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SeaDeep
member


Reged: 05/11/06
Posts: 87
Loc: Colorado
Collimating a NextStar 114-GT (with Barlow)
      #1366974 - 01/17/07 12:46 PM

I have (as shown in my signiture) a Celestron NextStar 114 (4.5") GoTo scope. A friend is interested in buying it (and I really have no reason to keep it as I am quite enjoying my 12" Lightbridge, with mods and DSC).

This 4.5" scope is what really got me sold on astronomy (and now I am nuts enough to "enjoy" a night under the stars at 6 degrees F, like last night). Yet, I understand that this 4.5" scope is hard to deal with in terms of collimation, due to that barlow that is stuck in the focuser.

For tools, I have a laser collimator and a cheshire (which, when using a barlow lens for the laser, I can get my Lightbridge collimated in good order). However, such techniques seem to not work well with this 4.5" scope.

So, I have verified that the secondary is centered in the OTA, that the center of the secondary is lined up with the cross bars on the cheshire. After that, I am not sure what to do to get this darn thing collimated.

I have it now about as good as its been. I am not seeing pinpoints, and a slight out of focus shows a not exactly centered center in the rings.

Does anyone have a strategy for getting this thing lined up better?

Thanks!
SeaDeep.

--------------------
Meade Lightbridge 12" Deluxe (with 26mm EP)
(Lightbridge has DSCs, using PalmDSC)
Celestron NexStar 114 GT
EPs = Hyperion 13mm, 8mm, 5mm
Garrett Optical Gemini 9x60 LW Binoculars


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sixela
Postmaster


Reged: 12/23/04
Posts: 13991
Loc: Boechout, Belgium
Re: Collimating a NextStar 114-GT (with Barlow) new [Re: SeaDeep]
      #1367139 - 01/17/07 02:31 PM

Setting the tilt of the primary is done as on a normal Newt (and in fact, if you have a windowed laser, you can do an unbalanced barlowed laser routine "for free").

The only problem is that you do need to set the focuser axis correctly to make the optical axis square to the internal barlow, and that point source lasers can't be used for setting the focuser axis.

I don't find using sight tube cross hairs that precise, and I find focusing on the distant objects also a bit harder with the lens in there.

I don't know if you could remove then reinstall that lens.

--------------------

400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Skywatcher 130mm f/5 BlackLine (finder, widefield scope and solar continuum scope)
Sumerian 250mm f/4.8 Alkaid (as travelscope without platform and on Tom O. platform above).


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SeaDeep
member


Reged: 05/11/06
Posts: 87
Loc: Colorado
Re: Collimating a NextStar 114-GT (with Barlow) new [Re: sixela]
      #1367311 - 01/17/07 03:58 PM

Thanks for the input. Indeed, I had considered that one way would be to take the barlow out, temporarily, use the laser and get the secondary set up. Then reinstall the barlow and use the "windowed laser" to set the primary. It seems rather possible. The issue is whether or not the barlow is a "unit" that I simple unscrew, or if I would have to actually touch the barlow lens is some way.

I did not previously give thought to the barlow lens being not square with the optical axis, although I surely see this as a potential issue. I don't have a clue how to check for that.

SeaDeep.

--------------------
Meade Lightbridge 12" Deluxe (with 26mm EP)
(Lightbridge has DSCs, using PalmDSC)
Celestron NexStar 114 GT
EPs = Hyperion 13mm, 8mm, 5mm
Garrett Optical Gemini 9x60 LW Binoculars


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sixela
Postmaster


Reged: 12/23/04
Posts: 13991
Loc: Boechout, Belgium
Re: Collimating a NextStar 114-GT (with Barlow) new [Re: SeaDeep]
      #1367591 - 01/17/07 07:01 PM

It's going to end up being square with the optical axis if it's square with the focuser draw tube -- at least if axial collimation is perfect.

Diagonal placement doesn't even have to be optimal for this - even with an intercept angle of something other than 90 for the secondary, if the optical axis is along the focuser draw tube, it's square to whatever is square to the focuser draw tube.

--------------------

400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Skywatcher 130mm f/5 BlackLine (finder, widefield scope and solar continuum scope)
Sumerian 250mm f/4.8 Alkaid (as travelscope without platform and on Tom O. platform above).


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SeaDeep
member


Reged: 05/11/06
Posts: 87
Loc: Colorado
Re: Collimating a NextStar 114-GT (with Barlow) new [Re: sixela]
      #1369967 - 01/18/07 11:33 PM

Just a follow-up on where things stand, for those interested:

I took the focuser off. The barlow lens is easily removed by a screw ring. The lens itself is about 0.5 inch thick (I was surprised it was so thick). It seems to be made from actually two lenses that are somehow glued together.

I then took out the primary, removed it from the holder, and made a paper disc, found its center, put a reinforcement donut on the paper, cut it out. Then using the paper with the hole and another reinforcement donut I marked the center of the primary. Then reinstalled the primary.

Then I put the focuser (without barlow) back on and used a laser collimator to align the secondary onto the primary's donut center.

Then I took the focuser off, reinstalled the barlow lens, reinstalled the focuser. Then I used the laser again to collimate the primary (a "barlowed laser", essentially). I centered the image of the donut on the laser window's hole.

Then I gave the thing a star test (yeah, it was actually clear - not the best seeing, but clear).

Much better... way much better. Stars where no longer looming all over the place.

So, I guess that is one way to do it.

SeaDeep.

--------------------
Meade Lightbridge 12" Deluxe (with 26mm EP)
(Lightbridge has DSCs, using PalmDSC)
Celestron NexStar 114 GT
EPs = Hyperion 13mm, 8mm, 5mm
Garrett Optical Gemini 9x60 LW Binoculars


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Bob Griffiths
Getting Grouchy


Reged: 10/10/05
Posts: 10103
Loc: Frederick Maryland
Re: Collimating a NextStar 114-GT (with Barlow) new [Re: SeaDeep]
      #1371441 - 01/19/07 06:22 PM

Glad you got it under control...

My 1st scope was also a 114 GT
& I still use it sometimes..

As you discovered removing the
barlow lense is the trick to
getting her done...

Sorry for the late reply I do not use reflectors all that much anymore so I do not read this forum religiously.

Bob G

--------------------
CPC1100
Nexstar 8i + GPS & Rays Brackets
Denk S1 power switch
Orion 100 mm Refractor
Meade LXD 55 ...AR-5 127 mm Refractor
Gerbring heated clothing in the winter
Exploradome Observatory S.I.E. (Smiling Irish Eyes)


39*21'03" N
77*28'12" W

The sky over my head....





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Starman1
Vendor (EyepiecesEtc.com)


Reged: 06/24/03
Posts: 17639
Loc: Los Angeles
Re: Collimating a NextStar 114-GT (with Barlow) new [Re: Bob Griffiths]
      #1372156 - 01/20/07 12:49 AM

You did it just right.
Unfortunately for others, a lot of scopes with the built-in barlow have the lenses cemented into the focuser. These scopes are nearly impossible to collimate.

--------------------
Don Pensack
www.EyepiecesEtc.com
12.5" Teeter/Zambuto, 5" Maksutov
Sustaining Lifetime IDA member



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