#12 gnowellsct

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Posted 08 August 2018 - 03:16 PM

Thanks for the feedback!

 

The ring was definitely loose. It was easy to turn it about 120 degrees clockwise, until it was firmly snug. My wrench is a square piece of wood, only a bit bigger than the ring, with a hole to fit over the slider tube, and a pair of screws going though it that fit into the holes in the ring. I built it so that it wouldn't provide a significant lever arm. I tightened it with about the amount of force that I would use in snugging a large filter onto a telephoto lens.

 

The result was a substantial decrease in flop, similar to what was reported in other threads and the instructions that were originally posted on Yahoo. I haven't seen any distortion in the diffraction rings after doing this (as best I can tell with the poor seeing). 

 

But the question is still whether the remaining flop is out of spec for an older C14, or if people are used to pointing carefully with a finder at nearly 400X and not having objects always inside the inner 50% of the FOV in the eyepiece.

 

I recalculated the displacement after posting. At 3/4 out in an 11' field, it is more like 4.1'. Given that focus shift is about 0.4', which I assume is mostly the slider tipping on the baffle tube, that would leave 3.7' that is from flop of the mirror with respect to the slider.

 

I was just hoping to hear from enough people about what they see in their scopes to establish a range of what is normal in the field. I think that if we can gather that data, it will also be a useful benchmark for the community.

 

As to the suggestions for splitting it up and packing it to ship, that looks great for general shipping. But Celestron wants to do a full cleaning and collimation (including ensuring that the Fastar housing and corrector are properly aligned and centered). They don't want to then disassemble the scope and return it in pieces, and they only reuse the customer's packaging. So they want it packed as an assembled unit in a way that they can easily repack for the return trip. I could pay UPS to custom build a box, but I'm still nervous about the chip in the corrector becoming a crack when subjected to shock.

 

With regard to regreasing, I also have my doubts, as it seems like a temporary solution. Although a thicker grease like Slick 50 would probably extend the effectiveness for long enough to be quite useful. I've also seen John's thread where he and another member talk about drilling holes in the slider and inserting nylon screws to take up the slack, but I'm not yet ready to pull the mirror out and go to that length for what would be less than half a minute of movement.

 

With respect to the fact that newer units don't have retaining rings, I wonder if there might have been a reformulation of the RTV in the meantime that makes it more rigid. Is it possible that the older RTV had a tiny bit of flex and that the ring and cork washer were meant to supply a bit of compression? I've seen some of John's earlier posts where he talks about the effects of microns in movement being amplified by the secondary into very visible effects.

 

My main interest is being able to build a good pointing model, and retaining collimation. I wanted to step up in aperture over my C11 so that I could go after visual observations of fainter galaxies and to get better resolution on planets and globulars. That requires accurate gotos and good collimation. 

 

I will also say that, when the seeing was good (which was before tightening), when it was at an angle where collimation was good, the views of Jupiter and Saturn were excellent. So optically, it seems to be a quality unit.

 

Regards,

Chip W.

Chip what happens is that they receive the parts the way you ship them and then they ship it back using their preferred techniques and packaging.  That's what happened on my c8.  What you see is the c8 in the pix going out for recoatings with an aftermarket company.  That did not work well (it came back with coatings but poorly aligned).  Some years later I again packed the c8 and shipped it off to Celestron--in fact, in 2015.  They offered me new optics (for a price).  So they stuck the optics in my tube and sent it back THEIR way.

 

As it happens they have this foam stuff they squirt around the tube that hardens and creates a really tight solid packing but doesn't crud up anything.  You can buy this stuff and I took a hard look at it but the cost at the retail level is very high.  So high that I removed it from further consideration.  But if you were a company buying it in bulk and had to consider the tradeoff between using it and insurance claims and what have you, it becomes effective to use.

 

So my advice is, let Celestron worry about getting it back to you.  They know how to ship SCTs and have a lot of industrial techniques for doing so.  The series I posted is what to do if you're some rube wandering around Home Depot trying to figure it out.

 

Greg N

 

p.s. all the boxes and stuff you use to ship to them don't come back.  One way trip.


Edited by gnowellsct, 08 August 2018 - 03:28 PM.


#13 gnowellsct

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Posted 08 August 2018 - 03:50 PM

So far as tightening the ring goes when I wrote to Celestron for advice they told me not to do it. They said I would PINCH THE OPTICS.   However, I was in a mood to break the rules.  I knew what they meant.  I have seen pinched optics results for both a poorly mounted Newt primary and another Newt's poorly mounted secondary.   

 

Both those mirrors survived and were un-pinched and remounted.  So I figured I would just back the ring out if it was an issue.  That was  a very critical juncture, if I had had to back the lock nut out and returned to viewing the scope losing collimation every time I went to a new part of the sky, I would have given up on the C14 and chalked it up to life experience.  I'd probably be one of the anti-SCT trolls on this list.   I actually don't know what I would have done given the cost and aperture limitation of apos and my strong preference for GEMs, and the difficulty of using Newts on GEMs.  

 

Anyhow Chip, if the error is 3 or 4 arc minutes you're sort of in a zone where you can go either way.  The live-with-it zone is something like this: use it as is and when you want super-good collimation (for example, on Jupiter) collimate *locally*, in this case on a Jovian moon.  If you want super good collimation on a deep sky object collimate on some mag 8 star in the neighborhood.    All of this collimation and recollimation only makes sense if you are settling in to image something for an hour or two or three.

 

The other thing you can do is get one of the after market cures to keep the mirror from moving around.  This usually involves some kind of mirror lock along with an aftermarket focuser.

 

All of these possibilities have their own pluses and minuses.  

 

Unfortunately for Celestron, the lack of clear cut method for dealing with these issues opens the market to $21,000 alternatives.  

 

So far as my own issue goes, and how I handled it, I sometimes get to a point where I feel that I have nothing to lose.  When my c8 came back messed up after its recoat I didn't deal with it for several years until finally I said well throw it out or fix it.  Not having anything to lose I tried to un-mount it from the after-market RTV and that was beyond my capability.  Judging from what I read on the net undoing RTV once it's set is beyond a lot of people's capabilities.  THEN I shipped it off to Celestron.  They didn't have time to mess with solid RTV any more than I did.  So the old optics got chucked and in went the new.  

 

A c14 is obviously pricier, but I did reach that moment in 2001 where I figured the situation was unreasonable.  And into the OTA I went.   If I were in that frame of mind on your scope I would back out the primary lock nut, put some slick 50 on the threads, and see if I can get it to go another 1/4 turn or so.  Because the machining on the threads of the lock nut allows for some binding, you might be able to tighten it more without deformation of the optics.  

 

But I am more conservative now.  The scope is a $4 or $5k OTA and not a c8 you picked up for $300.  Taking all things into consideration, and John's persistent protests, I would ship it off to Celestron.  That's what I would do at my current stage in life.

 

There is the risk that it comes back to you with the identical problem or worse, because when they "fix" it, they might not be aggressive enough in tightening the lock nut....or doing whatever it is they do to control the problem.

 

Greg N



#14 gnowellsct

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Posted 08 August 2018 - 04:15 PM

Incidentally when I do a ten star pointing model using Argo Navis and 10k tic encoders, I usually end up with a 2.5 ish arc minute RMS and a PSD of anywhere from 4 to 5 arc minutes.  This is with the C14.  So it's pretty easy for me to get all-sky pointing accuracy that allows me to use an eyepiece with a 10 arc minute true field.

 

That is with the computer compensating for *many* sources of pointing error.  I dunno, 8 or 10 mount related variables and also a collimation error value.  

 

If I had the same issue you are reporting, that is 4 arc minutes of mirror error moving around all sky, just from the collimation/mirror movement, the RMS would be larger and so would the PSD, and my pointing accuracy would be considerably less.  And in fact, FWIW, the CH variable (collimation) is usually turned off in my pointing models.  All of this is to say that I think the primary lock nut tightening did a good job back in 2001. 

 

HOWEVER, in those days Argo Navis was not on the market and a computerized analysis of pointing error variables would have required something like the SKY software hooked up via a laptop.   I didn't do that.   And even so there is a certain *intrinsic* variability due to the (relatively) low resolution encoders....about 1.5 arc minutes if I recall.   So basically I'm getting only an arc minute or two of variability all sky factoring in every source of flecture on the mount as well as the optic.  I mean....using a finder to double check a c14 is going to have a certain intrinsic error due to eye positioning even if the finder is 100% rock solid (which mine never are by the way, and I use a variety of brackets...Losmandy, Tak, what have you).

 

Let me put it this way.  When I build a pointing model using my f/6.3 130 mm apo, the reported pointing accuracy of my mounts DECREASES.  Why is this?  Because when I'm centering the f/6.3 with a 10 mm XW the field of view is 51 arc minutes.  The field of view I usually center in with my C14 is 14 arc minutes.  When you visually center in a 50 arc minute field of view being "a smidgeon off" will be quantified as a larger error.  That doesn't matter though because the required accuracy depends on the intrinsic fields offered by the scope.

 

THE CENTERING ERROR I MAKE WITH MY EYE IN A 51 ARC MINUTE FIELD is small, but it is LARGER than the centering error I make in a 14 arc minute field in my c14.  You might be *duplicating* this mistake by going from the low power finder to the C14.    It's not really a mistake, you just have to know what's going on.  If I get an RMS of 6 or 8 arc minutes and a PSD of 10 or 12 when I'm using my GT130 that's not a problem.  Functionally, it is as accurate as 2.5 arc minutes RMS and 5 arc minutes PSD on the C14.  The reason being that I'm *almost always* using lower magnifications in the refractor and there's no real reason to build a model using a 16 arc minute field of view (what I would get with a 3.5 XW).  All it does is make it harder to sight the star into the field without delivering functionality.    

 

It seems to me, therefore, that your centering ability in a finder is going to be intrinsically worse no matter how well it is lined up.  And not by a little.  Maybe by 30 arc minutes or so.

 

So I don't know what the confidence level of your estimation technique is.  If you have the means to do so I would recommend a 50 or 100 point model all-sky where you get a computerized read out of the pointing error sources, and check CH to see just how significant it is.   I'd probably do this before sending the tube off to Celestron unless you are confident that there is an egregious loss of collimation with all sky pointing or after meridian flips.  You would have to review how pointing models or done.  Its buried in the Argo Navis instructions which are on line, and when you do a Gemini pointing model you get feedback on the same variables.  I would have to think that The Sky or other laptop pointing models give similar information.

 

Greg N   


Edited by gnowellsct, 08 August 2018 - 04:19 PM.


#15 jhayes_tucson

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Posted 08 August 2018 - 04:19 PM

.... What the heck is potting compound?  Do you have a pic?

 

John don't forget to look at the C14 autopsy report link.  (posted above, post #4) I'd like your view on the weird markings on the secondary holder.  

 

thanks

Greg N

 

You can read about potting for electronics components here:  https://en.wikipedia...ng_(electronics).  In optics, potting compound is typically a two part silicon based epoxy with the desired thermal characteristics to prevent distorting a component over a wide temperature range.  By properly treating the mating surfaces, the potting compound will stick to glass and metal so that it bonds to the surfaces, fills gaps, and provides a sufficiently rigid mount so that the components don't move.

 

As for the weird marks on your secondary holder, they look like something that might have been done at the factory.  I would guess that some goofball worker decided that since two parts weren't fitting together, he'd take a hammer to it.  It looks to me like he might have been trying to "adjust" the roundness of the part.  I've seen some very high quality parts come out of China but this is one of the reasons that Chinese parts are so cheap.  At many factories, they still hire and train some very unsophisticated workers...for extremely low pay.

 

China has come a long way since my earliest visits to disk drive factories where they would hire young women with excellent vision and small fingers to hand wind the magnetic poles on read/write heads in very early hard drives.  They paid these women about 1$/day and the women could only do the job for a few years before their vision diminished enough that they couldn't see the parts any more.  It was crazy but they had gobs of women who would come with no education or training from the countryside to stand in line for the jobs.  I still remember that at night, the women would get all dressed up to go on the town looking for guys...but there mostly weren't any.  The flip side was that the Chinese guy that I worked with at the same factory had a Ph.D. from Harvard and he was one of the smartest guys I knew.  So, I'm sure that Synta has some very good people but it's hard to believe that one of them would have taken a hammer to that part!

 

 

John



#16 jhayes_tucson

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Posted 08 August 2018 - 04:22 PM

So far as tightening the ring goes when I wrote to Celestron for advice they told me not to do it. They said I would PINCH THE OPTICS.   

...

 

Yep, that's exactly what I'm talking about.

 

John


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#17 gnowellsct

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Posted 08 August 2018 - 04:26 PM

You can read about potting for electronics components here:  https://en.wikipedia...ng_(electronics).  

 

As for the weird marks on your secondary holder, they look like something that might have been done at the factory.  I would guess that some goofball worker decided that since two parts weren't fitting together, he'd take a hammer to it.  It looks to me like he might have been trying to "adjust" the roundness of the part.  I've seen some very high quality parts come out of China but this is one of the reasons that Chinese parts are so cheap.  At many factories, they still hire and train some very unsophisticated workers...for extremely low pay.

 

 

John

Thanks for the link, and taking  a look at those marks.    My C14 was pre-China syndrome, so the goofball worker, if that's what it was, was probably a stoner from Gardena.  -Greg


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#18 gnowellsct

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Posted 08 August 2018 - 05:30 PM

Re-reading, Chip, from above, it appears that you are using some kind of pointing software but you haven't indicated what the *other* sources of pointing error are in your model.  There could be a backlash adjustment problem, for example.  A worm gear might have too much play.  There might be other sources of flex in the mount.  GN



#19 Tom Glenn

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Posted 08 August 2018 - 05:36 PM

As I've commented in other threads on this topic, I also experience collimation shift with my SCT, in this case a C9.25 Edge.  I just deal with constant collimation, because I use the scope almost exclusively for planetary and lunar imaging, and so after collimating on a nearby star I don't touch the scope for an hour or so.  But now I have an additional related question that perhaps those experienced with these issues can address.  One of the three collimation screws is very tight, while the others are much less so.  Naturally, this tight screw is usually the one that would need to be tightened even more to adjust collimation, but since it is already very tight, I have to loosen the other screws slightly to achieve the same result.  This is not ideal.  I have on several occasions backed all the screws out to start completely from scratch, only to arrive at the exact same situation in which one screw (always the same screw) is overly tight compared to the others.  Are these problems related, or different?  Would whatever is causing this asymmetry in screw tightening also be causing the collimation to shift when moving the OTA to different angles in the sky?



#20 gnowellsct

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Posted 08 August 2018 - 06:08 PM

As I've commented in other threads on this topic, I also experience collimation shift with my SCT, in this case a C9.25 Edge.  I just deal with constant collimation, because I use the scope almost exclusively for planetary and lunar imaging, and so after collimating on a nearby star I don't touch the scope for an hour or so.  But now I have an additional related question that perhaps those experienced with these issues can address.  One of the three collimation screws is very tight, while the others are much less so.  Naturally, this tight screw is usually the one that would need to be tightened even more to adjust collimation, but since it is already very tight, I have to loosen the other screws slightly to achieve the same result.  This is not ideal.  I have on several occasions backed all the screws out to start completely from scratch, only to arrive at the exact same situation in which one screw (always the same screw) is overly tight compared to the others.  Are these problems related, or different?  Would whatever is causing this asymmetry in screw tightening also be causing the collimation to shift when moving the OTA to different angles in the sky?

I've never had the "do a reset on the screws" system fail so I'm not sure what to say.  It is possible that various issues with the secondary/corrector can cause all sky shift.  If the corrector can actually shift around that could be an issue.  If the secondary housing can move within its spot within the corrector, that's an issue.   It might be worth some time to back out the tight screw, measure its length, then measure the other two.  ONLY ONE AT A TIME THOUGH!  You don't want secondary to fall off.  It would be *weird* if they were not all the same length, but lots of weird things can happen.  As with telescopes, screws are manufactured objects that are produced to within tolerances.  You may have a screw that is not within spec.  Best done with a micrometer.

 

If the secondary, which is held by some kind of adhesive sticky surface, is actually having some slop in its attachment to the backing, and is sort of flopping, that's a potential FATAL ERROR because if it falls off it will wreak havoc.  You can see a picture of what holds the secondary on if you read the report in post #4.  A visual inspection and a moderate poke with a pencil or pen from the side to make sure it's holding firm might not be a bad idea.

 

Greg N



#21 Tom Glenn

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Posted 08 August 2018 - 06:24 PM

OK thanks Greg.  I'll have to do some poking around.  These issues are super frustrating for sure, and would probably drive me away from SCTs if it weren't for the issues that you mentioned above....namely that I need a fairly large aperture scope that is easily mounted on an EQ mount, and that has a wide flat field because I do lunar imaging with moderately large sensors with small pixels (hence the Edge scope).  And the scope does produce excellent images, but requires a lot of fussing around before each session.  



#22 gnowellsct

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Posted 08 August 2018 - 06:36 PM

Incidentally when you've been messing with all three screws you're likely to get the comet star effect.  The secret is to "chase the tail" of the comet star, that is, make adjustments such that as the collimation star moves, it is chasing its tail.  Astromart appears to be down or I'd point you to an article.

 

One random thought is to mark the problematic screw and its position with some nail polish so you don't forget where it goes.  Swap it one of the other two screws and see if the problem moves with the screw.  

 

Greg N



#23 ccwemyss

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Posted 08 August 2018 - 07:36 PM

Re-reading, Chip, from above, it appears that you are using some kind of pointing software but you haven't indicated what the *other* sources of pointing error are in your model.  There could be a backlash adjustment problem, for example.  A worm gear might have too much play.  There might be other sources of flex in the mount.  GN

The problem building a model was before I tightened the ring. I was just building a basic Gemini model with 6 stars on either side of the meridian. But with nearly half a degree of shift in the field, it was struggling to get each star in the widest field eyepiece. I was dutifully centering each star, while puzzling over whether the weight of the C14 was stressing it, or if there was something else keeping it from doing a better job of slewing to each new star.

 

Once the model was built, I started having it slew to objects, and they were still barely in the 42' field. If I centered and aligned on them and slewed to something nearby it worked OK since there wasn't much flop. But if the slew was more than about 30 degrees, the pointing was off.

 

That's in contrast to my experience with a 6" f9 refractor on the mount, where it consistently put objects close to the center of the field. So the mount is known to work well, albeit with a lighter load (with balancing weights, the refractor is about 30 pounds, and has a much longer moment arm than the C14). 

 

Like I say, I just want to get a sense of whether the 4' flop that I'm seeing is typical or unusual. Do other people see similar amounts?

 

Regards,

Chip W.



#24 ccwemyss

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Posted 08 August 2018 - 08:08 PM

 I mean....using a finder to double check a c14 is going to have a certain intrinsic error due to eye positioning even if the finder is 100% rock solid (which mine never are by the way, and I use a variety of brackets...Losmandy, Tak, what have you).

.

.

.

It seems to me, therefore, that your centering ability in a finder is going to be intrinsically worse no matter how well it is lined up.  And not by a little.  Maybe by 30 arc minutes or so.

 

So I don't know what the confidence level of your estimation technique is. 

 

Greg N   

I've used three different finders, and for each test, I flipped back and forth at least three times to ensure repeatability. If I didn't get it, I would realign everything and start again. One of the finders did have some issues with a loose dovetail shoe, but the other two are very solid. I went to the third one because it has very fine crosshairs. 30' is about 1/6 of the field of the finder, so that much offset would be very far from the crosshair.

 

When the scope is on one side of meridian, centering on the crosshairs in the finder consistently puts the object within the inner 10% of the 11' field of the eyepiece. Centering the same way on the other side of the meridian puts the object 2/3 to 3/4 of the way out from center, always in the same direction. As noted in my original post, I methodically eliminated other sources of movement or flexure until, in consultation with Celestron, it was narrowed down to mirror flop. 

 

Before I tightened the retaining ring, the finder would center objects on one side of the meridian, but on the other side, the object would be beyond the 42' field of a 41mm Panoptic. Since I tightened it, I've just been trying to measure the residual flop, and get a sense of the effect on collimation.

 

I haven't tried building a model with the mount yet. Interestingly, given my experience, I might actually get a better model from carefully using the finder for the alignment if I start by aligning it when the scope is pointed near the zenith, rather than laying on one side or the other. It would stay more consistently aligned with the axis of the tube than the primary, and objects in the eyepiece might have less displacement at other angles. 

 

Regards,

Chip W.



#25 ccwemyss

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Posted 08 August 2018 - 10:31 PM

Chip what happens is that they receive the parts the way you ship them and then they ship it back using their preferred techniques and packaging.  That's what happened on my c8. 

.

.

.

 

So my advice is, let Celestron worry about getting it back to you.  They know how to ship SCTs and have a lot of industrial techniques for doing so.  The series I posted is what to do if you're some rube wandering around Home Depot trying to figure it out.

 

Greg N

 

p.s. all the boxes and stuff you use to ship to them don't come back.  One way trip.

I guess it is worth asking again. In our back and forth, I asked about packaging, and they said that they don't supply it. But maybe that just meant for the trip there. 

 

At this point, however, I will have to wait until December because I intend to use the scope for a class I teach in the fall, and I know that if I send it in now, it won't come back in time for that.  

 

If it does have an accident on either side of the trip (which does happen, even for new scopes), the insurance for an 18-year old scope certainly won't cover replacing the optics. 

 

In the meantime, if I can make it usable, maybe I'll just learn to live with it.

 

Regards,

Chip W.




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