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Thread: Collimating an Orion XT8

  1. #11
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    Default Re: Collimating an Orion XT8



    Thanks again PhilipLangly for your great info.
    ES AR 102 102mm, f/6.5, Orion Starblast 4.5 114mm,f/4 , Celestron 10x50, GSO SV 30mm, ES 68° 20mm, ES 82° 14mm, 11mm, 8.8 mm, 6.8mm, 4.7mm. Twilight 1 mount.

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  2. #12
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    Default Re: Collimating an Orion XT8

    In re-reading this thread, it seems that we have been discussing the merits of collimating with the Cheshire, and whether I might keep the one that I recently purchased. That's all well and good, but my real concern is whether I have a telescope that is defective. I bought the scope from Ebay and have until about the 5th of April to decide whether to return it(it was categorized as new, but might be damaged or a factory second).

    From my perspective, I have been able to successfully collimate it from about 30 degree inclination up to vertical. You have indicated that we need to determine what is moving that caused it to lose its collimation from 30 degree down to horizontal.

    I found that the primary rests on 3 cork pads, that are between the primary and the cell. I cannot see any other pads.

    You indicated the following:
    "The following can cause the reflection of the peep-hole to move

    1) The primary mirror which sits loosely in its cell and is moving freely around within it as the scope is tilted. This would be a manufacturing defect and entail missing pads and spacers."


    I did the "poke test" and found that when I push on the primary at the 9, 12, and 3 o'clock position that the primary does move, as seen on the primary end of the scope as well as seen through the eyepiece.

    This is what you said about this:
    "This test is designed to ensure that the primary is not prone, due to manufacturing defect or missing pads/buffers, to tip over forward in its own cell as the scope is tilted downwards to the horizon (level) or perhaps, even below that."

    I've looked all over the internet today to find some description of these pads or buffers, and I can't find anything. Can you tell me how I can determine if my scope is missing pads and spacers? I have read something about how to remove the primary from the OTA. Should I do this, and if so, where might I look for these pads and spacers? They are not visible in my scope when I shine a flashlight down the open end toward the primary. Are these pads behind the 3 clips, to prevent the primary from moving side to side? If I place my fingers on the outside of the primary, and try to slide or move the primary in a sideways direction, I don't get any movement.

    In my last post, I presented some indication that when using the Cheshire eyepiece, it appears that it is the reflection of the Cheshire & peep hole that are moving toward the open end, and that the reflection of the donut is either staying put under the crosshairs, or moving very slightly toward the primary. Also, I found that the movement of the reflection of the Cheshire is significant, in that the entire reflection of the Cheshire moves away from the reflection of the donut when the scope is lowered from 30 deg to horizontal.

    I look forward to your next post as I am eager to resolve this dilemma.
    Thank you in advance

  3. #13
    PhilipLangley's Avatar
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    Default Re: Collimating an Orion XT8

    Quote Originally Posted by sopryth View Post
    I found that the primary rests on 3 cork pads, that are between the primary and the cell. I cannot see any other pads.

    I've looked all over the internet today to find some description of these pads or buffers, and I can't find anything.

    Should I do this, and if so, where might I look for these pads and spacers?

    They are not visible in my scope when I shine a flashlight down the open end toward the primary.

    Are these pads behind the 3 clips, to prevent the primary from moving side to side?

    If I place my fingers on the outside of the primary, and try to slide or move the primary in a sideways direction, I don't get any movement.
    Please see the attached photos showing the vertical felt pads also known as spacers or buffers. On the Orion XT8 there appear to be three of these hidden under the primary mirror clips.

    See also post#14 in this discussion here at AF.net

    Here

    To determine if the primary can move laterally or whether it can tip when the scope is inclined towards the horizontal may require that you remove the primary cell and gain direct access to the mirror within the cell.

    Your finger-poking test seems to indicate that you have some unwanted "vertical " movement within the cell. That, or the entire primary cell was tipping when you pushed on the back of the mirror itself.

    Perhaps other Orion XT8 owners might l want to jump in here.

    I will continue with my posts as soon as I can.
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  4. #14
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    Default Re: Collimating an Orion XT8

    Quote Originally Posted by sopryth View Post
    In my last post, I presented some indication that when using the Cheshire eyepiece, it appears that it is the reflection of the Cheshire & peep hole that are moving toward the open end, and that the reflection of the donut is either staying put under the crosshairs, or moving very slightly toward the primary. Also, I found that the movement of the reflection of the Cheshire is significant, in that the entire reflection of the Cheshire moves away from the reflection of the donut when the scope is lowered from 30 deg to horizontal.
    The emphasis in red is mine.

    Secondary

    The sideways displacements of the reflections that you report are a clear indication that the tilt of the secondary is moving in relation to the axis of the focuser. If you had reported vertical up and down displacements instead, that would have indicated that the secondary was rotating relative to the longitudinal axis of the scope.

    Look through the empty focuser at the secondary and concentrate on the extreme right hand side of the secondary, the side that would be at the 3 o'clock position. This is the edge of the secondary on the major axis that is closest to the primary and furtherest away from your eye.

    If this edge is tilted upwards, towards your eye, the reflection of the Cheshire and of the doughnut would appear to move sideways towards the primary at the back end.

    If this edge is tilted downwards, away from your eye, the reflection of the Cheshire and the doughnut would appear to move sideways towards the spider-hub at the front open end.

    These two reflections do not shift at the same rate. The reflection of the Cheshire plate and it's peep-hole appears to move faster, and further, than the reflection of the doughnut when the secondary is thus tilted.

    Form your report of what you see in the Cheshire eyepiece/sight-tube combo tool, it appears that the secondary is tilting downwards as the scope moves from 30° to the horizontal.

    When the scope is horizontal, the secondary is cantilevering entirely off the back face of the spider-hub. Its weight is possibly causing it to sag. This possibly as result of some differential displacement between the secondary, it's stalk and the back-face of the spider-hub. Perhaps the spider-hub is moving relative to the scope despite your observations to the contrary.

    Focuser axis

    Recall that I've written that the secondary is tilting relative to the axis of the focuser to produce the reflection displacements that you see. It is also possible that the secondary is motionless and it is the axis of the focuser that is moving relative to the secondary.

    When the scope is vertical, the focuser is horizontal and parallel to the ground. It hangs off the side of the vertical scope tube and its weight is carried in bending of the scope's tube. The scope's tube is bending about its own altitude axis.

    When the scope is horizontal, the focuser is in the vertical plane but inclined at 45° to the ground. It hangs off the side of the horizontal scope tube and its weight is carried in rotational torque or twisting within the cross section of the scope's tube. The scope's tube is twisting about its own longitudinal axis which is anchored at the altitude bearing. When the cross section of the scope, at the open front end, twists in this manner, it will also warp.

    It is this warping that causes the axis of the focuser to be displaced relative to the secondary which in turn results in the secondary appearing to tip away from the axis of the focuser, yielding the same visual affects as above.

    Remedy

    Another review of the relative movements between the secondary and the focuser may be in order.

    My anecdotal experience in the extreme

    When suffering a similar dilemma as yourself with my thin-walled, sheet-metal, solid-tube 8" 1200mm f/6 Skywatcher Dobsonian, my eventual solution was to install a 19" x 7" curved aluminum stiffener plate inside the front end.

    This 2mm thick aluminum plate was rolled to match the inside radius of the scope, is centered on the focuser and spans 1/3rd of the inside circumference in each direction. It is attached to the inside of the tube with the 4 bolts securing the focuser base plate, the 2 bolts securing the finder-scope bracket and 21 pop-rivets equally spaced throughout. A hole matching the 90mm diameter nub on the focuser base was cut through the stiffening plate to allow the nub to pass through it.

    The good news

    Newtonian reflectors on equatorial mounts that are used for imaging are very often stiffened inside or outside with plates or rings.

    I have not encountered too many 8" visual-only Dobsonians that required such a Draconian solution to reinforce and stiffen up the front end. I guess my wonky scope was manufactured very late on a Friday afternoon or very early on a Monday morning.

    Generally the source of the instability can be diagnosed in a tedious step-by-step process starting at one end and working your way through to the other.

    See a similar discussion elsewhere.

    Here

    Your new Cheshire eyepiece / sight-tube is the ideal tool to help you do this. Knowing how to use it properly when diagnosing these instability deflections is key to its value to you.

  5. #15
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    Default Re: Collimating an Orion XT8

    Today, I looked into whether the secondary is sagging, and whether the focuser is sagging/moving.

    Note-the scope is collimated when upright, and down to about 30 deg from horizontal. I notice that there was at least one time when the scope remained in alignment down to about horizontal. It did not remain so, though, when elevated to vertical, and then lowered to 20-30 degrees.

    So, with the scope at horizontal, and with the reflection of the Celestron Cheshire well toward the open end, and the reflection of the donut in approx its original place in the center, I did the following:
    I reached into the scope and grasped the support of the secondary, and tried to lift it upward(vertically) to see if the reflection of the Chesire would move back into the center. It did not. The secondary is pretty solid. If pushed on enough, the reflection moves a tiny bit, but when released, it and the reflection returns to where it was. So, my conclusion is that the problem is NOT the secondary.
    I grasped the focuser assembly and tried to lift it in the direction against gravity, and even tried to twist it in the direction around the OTA tube, and found that this effort made no movement in the reflection of the Cheshire.

    So, I turned my "focus" (bad pun) to the primary. Let me describe what my primary support cell looks like.
    There is a 4" diam ring with three ribs which extend out to the 8" ring. At the junction of each rib with the 4" ring, there are circular cork pads(between the mirror and the 4" ring), on which the primary mirror rests. As an aside, the junction of the 3 ribs with the 8" ring is where the 3 sets of collimation and lock screws are located. These three ribs are roughly in the 2 o'clock position, the 7 o'clock pos. and the 11 o'clock pos.

    When the scope is nearly vertical, I tried to slip a business card between the primary mirror and each of the three cork pads. In this position, the card would not slip into this space, meaning that the primary mirror is resting on each pad.
    When I lowered the scope to about 30 deg form horiz., I found that the primary had tipped forward from the pad at the 11:00 position, but not the other two pads. (the business card is about .011") I think at this point, I looked to see that the reflection of the Cheshire had begun to move a little bit toward the open end
    When the scope has reached the horiz. position, I slipped two, store gift cards (which measure about .037") at the 11:00 position with a little resistance(in other words, at this thickness, it goes through, but it's a little tight). Note that at this time, even the business card would still not slip under the mirror at the 2:00 and 7:00 positions. At this time, the reflection of the Cheshire had moved the full width of its reflection toward the open end of the OTA, and the reflection of the donut is in the center(under the crosshairs) and the reflection of the donut is on the edge of the reflection of the Cheshire.

    So, what this tells me is that at about 30 deg from horiz, the primary begins to tip forward, and it tips even more as the scope is lowered to horiz., but that it tips back into its original place when the scope it raised again.

    This evening, I may remove the primary mirror assembly to determine if there is something wrong with the clip that is nearest the 11:00 position on the outside. At this time, I still don't know if there are supposed to be pads between the clips and the sides of the mirror. I called Orion, and the person I "chatted" with didn't know. I may call them this afternoon.

    I don't know if I can post a pic, but one that I have taken shows me that the clips do seem to be on the inside from the collimation screws, and it looks like the one at the "11:00" position (as seen from the outside of the OTA) is "askew". It look like one side of it is barely covering the mirror. (if you look at this pic, note that the seam of the OTA (where the metal was rolled and joined) should be in the 6:00 position.IMG_4421.JPG

    Thoughts?
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    Default Re: Collimating an Orion XT8

    I apologize, as I did not see the pics of the primary cell and its pads in your 3/28 post. My scope has this same configuration.

    Now I can see where the pads should be when I disassemble by primary to check the pads and clips.

  7. #17
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    Default Re: Collimating an Orion XT8

    Excellent diagnostics so far, well done!

    Removing primary mirror at the back end

    Please be warned that an 8" primary mirror is a serious chunk of glass with a healthy weight of its own. It is heavy. When removing it, together with its back end support structure, expect it to be heavy.

    Think of a teeter-totter (see-saw) in a children's playground. When the heavy kid sitting on the lower end suddenly gets off, the lighter kid up on the higher end will instantly come crashing downwards into the ground. The same with your scope. The primary mirror is part of the counterweight that balances the focuser and secondary mirror at the front end. If the primary is suddenly pulled out, the front end of your scope will go crashing downwards.

    Be sure to place a chair and a cushion under the front end of the horizontal scope before removing the primary at the back end.

    Personally, I remove the OTA from the Dobsonian base and lay it horizontal in two half-round sponge foam cradles on a solid table before removing the primary. This way, I only have to worry about two things 1) the floppy back end of the OTA and 2) the expected weight of the primary/ cell/ bezel support ring.

    Primary clips

    See the attached photo showing some information about the clips.

    Notice that the plastic L-shaped clips are each attached to the cell by two machine screws. There is a steel plate across the top of each clip. This plate acts like a washer to stop the clip pulling off past the heads of the screws in the event that the scope is ever up-turned and the clips catch the primary as it falls.

    The important thing here is that when removing the screws, if that is what you need to do, is to protect the reflective surface of the mirror just in case the screw-driver slips or something like that. Also, please expect that there might be some hardened, sticky gunk that makes it difficult to turn the screws loose. If you have difficulty getting the screws to turn loose, please do not force them. Get advice elsewhere on the 'Net or come back here.

    When reinstalling the clips, plates and screws, there must be a gap in between the lip of the clip and the reflective surface of the mirror. Due to this gap you will not need to tighten the screws all the way down. This will probably leave the clip able to float up-and-down on its two screws. This is proper.

    Important: You might find that, since the screws are somewhat loose, the clips ill slide down the screws and rest on the reflective surface of the mirror. If they do, do not worry, this is OK. All that matters is that if you lift the clip up off the surface of the primary, it can and will move upwards, leaving a gap about the thickness of a business card between its lip and the reflective surface.

    Some will use a thick viscous lubricant such as optical grease on the threads of the screws to stop them from turning and shaking loose since they're never tightened down. Many other, most others, do not.

    Vertical pads on the sides of the mirror pocket in the cell

    There are usually the same number of these pads (also called buffers or spacers) in between the edge of the primary and the inside edge of the pocket. It is these pads that normally fill the gap and prevent the primary from moving around within the pocket. They are close fitting pads, not tight fitting pads.

    It is typically these pads that prevent the primary from tipping forward when the scope OTA is close to the horizontal position. If your pads are worn squished or missing, you can replace them with strips of the pile-side of Velcro. You do not want to use the hook-side. Others have used strips of rubber or plastic cut from 4L milk jugs.

    Reinstalling the primary

    Make sure to align the slot in the outer bezel support ring with the folded seam running along the bottom and length of the OTA.

    To ensure that the primary has settled fully into the pocket in the cell, and down onto the cork pads, when being installed into the primary, this is what I do.

    After all the screws are in and tight, I place the scope OTA back in the Dobsonian mount and aim it vertically towards the zenith. I then tap the outer bezel support ring of the cell, multiple places, with the rubber heel of one of my running shoes. These quick gentle taps, together with gravity, will cause the weight of the primary to settle it fully downwards, sliding past the vertical side buffers/spacers to rest fully onto the cork pads in the bottom of the cell.
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