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Secondary mirror on a 12"

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#1 nevy

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Posted 22 April 2014 - 12:54 PM

Hello , I just got my new secondary , my old one was 2.7"ish , the new one is 2.6" .
Could any one who has a 12" scope that uses a 2.6" secondary give me a rough position as to where I mount it regarding offset on the stalk , if I was using the same size I would just measure where the stalk is from the edges (the long side) I can't understand all the maths so is it possible for someone to give me a measurement as to what the gap is between the back & or front of the mirror back ( in millimetres ) to the edges of the stalk ( I will be sticking it with 3 dabs of silicone , many thanks.

#2 Pinbout

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Posted 22 April 2014 - 01:39 PM

http://www2.arnes.si...10/diagonal.htm

#3 nevy

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Posted 22 April 2014 - 05:08 PM

Thanks pinbout but I'm still not any wiser, I don't know what some of it means and where it's referenced from , I'm after measurements from edge of mirror to edge of stalk.

#4 Pinbout

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Posted 22 April 2014 - 05:51 PM

Ah you want someone to give you a layout. How big is your stalk. And how far from the secondary to the top focuser racked in.

#5 sixela

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Posted 23 April 2014 - 07:17 AM

Could any one who has a 12" scope that uses a 2.6" secondary give me a rough position as to where I mount it regarding offset on the stalk ,

It doesn't really matter. If you centre it, you'll just end up with an offset in one dimension only instead of two and a slightly elliptical fully illuminated field of view, but it's largely irrelevant (unless you have digital setting circles and want the optical axis exactly along the tube axis).

If you want the correct offset, it's almost always close to the minor axis of the diagonal divided by (4 times the f/ratio). That offset is away from the focuser _and_ towards the primary (that latter is something you will set automatically during collimation), so if you measure it along the face of the diagonal, it's 1.4 times the value that you got with the above formula. Away from the focuser (assuming obviously that the stalk on which you glue is centred in the tube).

#6 Pinbout

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Posted 23 April 2014 - 08:24 AM

download the attachment

don't print to fit and let the pdf choose the paper size.

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#7 nevy

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Posted 25 April 2014 - 05:02 PM

Thanks pinbout ,that I can understand , I can do a bit of measuring and transfer the offset so I can measure from edge of mirror to edge of holder,
I just fitted a new focuser ( still got the old secondary in the scope) I got everything collimated spot on & confirmed with a star test, but I've noticed that when I rack the draw tube up ,the laser dot moves up in the hotspot hole, also with the tublug & laser the hotspot shadow also moves out of collimation on the tublug.
It can't be the brand new moonlight focuser is badly made could it?
I'm hoping it's the focuser not being square as I didn't square it , so would a un square focuser cause this ?

#8 Pinbout

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Posted 25 April 2014 - 05:44 PM

Your focuser isnt square to the optical axis

#9 Vic Menard

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Posted 25 April 2014 - 07:18 PM

...I'm hoping it's the focuser not being square as I didn't square it , so would a un square focuser cause this ?

No.

What you've described indicates either the focuser drawtube is not moving in a linear motion (unlikely) or the laser is not registered in the drawtube correctly causing the laser axis (beam) to be tilted relative to the drawtube axis. I see that you're using a Glatter laser--have you verified the laser/focuser registration by rotating the laser in the drawtube? Be sure to tighten the focuser locking screw before you make the alignment "read".

Do you find the misalignment happens progressively as the focuser drawtube position is changed, or perhaps the misalignment occurs more suddenly near the fully racked out or fully racked in position?

#10 nevy

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Posted 25 April 2014 - 07:41 PM

...I'm hoping it's the focuser not being square as I didn't square it , so would a un square focuser cause this ?

No.

What you've described indicates either the focuser drawtube is not moving in a linear motion (unlikely) or the laser is not registered in the drawtube correctly causing the laser axis (beam) to be tilted relative to the drawtube axis. I see that you're using a Glatter laser--have you verified the laser/focuser registration by rotating the laser in the drawtube? Be sure to tighten the focuser locking screw before you make the alignment "read".

Do you find the misalignment happens progressively as the focuser drawtube position is changed, or perhaps the misalignment occurs more suddenly near the fully racked out or fully racked in position?

Yes I've verified the laser and it accurate like it should be,I noticeThe misalignment just after half way and it is progressive , I collimated it about half way through the focuser travel and yes I tighten the compression ring on the focuser.

#11 Vic Menard

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Posted 25 April 2014 - 08:04 PM

...I've verified the laser and it accurate like it should be,I noticeThe misalignment just after half way and it is progressive , I collimated it about half way through the focuser travel and yes I tighten the compression ring on the focuser.

If the focuser is positioned horizontally, it almost sounds like it could be gravitational flexure. Have you tried positioning the focuser so the drawtube motion is vertical?

I assume the Moonlite focuser has some means to load the drawtube to minimize flexure when the drawtube is extended--but I wouldn't expect much flexure from a 2-inch Glatter (a 2-inch Glatter in a Tublug is a bit more load--especially when the focuser axis is horizontal).

#12 nevy

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Posted 26 April 2014 - 10:40 AM

I did the test with the scope on the rocker box as if I were using the scope & I did it at various angles , I'm carefull to make sure the laser is sitting flush with the top of the draw tube & I push on the top while securing the compression ring to keep it square , if it is flexture then I don't think that's acceptable because my no frills standard skywatcher focuser never had that problem and it was set way looser than the moonlight is ( the moonlight is left at the same setting it left the factory which is 3lbs , ) I'll have to try & get to the bottom of this , the first thing I'll do is check the draw tube with digital callipers to see if it's uniform all the way up & then before I put the new secondary in ill square the focuser then I'll be able to eliminate one or the other.

#13 Pinbout

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Posted 26 April 2014 - 12:57 PM

Call Ron at moonlite he'll be able to help you. He stands behind his focusers.

#14 Starman1

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Posted 26 April 2014 - 01:24 PM

I did the test with the scope on the rocker box as if I were using the scope & I did it at various angles. I'm careful to make sure the laser is sitting flush with the top of the draw tube & I push on the top while securing the compression ring to keep it square. If it is flexure then I don't think that's acceptable because my no frills standard Skywatcher focuser never had that problem and it was set way looser than the Moonlite is (the Moonlite is left at the same setting it left the factory which is 3lbs). I'll have to try & get to the bottom of this. The first thing I'll do is check the draw tube with digital calipers to see if it's uniform all the way up & then before I put the new secondary in I'll square the focuser then I'll be able to eliminate one or the other.


Ah.

The flexure at various angles of scope tilt probably isn't in the focuser, but the spider.

If the dot on the primary moves up and down when the scope moves up and down, tighten the pole attachments and poles, and tighten the spider attachment screws. If that reduces, but doesn't eliminate the dot movement as the scope moves up and down, tighten the focuser on the scope, tighten the bolts holding the UTA together, and tighten the spider screws a little more. If that eliminates or reduces the laser movement to nearly zero, you've eliminated some of the flexure you see as the scope is moved up and down. Sometimes the center bolt of the stalk needs to be shimmed (by tape, etc.) where it passes through the spider.
If you have curved spider vanes, you may not be able to eliminate this movement.

If the movement occurs as you move the focuser in and out, and it's linear (moves back and forth on a small line as you move the focuser in and out), then you still don't have the focuser axis coincident to the optical axis. Then it's a matter of getting the alignment closer. The Glatter laser beam hits the primary with a dot in the center, surrounded by faint diffraction rings (easier to see in twilight). These diffraction rings can allow you to position the dot on the primary to a very accurate degree because you can use the diffraction rings on the center marker to help accurately assess the centering of the laser dot.

There is the possibility your center marker is NOT coincident with the center of the mirror, and that should be checked, for starters.

Have you gone back and forth between the tublug and the laser, making adjustments each time? Does it get closer with each repetition? And, if it gets closer with each repetition, does that reduce the movement of the dot as the focuser moves in and out? If so, then your problem is literally FAE.

But if the scope is completely collimated and you still see the movement as the focuser is racked in and out, though it's unlikely, you could have an improperly-machined flat surface on the drawtube. The Moonlite drawtubes with compression rings in them are quite thick. I see very little flexure with the weight of a 21 Ethos plus a Paracorr (close to 3.5 lbs), so it would have to be a machining error rather than flexure in the tube itself.

So I think it's likely to be that the alignment still isn't up to snuff.

Ironically, using a Catseye Infinity Autocollimator, I can see some "wiggle" in the movement of my Moonlite focuser as the drawtube is moved in and out. But the variation is in the one or two thousandths of an inch range, and has no visible effect on a laser beam, so the wiggle is smaller than can be resolved by a laser on the primary. I suspect you'd see that kind of variation in ALL focusers. The scale you're talking about is more of an indication of a lack of focuser alignment to the axis of movement along the optical axis.
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As for positioning the secondary in this scope, since the offset is 1/8", it's going to be really hard to accurately offset the secondary the necessary number of millimeters, so I recommend gluing it to the stalk completely centered. It won't make any difference in the accuracy of collimation, and such a small offset won't even affect DSCs with any significance. Mark the exact center of the back, and make sure the exact center of the stalk corresponds when the stalk is lifted by the 3 blobs of cement. If you don't, the cement will offset the stalk toward the bottom of the secondary when it's glued, and if you have any offset, you want it to be the other way.
What I suggest is to measure the center of the back of the secondary and draw a pencil line down the long axis of the ellipse. Place a dot 1/8" to 3/16" toward the upper end of the secondary mirror on the long axis of the ellipse. Center your stalk on that point. When the 3 blobs of cement intervene between the mirror and the stalk you will end up with the centerline of the secondary holder and stalk approximately in line with the center of the secondary mirror.
The reason to do this is that the blobs of glue holding the mirror on will have a thickness to them (they won't be completely squashed to no thickness). That means a centered stalk would not result in a centered mirror, but offset the mirror slightly toward the focuser. If you have any offset at all from the center of the UTA, it should be the opposite direction--away from the focuser. My recommendation should result in a mirror centered in the UTA at the same time the stalk is centered in the UTA.

#15 Vic Menard

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Posted 26 April 2014 - 01:30 PM

Just wondering, is this a 3-truss Dob?

"Squaring" the focuser is important for optimal secondary mirror placement, but has no impact on the axial alignments. The problem you've described is a change in axial alignment. Since the change occurs at the out focused position, I would start there...

#16 nevy

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Posted 26 April 2014 - 08:41 PM

The scope is a 12" solid tube dob with a 4 vein standard skywatcher spider (quite good & strong) I had a little play about with it today after reading the last couple of posts , I tested for flexture by first racking the draw tube up & down ( whilst the scope was laying horizontally & the focuser was vertically straight up ) & watching the shadow on the tublug , and that turnd out to be ok so I then put the scope on the rocker base as if I were using the scope , whilst racking the draw tube up & down there was a little shift so I tried pushing on the end of the tublug & I got more shift as I pushed.
So I tightened the two tension bolts a bit , the focus knob is a little tighter than I would like but never mind , the main thing is it's cured that , so there was a little flexture there.
I then racked the draw tube up & down again but this time with just the glatter laser with the small dot attachment in the focuser & this time I was watching the dot in the middle of the donut on the primary & the same thing was happening , the dot was gradually moving from the centre to the edge of the hole in the hotspot , I did this when the scope was laying down & also whilst in the mount. But one thing I noticed was that if I didn't clamp the laser in then the dot would hardly move but it would move after I tightened it , I tried using one clamp screw then two then Three , I also varied the amount of tension on each & it made no difference.

#17 Vic Menard

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Posted 26 April 2014 - 09:19 PM

I'm guessing your Moonlite focuser drawtube has a compression ring for locking. You might try twisting the compression ring around until the gap lines up with one of the locking screws and then use that locking screw to secure the focuser. This effectively removes the compression ring from the equation and forces the locking screw to push the laser solidly against the opposing side of the drawtube. Ideally, I would prefer to see you use the one of the three locking screws that will push the laser "downward" (and let gravity pull the inserted end of the laser downward), forcing parallel registration and keeping the laser axis and the drawtube axis in alignment.

If the fit between the 2-inch laser outside diameter and the 2-inch drawtube inside diameter is too loose, there are other options. But don't forget, you only need to meet the prescribed axial tolerances necessary for the way you intend to use the scope--the alignment doesn't need to be perfect.

#18 nevy

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Posted 26 April 2014 - 09:44 PM

It has a compression ring with three screws to tighten it but it's got a cutout in the top of the draw tube so the 1 1/4" adapter screw sits in it , it's a low profile cr2 trinob.
The compression ring stops at both sides of the cutout , it will spin round but you would see the compression ring bridging the gap.
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#19 nevy

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Posted 27 April 2014 - 08:12 AM

Do all moonlites do this? Or did I get unlucky ?

#20 Vic Menard

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Posted 27 April 2014 - 09:03 AM

It has a compression ring with three screws to tighten it but it's got a cutout in the top of the draw tube so the 1 1/4" adapter screw sits in it , it's a low profile cr2 trinob.
The compression ring stops at both sides of the cutout , it will spin round but you would see the compression ring bridging the gap.
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Depending on the way your focuser is mounted to the OTA, one of those three screws is more topside. Since you're not using the 2- to 1.25-inch adapter when you're using the 2-inch laser, I suggest rotating the compression ring around until the gap is aligned with the top most screw. This should (temporarily) eliminate the compression ring from the registration and force the laser into registration with the drawtube.

#21 Vic Menard

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Posted 27 April 2014 - 11:03 AM

Do all moonlites do this? Or did I get unlucky ?

All compression ring locking systems seem to have this peculiar problem to some extent. When the compression ring locks down on the accessory (laser in this case), it centers the accessory in the drawtube. This leaves a small gap around the periphery of the accessory, which then requires a robust lock/registration to the top edge of the drawtube or the centered accessory can wobble a bit in the drawtube. If you remove the compression ring from the registration, the accessory will be pushed against the inside wall of the drawtube opposite the locking screw. If the lock screw is underneath the accessory, the accessory will be pushed upward to the opposite wall, which can still allow gravitational force to pull the deeper end of the accessory to the opposite wall (if the registration isn't robust enough at the top edge of the drawtube), effectively moving the laser axis away from the drawtube axis. The problem can be compounded by the way the accessory registers to the top edge of the drawtube--the Glatter laser (beveled edge) may contribute to this registration issue when the OD/ID fit is too loose.

The simple solution is to remove the compression ring and replace the locking screws with plastic/delrin knurled head screws and always start with the top most screw (unless the load is outside of the drawtube) to remove gravitational torquing issues. Since your Dobsonian never allows the focuser to point downward, there's no chance for an eyepiece to slip out of the focuser and fall to the ground, so the locking screws are there just to keep everything fully registered and reasonably "secure".

Hope this long ramble makes sense...

#22 nevy

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Posted 27 April 2014 - 07:14 PM

Thank you very much everyone for the advice especially don & vic ,
Now back to the secondary , I read your post you did earlier don, and I'm wundering if it's still relevant because I've took the secondary out and I was quite surprised of the size difference , it's smaller but not as much as I imagined ( on the minor axis it's only about 3 mm smaller) but it is also 3 mm thicker , would this difference in thickness make any difference as to how it's mounted? There also seem to be quite a big offset or maybe it looks big because of the small diagonal holder, I'm mainly now wundering if the difference in thickness is gonna have to change the measurements when sticking it on , here's a couple of pics of the old one still mounted.
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#23 Starman1

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Posted 27 April 2014 - 08:01 PM

That's a large discrepancy between the stalk diameter and the mirror diameter!
For sure the thickness of the glue pushes the secondary toward the focuser (and down, but that's adjusted out when you center the secondary under the focuser), but it's a small amount.
If you draw a straight line down the bolt, through the center of the stalk, and through the mirror, you'll see that the center of the bolt appears to go right through the center of the optical surface of the mirror. So all that offset on the rear was necessary because of the thickness of that mirror.
I haven't taken the image and actually measured it, but it looks like they pretty much centered the stalk's center on the center of the optical surface.
Kudos to them for such effective calculations.

#24 Vic Menard

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Posted 27 April 2014 - 08:14 PM

I agree with Don--the secondary mirror in the images is mechanically centered relative to the center mounting bolt, it's not optically centered (offset).

#25 nevy

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Posted 27 April 2014 - 11:43 PM

Ah thanks don , so I should copy what they've done on the new mirror & it should all work well?






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