Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Is there a way to fix SCT Mirror shift

  • Please log in to reply
266 replies to this topic

#226 StarmanDan

StarmanDan

    Aurora

  • *****
  • Posts: 4537
  • Joined: 27 Aug 2007
  • Loc: Deep in the heart of Texas

Posted 12 September 2015 - 09:26 PM

I can't see why the engineers at Celestron can't resolve this problem that's been plaguing them from the get go. My Intes Micro 10" Maksutov had a simple helical type threaded  system that worked great. I don't recall any image shift. See this link and scroll down.

 

 

 

http://www.photoinfo...ter-M603-03.htm

Interesting design.  How do they keep the entire mirror assembly from rotating though?



#227 RandyC

RandyC

    Viking 1

  • *****
  • Posts: 665
  • Joined: 01 Apr 2013
  • Loc: RI

Posted 12 September 2015 - 09:36 PM

http://www.celestron...barticleid=2220



#228 PowellAstro

PowellAstro

    Vanguard

  • *****
  • Posts: 2109
  • Joined: 14 Oct 2009
  • Loc: Tennessee

Posted 12 September 2015 - 10:37 PM

The main baffle has a groove and the mirror mount uses a pin that rides in that groove. This keeps the mirror from rotating. You can see both on the last image.

Edited by PowellAstro, 12 September 2015 - 10:38 PM.


#229 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 6695
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 13 September 2015 - 12:21 AM

 

Hi John and PowellAstro, thanks for your help. I am going to try getting slightly longer bolts for the mirror locks so I can get to the second set of threads. I guided using the Celestron 80mm guidescope @600mm focal length and QHY5L-II mono camera. No focal reduction. Do you think one of the higher end collimators is worth it. http://www.ebay.com/...=item20dff8dca6


Hi Randy - you can use longer bolts as you suggest, this is a technique I used on my C14 previously to ensure a stable primary mirror which basically addresses any movement issues in this optical element...but of course once you do this you will not be able to shift the primary mirror!!! :(

For this reason I don't recommend using this method - why I've made the modifications I just have above - but it will certainly work well: most of the planetary imaging on my site was done with the additional 2 bolts in situ on my C14 - the only caveats I'd place is (1) the aforementioned (2) hi-resolution planetary imaging is done with constant focus-checking & employs only a small region at the centre of the FOV...but it will hold both collimation & focus very well once set, it's just that when you image at around 10 metres "focus" is a completely different kettle of fish to DSO imaging!

It is quite difficult to insert these additional bolts through the rear-casting threads PLUS the threads in the 2 unused arms of the triple-tree. (the 3-armed unit behind the primary which has the threads you refer to)

This is because (as anyone with any engineering knowledge should appreciate) you need very accurate alignment between the 2 sets of threads to do so...but don't worry about screwing them right through & hitting the primary, that likelihood is pretty far-fetched!

You will need to place the scope face-down on a clean floor, this lets gravity "settle" the primary into the best alignment ie, for those 2 sets of thread-holes.

Screw both bolts into the rear-casting first & continue screwing until they touch those 2nd set of thread-holes...it helps if you have a scope which has the vents on the rear-casting because removing these covers allows you to see into these areas inside the scope...I'd also recommend having the primary as far back (clockwise on the knob) as possible when doing this - it makes it difficult to see these part through the vent-holes but it does shorten the distance between the sets of threads which makes things easier from an alignment perspective...

You then carefully (making sure the bolt is properly engaging the thread) screw one bolt in no more than a turn & a bit & then do like wise with the other bolt - doing this ensure you are not pulling these elements out of alignment marginally, which would make the operation difficult & risk cross-threading.

Repeat this, alternating from one bolt to the other until they are fully-engaged - if you can't look inside because your model doesn't have the vents to remove just mark the bolt thread with a marker pen when you reach the 2nd set of threads (this is easily felt) & work on another 5mm of engagement.

That's it! :)

There are collimators specifically made for SCT's such as those using projection screens but you can't use any generic types: I always recommend people use the tried & true star-test at very high magnifications...as planetary imagers we have the advantage of being able to do this onscreen & thus take into account the entire imaging train when we do so, as well as look at the screen while we adjust...but I still think you're better off with an ep rather than any proprietary SCT collimator...but then again as far as I'm concerned collimation is something you do for each & every imaging session! ;)

 

 

Ah..I think that see what you guys are talking about.  Some of the older C14s just had a clearance hole in the rear casting.  That's what I had in my former straight C14 and that's what I was thinking about.  In that case, it is indeed quite possible (and even easy) to turn a long screw into the back of the mirror.  If both holes are threaded, it sounds like you are trying to gently cross-thread the through-bolt.  So are you focusing the system to correctly set the BWD and then trying to lock the mirror?  Unless you have an external focuser, I'm not sure how you are going to achieve accurate focus without a whole lot of messing around.  As I've said, this whole approach has the potential to put a lot of torque on the baffle tube and to drive the system out of alignment.  I will also point out that the rear support plate for the primary is a cast aluminum part.  I haven't messed with it a lot but I'd be a bit cautious about cross threading screws into it.  I've observed that some of the metal parts in the system are pretty soft and I don't know if that cast part has been heat treated in any way so it may strip out pretty easily.  You can probably get a way with it if you are gentle but if you are tightening and loosening frequently, you may eventually strip the threads.

 

I completely agree that this is not a recommended way to fix anything.   If you want to fix the mirror position the right way, you are better off just taking the system apart and using three stout standoffs to screw the mirror down in the correct position.

 

John



#230 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 12205
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 13 September 2015 - 01:49 AM

John, we adjust focus by micron shifts in planetary imaging...I apologise if anyone thought we DIDN'T use an external focuser. (ie, the one in the pik;))

Just as focus is absolutely critical, naturally so is collimation & the ability to hold collimation over the span of the imaging session...focus nearly always changes in all 3 channels (although regularly overall) with increases up to culmination & vice-versa the usual trend...

As said in my last post when imaging at around 10 metres any idea of stable focus over several hours is unlikely...but we do rely on stable collimation after adjusting the secondary at the start of the session...

I don't recommend it to Randy for the reasons given - but it is a completely stable method: true the arms (triple-tree) is a cast item but we actuate the primary with this & some folks use it as their only focusing mechanism as per Celestron intentions...this would involve similar if not greater risks in the manner you mention tbh!

I'm not sure what you mean by "cross-thread" - in Oz lingo that means misaligning threads with a setscrew/bolt etc such that you jam them, causing thread-stripping or burring: the method I've described above is to make sure you don't cross-thread the bolt when trying to pass it through 2 differently spaced thread inserts as we describe...as I said, from an engineering perspective this is all to do with alignment... :)

#231 RandyC

RandyC

    Viking 1

  • *****
  • Posts: 665
  • Joined: 01 Apr 2013
  • Loc: RI

Posted 13 September 2015 - 08:08 AM

Kokatha, the reason I chose the XLT over the Edge, (aside from price) is I am not crazy about having a permanent field flattener inside the tube. I also wanted my 13.5" of clear aperture unimpeded by glass, dust buildup or whatever. So, let me ask you as a planetary expert, do you think the Edge would be as good as the XLT. My opinion was the XLT would be slightly better for galactic images near the center of fov.

 

Also, it's too bad they can't put the same mirror lock mechanism on the XLT as Edge.


Edited by RandyC, 13 September 2015 - 08:27 AM.


#232 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 12205
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 13 September 2015 - 09:02 AM

Randy, I appreciate your concern about "extra glass" although I wouldn't expect the dust build-up from the corrector lens to add much...although dust (& moisture/condensation) can be a p.i.t.a - & the subject of the additional modifications I'm doing right atm.*

 

Planetary imagers who I respect such as Glenn Jolly (Desert rat) use Edge C14's & I see nothing that indicates he is handicapped because he is using that particular model...about the only thing I can glean is that those coolers (can't remember their name atm) that blow air up the baffle & into the OTA proper will not work because the baffle is blocked by that lens element - but imho those types of coolers are pretty pathetic...although that could be because of the demands of Australian weather conditions.

 

DSO's aren't my expertise Randy so I can't comment thereon, but I guess the centre of the FOV isn't really why they created the Edge series! ;)

 

It does seem ridiculous not to incorporate the same basic features on each model version but I guess they consider the Edge their "premium" scope & wanted extra bells & whistles for it: I use the XLT also & it is a planetary imaging scope par excellence...thank goodness Celestron realised making 2 different back-castings was bad production economics for themselves & used the same vented casting for the XLT also...a pity they stopped at that point!

 

The vents made it easy to add a mirror temperature sensor without dismantling the scope completely...I've done enough of this dismantling with Celestrons to last me a fair while before I want to do it again: this is also the reason why I affixed the guide blocks for the mirror locks in the manner I did as per the pik a few posts back - not my preferred method but I did manage to make them appear quite reasonable cosmetic-wise & they'll function just as well...but drilling out the back-casting thread-holes & re-tapping for a larger diameter would've necessitated a complete dismantling which I wasn't prepared to do, not with the Uranus & Neptune apparitions as they are presently!

 

* We suffer from a lot of fogging/condensation INSIDE the OTA, even on the secondary where it is especially problematic: I am fitting 2 fans, one an intake & the other an exhaust to the vents a la "Tempest" etc...except my unit is incorporating an acrylic/Perspex canister on the intake which will be filled with self-indicating granular (2.5mm-5.0mm) silica gel - a neoprene gasket under the corrector as well...I don't want to introduce additional air into the tube (for quicker cooling) if it brings in loads of water vapour given our problems therein of late!



#233 RandyC

RandyC

    Viking 1

  • *****
  • Posts: 665
  • Joined: 01 Apr 2013
  • Loc: RI

Posted 13 September 2015 - 10:09 AM

The main difference with deep sky is it's a low-light condition. For example, a barlow will produce basically nothing in a dso image. Expanding aperture to a 14" brings out slightly more detail than an 11", for example. Adding a focal reducer will speed up imaging times but reduce magnification. In my opinion, a flattener will reduce effective aperture. Celestron makes some of the highest magnification (focal length) SCTs. Having a full frame sensor, gives you the entire fov to work with requiring only a crop.

 

John thanks for your help on adding the longer screws. I am just worried I cant make any focus adjustments. It seems a little ridiculous they couldn't make the stock screws fit the entire focus range. I am going to talk with Celestron tomorrow. Compared to my 11" the aberration is excessive.


Edited by RandyC, 13 September 2015 - 10:11 AM.


#234 Psyire

Psyire

    Apollo

  • *****
  • Posts: 1362
  • Joined: 24 Jun 2007
  • Loc: 55* North

Posted 13 September 2015 - 01:31 PM

This was my solution, mirror flop was greatly reduced due to the mirror being stabilized by three axis instead of just one.  Is it gone? No, but it's much better than original.  (nearly un-noticeable)

 

http://www.cloudynig...in-my-cpc-1100/

 

Of course, there is the added benefit of being able to lock the mirror on a non-edge C11.



#235 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 6695
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 13 September 2015 - 03:13 PM

The main difference with deep sky is it's a low-light condition. For example, a barlow will produce basically nothing in a dso image. Expanding aperture to a 14" brings out slightly more detail than an 11", for example. Adding a focal reducer will speed up imaging times but reduce magnification. In my opinion, a flattener will reduce effective aperture. Celestron makes some of the highest magnification (focal length) SCTs. Having a full frame sensor, gives you the entire fov to work with requiring only a crop.

 

John thanks for your help on adding the longer screws. I am just worried I cant make any focus adjustments. It seems a little ridiculous they couldn't make the stock screws fit the entire focus range. I am going to talk with Celestron tomorrow. Compared to my 11" the aberration is excessive.

 

Hmm... help me to understand why you believe that "a flattener will reduce effective aperture" and which pupil are you talking about?

 

John



#236 RandyC

RandyC

    Viking 1

  • *****
  • Posts: 665
  • Joined: 01 Apr 2013
  • Loc: RI

Posted 13 September 2015 - 04:00 PM

Because everything comes at a cost. You can't flatten the field and also maintain the same resolving power. Something has to give.

 

It's a shame Celestron went with the RASA instead of focusing on larger lighter scopes and these issues.



#237 DuncanM

DuncanM

    Vanguard

  • -----
  • Posts: 2480
  • Joined: 03 Nov 2009
  • Loc: Arizona Sky Village or the rain forest

Posted 13 September 2015 - 04:09 PM

I think that it is possible to reduce SCT mirror shift by using rods, or other methods, to stabilize the mirror cell, but, IMHO, it is not possible to eliminate it entirely.

 

SCT mirrors cannot be locked rigidly or they will deform with temperature change. The mirror is typically fixed in the mirror cell with a rubber/silicon, or other, compound that allows the mirror to expand/contract and thus flex slightly over time and the larger and heavier the mirror, the more this is likely to happen.



#238 DuncanM

DuncanM

    Vanguard

  • -----
  • Posts: 2480
  • Joined: 03 Nov 2009
  • Loc: Arizona Sky Village or the rain forest

Posted 13 September 2015 - 04:21 PM

Because everything comes at a cost. You can't flatten the field and also maintain the same resolving power. Something has to give.

 

It's a shame Celestron went with the RASA instead of focusing on larger lighter scopes and these issues.

I think what you are saying is that on-axis resolution will suffer if the scope is optimized for off-axis resolution. Celestron's ray trace diagrams in their white paper state otherwise:

 

http://www.celestron...paper_final.pdf

 

See page 5.



#239 drmikevt

drmikevt

    Viking 1

  • *****
  • Posts: 814
  • Joined: 22 Jun 2015
  • Loc: Burlington, VT

Posted 13 September 2015 - 06:21 PM

I've read through this thread but did not see this question:  Can you add some type of add-on mirror lock to older SCT's (N11GPS)?

 

Thanks

Mike



#240 RandyC

RandyC

    Viking 1

  • *****
  • Posts: 665
  • Joined: 01 Apr 2013
  • Loc: RI

Posted 13 September 2015 - 07:47 PM

 

Because everything comes at a cost. You can't flatten the field and also maintain the same resolving power. Something has to give.

 

It's a shame Celestron went with the RASA instead of focusing on larger lighter scopes and these issues.

I think what you are saying is that on-axis resolution will suffer if the scope is optimized for off-axis resolution. Celestron's ray trace diagrams in their white paper state otherwise:

 

http://www.celestron...paper_final.pdf

 

See page 5.

 

Thanks for referring to the paper that Celestron put a ton of effort into when designing the EdgeHD. I believe their off- and on-axis analysis relates to a flat versus spherical field. The corrected EdgeHD field will definitely display better optical characteristics. The resolution they are referring to is the accuracy of the light beams themselves. What I am saying is that to transform the spherical to flat field using a flattener has a cost. The cost is to the detriment of overall resolving power of the distant object being observed. It is the resolution as relates to the aperture of the scope. In other words, the Celestron analysis of optical resolution is going to be basically the same for an 8" or 14" scope (see pages 7 and 8). The resolution I am referring to is obviously much better in the 14".



#241 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 12205
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 13 September 2015 - 07:55 PM

I've read through this thread but did not see this question:  Can you add some type of add-on mirror lock to older SCT's (N11GPS)? 
Thanks
Mike


Hi Mike - not sure what the N11GPS looks like at the back - is it this first pik below..?

I'd imagine that all C11's & C14's are convertible...the 2nd image is my old C11 after installing active cooling & the long bolts/setscrews with the more primitive version of the mirror locks. (fixed primary, more effort to unscrew to move primary)

Complete with home-made knobs using soft drink caps :grin: - I was scratching a bit for money in those days!!!

This was an excellent planetary imaging scope: there is no reason any of the more sophisticated versions of the mirror locks such as those I've recently made couldn't be engineered to these scopes...the only other comment I'd make is that like Psyire you need to affix any rods/bolts to the triple-tree if it doesn't have threaded holes already there & of course drill the rear-casing/casting - Psyire used JB Weld...I drilled & tapped holes into those other 2 arms - "yes John" - you certainly could hit the primary glass with the drill when it goes through...but I'm an old dog who has picked up a trick or 2 over the last 60-odd years! ;)

As someone has pointed out these types of mods are NOT perfect...but it's the same with almost anything tbh, you can always find faults/issues - but to all intents & purposes their effect is very good/positive: we collimate on a star near our target planet & I have every confidence over the course of 1-2 hours either side of transit this collimation will be stable.

Web7567.jpg

TestFire2.jpg

#242 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 6695
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 13 September 2015 - 07:57 PM

Because everything comes at a cost. You can't flatten the field and also maintain the same resolving power. Something has to give.

 

It's a shame Celestron went with the RASA instead of focusing on larger lighter scopes and these issues.

 

No, that's not right.  The optical design process works to minimize various parameters by optimizing numerous variables.  The variables are:  radius of curvature, index, dispersion, aspheric coefficient, spacings, etc.  The more things that you want to correct, the more variables you need--and that may require adding components.  In the case of the Edge system, adding two lenses near the rear of the system, allows excellent correction over the whole field as well as a flat field.  The diffraction limited flat field extends over quite a large field.  For example, the 8" Edge has a roughly 0.75 degree diffraction limited, aplanatic flat field.  For further reading, I suggest that you look at "Telescopes, Eyepieces, Astrographs; Design, Analysis and Performance of Modern Astronomical Optics" by Smith, Cerogioli, and Berry.

 

John



#243 drmikevt

drmikevt

    Viking 1

  • *****
  • Posts: 814
  • Joined: 22 Jun 2015
  • Loc: Burlington, VT

Posted 13 September 2015 - 08:16 PM

Kolkath

 

Thanks for that - very interesting!  No, my scope is older then that - the rear panel is flat and metal.  

 

I guess my real question was if anyone sold some type of mirror lock kit or service (like Hypertune, but for old SCTs - someone needs to do this)?  It would not be a good idea for me to take a drill to my scope - at all.

 

Mike



#244 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 12205
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 13 September 2015 - 09:46 PM

Kolkath

 

Thanks for that - very interesting!  No, my scope is older then that - the rear panel is flat and metal.  

 

I guess my real question was if anyone sold some type of mirror lock kit or service (like Hypertune, but for old SCTs - someone needs to do this)?  It would not be a good idea for me to take a drill to my scope - at all.

 

Mike

 

Hi again Mike - if there are NO holes in the rear of your scope other than the one that the primary focuser knob connects through to the inside then you are pretty well stuffed re making mirror locking mechanisms...

 

There are a couple of other approaches but all probably involve taking a drill to your scope - & I can understand folks' reluctance to do what seems like risky business..! :shocked:  :lol:

 

This interesting approach http://www.marsastro...Mirror-Lock.pdf  could possibly be entertained if the screw-holes which are in pairs (with stub-screws) around the circumference of a lot of Celestron models' rear-castings were used as the mode of applying pressure on those "clutch plates" the author devised...the pairs of screws are there for fitting things such as finder-scope shoes etc...

 

Of course you would still need to take your scope apart for this..! :)

 

In those circumstances where nothing is/can be/risked being done  :grin:  I'd just reiterate the practise of an anti-clockwise twist of the primary focuser knob as your final focusing adjustment...regardless of whether you have an after-market focuser fitted or not. :)



#245 RandyC

RandyC

    Viking 1

  • *****
  • Posts: 665
  • Joined: 01 Apr 2013
  • Loc: RI

Posted 15 September 2015 - 07:29 PM

If I get 2 custom bolts made that are longer, can this easily be fixed in a newer C14. If I alternately loosen and tighten these nuts while adjusting focus, will it even out. Or do you have to put so much torque on the mirror plate, that something will give way. Is using the focuser as one of the torque points too risky that something will strip? Thanks. 



#246 Kokatha man

Kokatha man

    Voyager 1

  • *****
  • Posts: 12205
  • Joined: 13 Sep 2009
  • Loc: "cooker-ta man" downunda...

Posted 17 September 2015 - 04:27 AM

If I get 2 custom bolts made that are longer, can this easily be fixed in a newer C14. If I alternately loosen and tighten these nuts while adjusting focus, will it even out. Or do you have to put so much torque on the mirror plate, that something will give way. Is using the focuser as one of the torque points too risky that something will strip? Thanks.



...not sure who you're asking this question of Randy - apologies if it has nothing to do with my postings but it seems like you're referring to using extra bolts to support the other 2 arms behind the primary so I'll respond... ;)

"Yes" it is easy to add them to the later C14's but because you are running them through 2 sets of threads some distance apart (ie, those in the back of the scope in the rear-casting & those in the 2 arms) any slight misalignment will make this feat quite difficult...that is why I recommend resting the scope face-down & screwing them into the 2nd thread (where any misalignment will rear its ugly head) very slowly & carefully, switching from one bolt to the other alternating between each as you screw them in...

Focusing by shifting the primary via its knob becomes well nigh impossible & any variation in the amount of torque each of the bolts applies (including the focus screw on its arm) will very slightly alter the primary mirror alignment. (via the small sleeve/baffle tolerances allowing minute variations in said tolerance at any particular arm's position...)

This could of course be used as another collimation adjusting technique but not recommended at all! And trying to shift the primary's position by working away at the 2 additional bolts & the focuser knob screw would be very arduous & fraught with some risk...

Therein lies the crux of the problem with this method when you use fixed bolts imo - slight misalignment of the primary this way can be accommodated in collimation via secondary adjustment but it does throw orthogonality awry which isn't optimal...my main reason for abandoning the fixed bolts was to allow resetting the primary for the various pixel-sized cameras we constantly test & use & also because of not wanting to apply these slightly uneven torques on each of the 3 primary mirror support arms...the rods in guides is a much more sophisticated & "trued" setup if it is put together properly, with all the benefits of the bolts +...

Apologies if I have made presumptions as to who/what you are referring to in your last post... ;)

#247 Edd Weninger

Edd Weninger

    Apollo

  • -----
  • Posts: 1056
  • Joined: 16 Feb 2014

Posted 17 September 2015 - 09:26 AM

Hi John,

 

Here is a different concept.  A different approach for the focuser sliding tube arrangement. 

 

I sketched one up years age for the Questar 3.5".  The Q7 uses linear bearings to prevent focuser shift.  Not enough room in the Q3.5.  I never had time to make some pieces and experiment with the idea.  This drawing is just a quick sketch to eyeballed C14 dimensions. 

 

Cheers,

 

Celestron mirror stuff-Model.jpg

 

 

.................That's it! :)

 

etc.
 

 

Ah..I think that see what you guys are talking about.  Some of the older C14s just had a clearance hole in the rear casting.  That's what I had in my former straight C14 and that's what I was thinking about.  In that case, it is indeed quite possible (and even easy) to turn a long screw into the back of the mirror.  If both holes are threaded, it sounds like you are trying to gently cross-thread the through-bolt.  So are you focusing the system to correctly set the BWD and then trying to lock the mirror?  Unless you have an external focuser, I'm not sure how you are going to achieve accurate focus without a whole lot of messing around.  As I've said, this whole approach has the potential to put a lot of torque on the baffle tube and to drive the system out of alignment.  I will also point out that the rear support plate for the primary is a cast aluminum part.  I haven't messed with it a lot but I'd be a bit cautious about cross threading screws into it.  I've observed that some of the metal parts in the system are pretty soft and I don't know if that cast part has been heat treated in any way so it may strip out pretty easily.  You can probably get a way with it if you are gentle but if you are tightening and loosening frequently, you may eventually strip the threads.

 

I completely agree that this is not a recommended way to fix anything.   If you want to fix the mirror position the right way, you are better off just taking the system apart and using three stout standoffs to screw the mirror down in the correct position.

 

John

 

 


  • bilgebay likes this

#248 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 6695
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 17 September 2015 - 12:08 PM

 

Hi John,

 

Here is a different concept.  A different approach for the focuser sliding tube arrangement. 

 

I sketched one up years age for the Questar 3.5".  The Q7 uses linear bearings to prevent focuser shift.  Not enough room in the Q3.5.  I never had time to make some pieces and experiment with the idea.  This drawing is just a quick sketch to eyeballed C14 dimensions. 

 

Cheers,

 

attachicon.gifCelestron mirror stuff-Model.jpg

 

 

 

Edd,

Thanks, that looks like an interesting idea.  The challenge is to get everything zero clearanced and smooth.  If you look at the tolerances needed to hold the image steady to within say an Airy radius, the angular slop allowed back at the primary is absurdly tight.  So a good solution really needs to be kinematic to insure that the constraints are about the correct axis.  It's also important to make sure that the stiffness of the system limits mechanical flexure, which relates to the way force is applied to the system.  Ultimately, there is more going on than simply constraining the slider motion.  It's important to recognize that this problem gets considerably more difficult for the larger systems like the C14 because of the larger mass and longer EFL.

John



#249 RandyC

RandyC

    Viking 1

  • *****
  • Posts: 665
  • Joined: 01 Apr 2013
  • Loc: RI

Posted 17 September 2015 - 01:14 PM

After carefully threading the two bolts in as you suggested, the focus knob could still turn 3-4 times in each direction before encountering any resistance. This really makes it impossible to get a quick fix since you would have to gauge where to put the bolts in so you could achieve correct resistance at 3-4 turns. I guess I could make custom bolts of variable length that would only thread into the second set of threads. It's a tricky endeavor since it depends on the focus position. Simplistically, I just need some bolts that are a little longer, like a half inch or so. I'm really not inclined to open my scope up since I don't trust myself to get into something like that.



#250 rolo

rolo

    Cosmos

  • *****
  • Posts: 8202
  • Joined: 14 Jan 2007
  • Loc: GA

Posted 17 September 2015 - 01:31 PM

Here's a way to permanently fix mirror shift, mirror flop, mirror anything. 

 

http://www.mcafabric...lding-3-400.jpg


  • RandyC likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics