Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Reflectors

Pages: 1 | 2 | 3 | (show all)
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Secondary mirror on a 12"
      #6481292 - 04/22/14 01:54 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Secondary mirror on a 12" new [Re: nevy]
      #6481376 - 04/22/14 02:39 PM

http://www2.arnes.si/~gljsentvid10/diagonal.htm

Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Secondary mirror on a 12" new [Re: Pinbout]
      #6481760 - 04/22/14 06:08 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Secondary mirror on a 12" new [Re: nevy]
      #6481846 - 04/22/14 06:51 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
sixela
Postmaster
*****

Reged: 12/23/04

Loc: Boechout, Belgium
Re: Secondary mirror on a 12" new [Re: nevy]
      #6482737 - 04/23/14 08:17 AM

Quote:


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).


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Secondary mirror on a 12" new [Re: nevy]
      #6482851 - 04/23/14 09:24 AM Attachment (34 downloads)

download the attachment

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Secondary mirror on a 12" new [Re: Pinbout]
      #6488481 - 04/25/14 06:02 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 ?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Secondary mirror on a 12" new [Re: nevy]
      #6488545 - 04/25/14 06:44 PM

Your focuser isnt square to the optical axis

Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Secondary mirror on a 12" new [Re: nevy]
      #6488710 - 04/25/14 08:18 PM

Quote:

...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?


Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Secondary mirror on a 12" new [Re: Vic Menard]
      #6488744 - 04/25/14 08:41 PM

Quote:

Quote:

...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.

Edited by nevy (04/25/14 08:43 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Secondary mirror on a 12" new [Re: nevy]
      #6488785 - 04/25/14 09:04 PM

Quote:

...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).


Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Secondary mirror on a 12" new [Re: Vic Menard]
      #6489690 - 04/26/14 11:40 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Secondary mirror on a 12" new [Re: nevy]
      #6489938 - 04/26/14 01:57 PM

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

Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Secondary mirror on a 12" new [Re: nevy]
      #6489999 - 04/26/14 02:24 PM

Quote:

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.
-------------------------------------------------------------------------------
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Secondary mirror on a 12" new [Re: nevy]
      #6490016 - 04/26/14 02:30 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...


Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Secondary mirror on a 12" new [Re: Vic Menard]
      #6490820 - 04/26/14 09:41 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Secondary mirror on a 12" new [Re: nevy]
      #6490912 - 04/26/14 10:19 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Secondary mirror on a 12" new [Re: Vic Menard]
      #6490940 - 04/26/14 10:44 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.


Edited by nevy (04/26/14 10:52 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Secondary mirror on a 12" new [Re: nevy]
      #6491449 - 04/27/14 09:12 AM

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

Post Extras: Print Post   Remind Me!   Notify Moderator  
Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Secondary mirror on a 12" new [Re: nevy]
      #6491504 - 04/27/14 10:03 AM

Quote:

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.




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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | (show all)


Extra information
16 registered and 29 anonymous users are browsing this forum.

Moderator:  ausastronomer, Phillip Creed, JayinUT, okieav8r 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1040

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics