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Collimating My TV NP-101

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#26 Jon Isaacs

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Posted 17 November 2010 - 12:47 PM

I'm not sure why TV continues to use this method of mounting the objective



While we can only speculate...it seems that one of TV's design-style goals is having the smallest OTA diameter they can get away with for the objective size. Their cell attachment design and the use of flocking paper instead of baffles both seem to be chosen with this in mind. The TV-85 is the most extreme example of this philosophy, where even the tube itself is smaller in diameter than the objective.

Mike


Mike: It seems to me that the current cell design could be modified with some set screws that push against the tube much in the same way the jig I made did that would provide for collimation adjustment as well as locking it in place.

I considered removing the cell and having a machinist friend drill and tap some holes but this was simpler.

Jon

#27 KerryR

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Posted 17 November 2010 - 01:39 PM

That's a really great idea! I wouldn't even be that difficult to do at home.

#28 hudson_yak

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Posted 17 November 2010 - 01:48 PM

perhaps centering



Seems like that's most likely. Doesn't appear that the cell itself is collimatable that scope, so you just have to assume it's built ok as far as the tilt of the lens elements goes.

Mike

#29 hudson_yak

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Posted 17 November 2010 - 01:51 PM

I considered removing the cell and having a machinist friend drill and tap some holes but this was simpler.



? Wouldn't your method require more overlap of cell and tube than currently exists? If it's anything like my TV-85 they don't overlap by much.

Mike

#30 spencerj

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Posted 17 November 2010 - 02:12 PM

Not to hijack the thread, but . . .

On a kind-of-similar topic that some people on this thread may have some good information on, I bought a new TV 102 a couple of years ago. The collimation is fine, but this time of year, when it starts to get cooler, that defocused stars have a bit of a "triangular" appearance.

This seems to indicate that something is a bit too tight. How difficult would it be to remove the dew shield and relieve a bit of that excess pressure? Any specific tips/ideas/thoughts?

#31 KerryR

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Posted 17 November 2010 - 02:46 PM

I don't think you need to remove anything-- on my Pronto and Genesis, the objective is held in by a slotted ring. I suspect it's a bit too tight in your scope. You'll probably need to make or buy a tool for this-- they're usually pretty tight, and so usually won't release by sticking a precision screwdriver in one of the slots and pushing, as can be done on some refractors...

You probably want to be sure it's not something happening in your diagonal, though, so verify the astigmatism is evident straight through.

Also, be sure you don't inadvertently rotate the front element as you do mess with this, in case lens wedge was mitigated by careful rotational placement of the front vs. the rear element. You'll know it if you do; you'll suddenly gain so new lateral specturm.

#32 hudson_yak

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Posted 17 November 2010 - 02:52 PM

Yeah, nothing to adjust under the dew shield. Messing with the ring, though, is sort of serious, since there's some globs of stuff on there that would indicate it had been tampered with. Could affect resale value, certainly.

Of course any slips with whatever tool you cobble up for this could be a disaster as well.

Kerry's advice to make sure it really is the cell is good, and if you are really sure it is, you probably would be better off calling TV and having them work on it.

Mike

#33 Jon Isaacs

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Posted 17 November 2010 - 04:13 PM

I considered removing the cell and having a machinist friend drill and tap some holes but this was simpler.



? Wouldn't your method require more overlap of cell and tube than currently exists? If it's anything like my TV-85 they don't overlap by much.

Mike


I think there is room between screws and end of the cell, not a lot but probably an inch or so.

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Jon

#34 hudson_yak

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Posted 17 November 2010 - 04:41 PM

Ok, yeah, I see that now. The TV-85 has the screws closer to the end of the cell and there isn't as much overlap. But that's due to the unique flared-cell design of the TV-85.

Still, when you are collimating your scope, there'd need to be more length-wise movement of the set screws against the tube surface then radial movement in the direction you are turning them. Mainly I think they'd want to dent the tube instead. Your jig has a much more favorable geometry (more leverage) for what you are doing.

Set screws as you describe would help the cell stay put more firmly after collimation is complete, would agree with that much, certainly.

Mike

#35 Sean Puett

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Posted 21 April 2012 - 12:34 PM

Thanks for sharing your ingenuity again Jon. I may need to try this on my genesis sdf someday. I am putting this thread on my favorites because that jig is worth alot to me. How many potential trips to televue would that save?
That jig looks very simple to make and seems like it is more precise than tapping on the objective. :bow:

#36 Pinbout

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Posted 21 April 2012 - 01:01 PM

I just took my focuser off my genesis sdf.

What do they use to cover the hex sockets on all the set screws?

Boy you wouldn't believe what I found inside this telescope. OMG!

but I basically got it for free so...

and I did a double pass ronchi test on the optical train and those lines are jailbar straight.

too bad there was a mouse in there...
how did a mouse get in the case when there's no evidence of it in the case. :foreheadslap:

#37 Moravianus

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Posted 27 April 2013 - 02:52 PM

Materials: Acrylic Tube 6 inches long x 4.25" x 4.50" (0.125"wall)
4 1/4-20 Nylon Screws 1" long (Nylon helps to prevent marring the scope finish)
Hose Clamp 5 inch diameter

McMaster-Carr has the tubing in 12 inch lengths for $19.


The tube is

part # 8486K381
link

Optically Clear Cast Acrylic Tube 4-1/2" OD X 4-1/4" ID, 1' Length

by now price is $29

just ordered it as my TV-101 suffered miscollimation on the way here ...

Hope to find the nylon screws and hose clamp at Home Depot..

Would not be better to have only 3 screws 120 deg apart ?

#38 Jon Isaacs

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Posted 27 April 2013 - 03:29 PM

Would not be better to have only 3 screws 120 deg apart ?



Either way works but having the screws at 90 degrees means you can dial in each axis independently. If this were a mirror cell where long term stability was desirable, three would be better but it's just a jig.

Jon

#39 FirstSight

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Posted 27 April 2013 - 10:44 PM

IMPORTANT WORD OF CAUTION: Be wary of inadvertently stripping the lens cell locking screws when you go about loosening them to begin the actual collimation. Of course, you already know that when Televue finishes the original collimation, they snugly tighten the screws and then insure their immobilization by filling the screw wells with epoxy, and that to re-collimate the scope yourself, you have to dig this epoxy out of the screw-well to access the screw. However, even when you *think* you've dug the screw-head out from the epoxy, be aware that the epoxy can potentially have worked its way underneath the screw head, increasing the challenge of working the screw loose. IF you do inadvertently strip the screw, you'll find it very challenging to drill it out with a reverse bit, and you risk marring the area around the screw well if the drill bit slips and "walks" on you. The good news is: if this worst-case scenario happens to you, you can send it off to Televue to safely tap out and replace the stuck screws and perform the collimation, the bad news is it isn't cheap (roughly $325 + shipping both ways).

Other than that potential issue, doing the collimation yourself with Jon's collimation rig works very well; even though I was able to free and work with one screw, I was able to correct about half the collimation error in my scope using it. That was not enough to achieve a satisfactory final state of collimation (I wound up sending mine off to Televue), but good enough to convincingly prove his jig and method work well enough that even a person of relatively modest mechanical aptitude could easily achieve good collimation with it.

#40 Moravianus

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Posted 28 April 2013 - 03:04 PM

Thanks for the warning. I remember your troubles. Any idea what tool to use for digging, how hard it is and what is the size of the hex <?> screw to be used ?

#41 james7ca

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Posted 23 August 2015 - 11:37 PM

There is good information here and since collimation issues have recently generated another thread under the refractors topic I'd like to re-open this issue as it relates to the Tele Vue NP series of scopes.

 

I have an NP127is that is about three years old and while it has taken some good pictures I've always wondered whether I was getting all that I could from this particular design. I've had some brief discussions with a vendor who has told me that ALL Petzval scopes suffer from some degree of astigmatism (including, from his direct experience, Takahashi) so based upon that bit of "advice" I've just assumed that I'm never going to get perfectly round stars in all areas of an APS-C frame with my NP127is. One caveat here, I'm imaging at 1.5 arc seconds per pixel with a 16 megapixel DSLR-type camera and I suspect that if I were using a camera with larger pixels (say 6 to 8 microns) then I might not even notice the star distortions that I get with my NP127is. Truth be told, I've never used a scope that produced perfectly round stars right out to the corners of an APS-C field (even with field correctors, although my Edge HD seems to do pretty well at prime focus), so maybe the high-resolution DSLR sensors that we have now are just too "good" for even the best scopes.

 

Okay, given the above how should one approach the lens collimation issue on the NP series of scopes? I'm doing tests to help determine where any POTENTIAL problems may exist (camera tilt, focuser tilt, collimation, or maybe as good as it's going to get -- that being residual aberrations that can't be fixed).

 

One issue that really worries me about the design of the NP scopes is that it SEEMS that the front lens collimation relies totally on the friction between a set of three screws and the main tube wall of the scope itself. Furthermore, it would SEEM that the first time that these screws are tightened (at the factor) that the screws will become more-or-less self-set into depressions in the tube wall. Thus, if the latter is true it could mean once a scope goes out of collimation from movement of the lens cell that you'll never be able to get the collimation to hold again after adjustment because the tube wall is already deformed (dimpled) and any re-tightening of the screws will just be working against a "slippery slope." So, you may think you can get it right after re-adjustment but then comes the first little shock and it's easily out of adjustment once again.

 

I guess the question here is what have users found concerning the stability of collimation after they have adjusted the front lens cell? Does it really hold, or are you back at it again in a few months or do you finally just have to give up?

 

A related question is this, can the NP127is really produce round stars right into the corners of an APS-C field (when working at under 2 arc seconds per pixel)? Since a picture is often worth a thousand words, here is the kind of results I'm getting now with my NP127is (reproduced here at 1.5 arc seconds per pixel and with using the aberration inspector script in PixInsight, there is some non-uniformity between the four corners and while the center is okay the edges are suffering a bit). This is also from a 36 image stack, so the eccentricity is probably a little better than for a single image and the seeing conditions on this night were only so-so which makes the stars a bit large and thus perhaps a little more round looking. At prime focus the NP127is has a 660mm focal length at f/5.2 and the angular field of coverage (diagonal) is about 2.4 degrees on my APS-C camera.

Attached Thumbnails

  • NP127is Abberration Inspector.jpg
  • NP127is Eccentricity and FWHM.jpg

Edited by james7ca, 24 August 2015 - 12:10 AM.



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