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Pages: 1 | 2 | (show all) Tel Postmaster Reged: 03/31/06 Loc: Wallingford England Corrector Plate Rotation--- Myth or Fact ?       #5159187 - 04/06/12 01:01 PM Edit   Reply   Quote Quick Reply   Hi Folks, Maybe you can help by way of an explanation ? I am not disputing the accepted procedure but I am suddenly curious as to why corrector plate orientation IS so important when replacing this component, after, say, an internal cleaning session. According to Celestron's FAQ :- "How do I align my SCT corrector plate and secondary mirror" the following answer is given. Quote. "Alignment of your secondary relative to the corrector plate and the primary mirror is important to the proper optical performance of your 'scope. There are two alignments: corrector plate-secondary mirror assembly to primary mirror, and corrector plate to secondary mirror. These alignments should be understood before attempting to take apart the optics". Unquote. It is widely accepted that the procedure for the removal of the larger (8"+) Celestron corrector plates entails the pre-marking of their position in their cells together with the marking of the positions any shims present to ensure the said corrector plate is replaced in exactly the same place as before disassembly. Now, while I can understand the need for shimming in order to place the corrector squarely and centrally above the primary mirror, I don't comprehend why its rotation is so important since it is to all intents and purposes, a solid, mass produced, piece of glass with a (normally) fixed central cell which, in turn, holds the secondary mirror; the latter being the only non-fixed component when it comes to collimating the optics of these SCTs. Thus considering the above answer to the component alignment question and particularly the second of these, (i.e. corrector plate alignment to secondary mirror), how can any alignment be effected between a solid piece of glass and a fixed cell at its centre ? In summary therefore, I can understand the necessary alignment between primary and secondary mirrors, (collimation), and also the centralisation of the corrector above the primary, but not the reason for the corrector plate's apparently necessary rotational alignment relative to the primary. Sorry guys, but if you can provide an explanation I'd be very grateful, Best regards, Tel Post Extras:       orion61 Vendor(Clear Edge Optical) Reged: 10/20/07 Loc: NW Iowa BURRRR Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Tel]       #5159221 - 04/06/12 01:22 PM Edit   Reply   Quote Quick Reply   Theis is true, to an extent. BUT on Meades the corrector only has to be locked in with the primary, secondary is a sphere On Celestrons here is the deal. Up untill the Chinese made scopes all 3 components need to be aligned together.They used to do small hand corrections on the secondary. The corrector was rotated to reduce spherical aberration. All the Chinese optics have spherical secondarys. The computer controled process is much more accurate and does not need hand figuring on a normal basis. I remember getting an old Meade 2120 10" scope very cheap because the guy wasn't happy with the performance. I happened to have one with a pretty beat up tube but good optics, I was going to swap the optics. When I got it home I tried rotating the tube in 1/2" measurements. About 1/4 of the way around the test image just popped into focus, making 1/8" then 1/16" measurements made the system very good! That is my personal experience with this issue.Someone must have cleaned it and never marked it. I know about Meads secondary when I bought one of the MCSOG (silvered) secondarys that tarnished. Meade rpl'd it free and they are the ones that told me not to worry about rotation. Post Extras:       Midnight Dan Postmaster Reged: 01/23/08 Loc: Hilton, NY, Yellow Zone (Bortl... Re: Corrector Plate Rotation--- Myth or Fact ? [Re: orion61]       #5159332 - 04/06/12 02:36 PM Edit   Reply   Quote Quick Reply   Hi Tel: From what I have read (I believe it was from Uncle Rod, our SCT expert), the corrector and the primary mirror are a matched set. Not sure if I have it quite right, but it goes something like this. While both the primary mirror and corrector are mass produced, like any high quality polished optics, they will not be perfect. At the factory, they select a primary and corrector plate with optical imperfections in the surfaces that can essentially cancel each other out - at least to some degree. Since these imperfections often affect one side or the other of the mirror or corrector, they must be aligned with each other to get the maximum benefit. If you rotate one relative to the other, the views will suffer. That's my story and I'm sticking to it! -Dan Post Extras:       Naturlich Carpal Tunnel Reged: 10/30/09 Loc: Dukinfield, Cheshire, UK Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Midnight Dan]       #5159851 - 04/06/12 09:00 PM Edit   Reply   Quote Quick Reply   Orion and Dan says it how I've read it in the past. Whilst both Meade and Celestron like to advertise them as matched sets, they're not really. Orion did just what they do, they drop a corrector and secondary in with a primary and rotate it until the image produced is within the given specs. Whilst this works to an extent, they stop once it's within specs, and it's often found that the image can be improved by going through the procedure yourself. The marks on my LX200 show where my corrector needs to be in order for it to be "in spec", but I could go and rotate it little by little and find a better position. So, are they matched sets, well I guess you have to say yes to that, BUT, they ARE NOT constructed to match, I mean really, can you imagine the time and expense to create a primary mirror, do a wave front analysis and then create a perfectly matching corrector to remove any distortion found. I mean I know they did that for Hubble but not for us lol What it does mean is that you should be able to use any primary/corrector combination as long as you take the time to find the best rotational position for the corrector to produce the best image. Secondary's are a slightly different beast which we've argu.. er.. discussed on the Meade forum not to long ago. I maintain that they are spherical and not designed to correct anything at all, thats the job of the corrector plate. Others disagree. I did though feel like there was a sweet spot with the secondary rotation, though the secondary and it's housing have no alignment markings. Anyway, thats my little addition to the thread Nat Post Extras:       Tel Postmaster Reged: 03/31/06 Loc: Wallingford England Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Naturlich]       #5160206 - 04/07/12 06:40 AM Edit   Reply   Quote Quick Reply   Hi Larry, Dan and Nat, Many thanks for all your comments. I don't know about you, but I think it sometimes pays to question the origins of what knowledge we absorb as authentic when, if we were to take a step back occasionally and analyse what we have read, heard and subsequently tell others, we might just see things in a slightly different light. Nat, your explanation fits almost exactly that which I had in mind given, as I tried to convey in the original post, the mass produced and relatively low cost nature of our SCT 'scopes. It would however be interesting to learn how the rotation of the corrector plate relative to the spherical primary ultimately falls, (if this indeed be "how it's done"), between " in-spec limits",(i.e. how is this achieved and what the "in-spec limits" are). Presumably, there is some factory based test by which optimisation can be measured but which, I assume, must ONLY match the corrector profile with that of the primary in the absence of the secondary, since the secondary's presence would surely introduce an almost infinite variable to the aim of corrector plate/primary mirror alignment ? If this is so, it intrigues me to know how one, with no particular test equipment, might go about optimising in the presence of all three components as you suggest might be carried out. Can this realistically be done "at home", given that under practical conditions, one would be trying to create the best "match" between corrector and primary in the presence of that adjustable secondary ? In other words, how would one know whether optimisation between corrector plate and primary had been achieved and that the secondary had played no role in this alignment? Please don't think I'm being flippant or dismissive in any way, but it does seem to conjure up visions of monkeys, typewriters and the complete works of Shakespeare. However, if there IS a practical, home-based way of achieving such optimisation, I would like to know so that may possibly apply it. Your thoughts most appreciated. My reason for asking all this, is that a colleague and I are envisaging an internal cleaning of his 8SE's corrector plate together with removal of the primary mirror for a screw repair. Not having ever performed this disassembly, it surely pays to be forearmed ? Many thanks to all of you for your input ! Any further comments you might have, would be most welcome. Best regards, Tel Post Extras:       Geo. Carpal Tunnel Reged: 10/01/08 Loc: Upstate NY Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Midnight Dan]       #5160240 - 04/07/12 07:24 AM Edit   Reply   Quote Quick Reply   Yes, I think Rod is correct. This is why Meade has always resisted selling individual correctors. The optics sets have to be matched. This is probably done by an inferometer soon after grinding. Matching your mirror might mean stripping it and putting in in the line for matching. This would cost more than just selling you a new set. Celestron was the same because of the hand figuring of the secondary to correct for primary and corrector imperfections. I have to think there was also some pre-matching of the primary to a corrector or a lot of figuring would have to be done. Since the switch to the "Meade method" of matching optics sets, they are prepared to just replace your corrector. Celestron used to claim that their optics would provide "good" performance if selected randomly, but they used a high precision inferometer (0.01 wave) to match and figure them. They guaranteed straight lines with 100 line per inch ronchi test and "similar" intra and extra defraction patterns with a 12.5 mm ocular. I haven't seen that guarantee on any Celestron literature in the last 20 years. Post Extras:       Don Trinko Carpal Tunnel Reged: 07/05/09 Loc: Ohio Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Geo.]       #5160316 - 04/07/12 08:32 AM Edit   Reply   Quote Quick Reply   I had a C5 apart and there were marks on the secondary for orientation. Were they necessary? I do not know but I used them to reassemble. Don T. Post Extras:       Tel Postmaster Reged: 03/31/06 Loc: Wallingford England Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Don Trinko]       #5162272 - 04/08/12 02:23 PM Edit   Reply   Quote Quick Reply   Hi George, Hi Don, Many thanks for your input too ! I'm pretty sure I now understand how our manufacturers go about "matching" corrector plate with primary mirror on these SCTs and thus a need to keep the alignment, (orientation), should the corrector need to be removed from the OTA for any reason:(the primary, assumingly, being held in the fixed position of its cell and by the screws or bolts which secure this to the tube. I am however not so sure that one has the means to tune the corrector further, (if this is possible and it might well be as Nat describes), on a home based level. Once again, many thanks to all of you for your input to this enigmatic question. It is very much appreciated. Best regards, Tel Post Extras:       motorcycle jack member Reged: 03/12/12 Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Tel]       #5162924 - 04/08/12 11:45 PM Edit   Reply   Quote Quick Reply   Hmmm. from my optics classes (now many years ago, repeat many) I learned the corrector plate is also a spherically ground (but with a hyperbolic facial curve) similar to how the mirror is done. So now let me think abou tthis for a moment. We have a computer ground spherical mirror that has a corrector plate less than 1/2 the focal length away and we are saying it is positionally aligned to the spherical mirror? I don't think so, the only alignment between the two would be the parallelness of their planes of grinding. This could be one reason to keep them oriented in the tube as to how they maybe mounted to the tube. I would more believe the orientation fixation has to do with the secondary to primary alignment. Those are the only reasons not to rotate the mounting not some "voodoo" the mfg'er does to the corrector or mirror. TMOAISWI Post Extras:       Tel Postmaster Reged: 03/31/06 Loc: Wallingford England Re: Corrector Plate Rotation--- Myth or Fact ? [Re: motorcycle jack]       #5163105 - 04/09/12 05:36 AM Edit   Reply   Quote Quick Reply   Hi Motorcycle Jack, A very warm welcome to CN and to this forum ! Equally so for your input to this question. It's equally a very long time since I forgot my school optical physics let alone learning anything, (in my case), of the subject ! So, in basic ignorance, my own poor thoughts are such that I am pretty sure the modern corrector plate is mass produced using the Tom Johnson/John O'Rourke vacuum/master block process. Certainly individual grinding and figuring must have long been abandoned due to the excessive cost. What I therefore envisage is, as Geo points out, some form of complemetary Ronchi or interferometer test at manufacturing level to get the best orientation between the two components and nothing much more than this, bearing in mind the mass production/cost aspect. My thoughts extend to the secondary mirror only then being introduced once the corrector to primary orientation has been optimised, otherwise, I come back to that "infinite variable" to the corrector/primary alignment produced by any earlier introduction of the adjustable secondary. If the above reasoning is sound, it would seem sensible to maintain the same orientation if the corrector should be removed post manufacture, if not for any other reason but that it would be difficult to test for any possible better orientation on the home front and where more likely, a worse alignment might well arise. Best regards, Tel Edited by Tel (04/09/12 06:06 AM) Post Extras:       Peter9 Post Laureate Reged: 10/30/08 Loc: Yorkshire - Born & Bred Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Tel]       #5163118 - 04/09/12 06:01 AM Edit   Reply   Quote Quick Reply   Hi Tel, My thoughts, as someone who is totally ignorant of the methods/maths used, was that any precise matching done between the corrector and the primary mirrow would be undone over time due to the fact that the primary mirrow is moved when focusing. I realise that the whole cell is moved squarely during focusing, but would not some slight wobble be introduced, again over time. The effect on the secondary would be conpensated for by collimation. If I am barking up the wrong tree, please feel free to say so, although I may, in a moment of weakness, fire mushy pea bombs in your general direction. Just a thought. Regards. Peter. Post Extras:       Tel Postmaster Reged: 03/31/06 Loc: Wallingford England Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Peter9]       #5163132 - 04/09/12 06:44 AM Edit   Reply   Quote Quick Reply   Hi Peter, With time I guess there might well be some side play introduced between primary and the baffle tube on which it slides, due possibly to a greater distribution of the grease, but I think it more likely this would probably excentuate mirror shift and/or flop rather than upset any rotational orientation between corrector and primary by way of the fact that as you say, the primary is only moved linearly with assumed, no rotation. Should the movement induce shift or flop, this would however equally disturb the image quality due, I would think, to at least two factors,: a) misalignment of the primary with the secondary and b) lateral rather than rotational misalignment between primary and corrector. At the end of the day though, I guess we're stuck with what we have; which is an excellent, if not still mass produced set of 'scopes which in turn, will inevitably be subject to wear and tear and need maintaining from time to time. Incidentally, to us posh Southerners and Peter Mandelson, mushy peas are known as Avocado Dip down here ! Best regards, Tel Post Extras:       Midnight Dan Postmaster Reged: 01/23/08 Loc: Hilton, NY, Yellow Zone (Bortl... Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Tel]       #5163162 - 04/09/12 07:21 AM Edit   Reply   Quote Quick Reply   What Tel said. There is no question that focusing shifts the mirror. Put in a high power eyepiece and watch a star move side to side when changing the direction of focus. But that movement would induce a lateral misalignment relative to the secondary, not a rotational misalignment. -Dan Post Extras:       Peter9 Post Laureate Reged: 10/30/08 Loc: Yorkshire - Born & Bred Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Midnight Dan]       #5163234 - 04/09/12 08:53 AM Edit   Reply   Quote Quick Reply   Hi Tel, Many thanks for the explanation. I posed the question in case others, along with myself, had wondered if lateral, as well as rotatonal movement, would have any effect. I will withhold my Avocado ..na.. Mushy pre bomb assult in light of this. Regards. Peter. Post Extras:       Tel Postmaster Reged: 03/31/06 Loc: Wallingford England Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Peter9]       #5327816 - 07/21/12 04:58 AM Edit   Reply   Quote Quick Reply   Hi Guys, Sorry to resurrect this one, but just in case anyone has a further interest, I had occasion last Saturday to witness the removal of the corrector plate from an 8SE. Now, as I always understood it, irrespective of its predetermined rotation position relative to the primary mirror, the plate would likely be shimmed, (with cork pads of varying thicknesses), which required additional marking to ensure accurate relacement. It was also my further understanding that the reason for shimming was to ensure the plate was placed centrally and squarely over the primary. Certainly this aspect was originally cited on Mike Swanson's website. However, in case you haven't visited his website recently, I draw your attention to his update on this issue which I've only just discovered for myself. If you've seen it before, then all well and good, but if not, you'll see that it puts a different perspective on the erstwhile accuracy apparently necessary in removing and replacing the corrector plates of earlier Nexstar SCTs as opposed to the more modern. http://www.nexstarsite.com/OddsNEnds/SCTCorrectorRemoval.htm The removal of this particular 8SE's corrector plate, last Saturday, revealed no more than a circular, rubber gasket on which the plate rested, the whole being held in place by the short screws of the outer securing ring. Doesn't this perhaps revive the question as to whether the original reported need for accuracy in removal and replacement of corrector plates was indeed based on myth rather than fact or have standards diminished since Celestron's manufacturing moved to China ? Reading between the lines of Mike Swanson's description, the former certainly seems to be largely the case. Best regards, Tel Post Extras:       Peter9 Post Laureate Reged: 10/30/08 Loc: Yorkshire - Born & Bred Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Tel]       #5327824 - 07/21/12 05:31 AM Edit   Reply   Quote Quick Reply   Hi Tel, I would agree with you that the presence of the single rubber ring indicates it this is all that is required to ensure opital alignment. There was a thread some time ago where a member had removed the entire front cell, thus negating the need to remove the corrector plate. Regards. Peter. Post Extras:       Tel Postmaster Reged: 03/31/06 Loc: Wallingford England Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Peter9]       #5327826 - 07/21/12 05:46 AM Edit   Reply   Quote Quick Reply   Hi Peter, This can, I believe, be done with the 6SE and presumably with the 5SE if not the 4SE Mak, by way of the screws used to secure both the corrector plate and indeed the primary mirror cell as Art proved. However, it can't be done with the 8SE because the cells are retained with bolts held in place by non-captive nuts. Any attempt therefore to remove the bolts would cause each nut to drop into the tube or if not, just rotate with the bolt. Reading Mike Swanson's update on the subject though, just brought me back to my original stand on this "myth or fact" issue. It now seems to me that there's a bit of both involved as I suspected there might be. Best regards, Tel Post Extras:       Peter9 Post Laureate Reged: 10/30/08 Loc: Yorkshire - Born & Bred Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Tel]       #5327829 - 07/21/12 06:06 AM Edit   Reply   Quote Quick Reply   Memory duly jogged Tel. Pressure of retirement Regards. Peter. Post Extras:       mclewis1 Thread Killer Reged: 02/25/06 Loc: New Brunswick, Canada Re: Corrector Plate Rotation--- Myth or Fact ? [Re: Peter9]       #5328134 - 07/21/12 11:52 AM Edit   Reply   Quote Quick Reply   Why would a change to the manufacturing of a scope now cause questions to the established fact that corrector positioning does indeed affect the optical correction of a commercial SCT? Because some folks don't have the visual accumen to see the difference or the technology to measure it doesn't mean it isn't something to be concerned about. Mike clearly states that a) things have changed with the latest scopes (made in the past few years) and b) that slop in most folk's mechanical setup (visual back, diagonal, focusers, etc.) renders precise centering somewhat unimportant. Remember as well that when we talk of positioning the corrector there are two variables, rotational position and centering. There appears to be less hand figuring going on as the scopes get smaller and there is a question whether this is actually occurring at all anymore. Better manufacturing procedures may have indeed rendered rotational positioning less important or even not required at all on the latest scopes. Centering of the corrector plate is also something that may have changed a bit with better manufacturing procedures (better mechanical positioning of the corrector with that rubber gasket like product and centering bolts). A corrector with hand figured areas would likely be very sensitive to centering and rotational position. A corrector with a highly accurate and symmetrical figure (no hand figuring) would not be as sensitive. Another benefit of accurate centering of the corrector is the accurate centering of the secondary mirror. Without any adjustment capabilities in the position of the secondary (outside of any play in the position of the secondary) the accuracy of the position of the corrector will affect the secondary as well. So clearly things have changed over the years. If you have a later model Celestron SCT and you disassemble your scope you might not need to be as concerned about the accurate positioning of the corrector plate but if you own one of the tens of thousands of older scopes it is an established fact that the positioning does matter. Post Extras:       Midnight Dan Postmaster Reged: 01/23/08 Loc: Hilton, NY, Yellow Zone (Bortl... Re: Corrector Plate Rotation--- Myth or Fact ? [Re: mclewis1]       #5328303 - 07/21/12 01:34 PM Edit   Reply   Quote Quick Reply   I think if I ever need to remove my corrector, I'll make every effort to put it back in the exact same location and orientation. It's simple enough to do, and whether it's actually needed or not, there certainly is no downside in doing so. -Dan Post Extras:       Pages: 1 | 2 | (show all)

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