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Collimating Newtonian versus SCT

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#1 scoale

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Posted 05 December 2021 - 07:39 PM

I'm contemplating the pros and cons of collimating a Newtonian versus an SCT.  Specifically, the fact that a Newtonian can be collimated with tools, during daylight hours, with less thermal issues, and without relying on above-average seeing seems very appealing to me.  The specific application I am considering is an ~ 8-10" ota with a longish F/6 focal length for visual planetary observing.

 

Am I thinking about this correctly?  Any other considerations?  Thank you.



#2 markb

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Posted 05 December 2021 - 07:51 PM

There are easy ways to collimate an SCT in daylight with an artificial star (distance is not particularly critical although it absolutely is critical for the entirely separate task of Ronchi or Suiter star testing).

 

Other opinions may differ, but I find the centering the shadow method to be completely useless to me, and use the following diffraction-based methods after starting with a casady daytime rough collimation, not normally needed.

 

Search CN for the Robin Casady method to find some of my posts, or simply Google for the starting method.

 

I crude collimate an SCT (only needed if there was a prior issue like Bob's knobs installation), and then go to a virtually free Duncan 'radioactivity symbol' mask cut out of cardboard/foamcore (sloppy ones work just fine!) or an inexpensive Tri-Bahtinov mask off eBay, and then get nearly perfect collimation by using the slightly defocused Poisson Spot Diffraction pattern (like the Casady reflections, just a concentric circle centering).

 

All are pretty seeing-insensitive.

 

If seeing is very good, I then do the usual tweak at night. But my seeing is almost never that good.

 

All in daytime, all easy.

 

I have shimmed my secondary housings to prevent movement within the perforation, and it is my understanding that reasonably tight collimation screws prevent many shift issues.

 

My only newt is a non-adjustable Astroscan, but it is my understanding that you told me and cannot be accommodated until they've been set up for the night since the primaries can shift a bit during relocating the scope. Regular newtonian users can tell you, I cannot. But SCT daytime collimation can work.


Edited by markb, 05 December 2021 - 08:00 PM.


#3 MitchAlsup

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Posted 05 December 2021 - 07:58 PM

Takahashi makes a telescope with a reticle that makes centration of any Cassegrainian easy.

 

After centration, SCT collimation is simply getting the secondary pointed precisely.

Centration is what is hard about initial collimation of an SCT--collimation when you start with a table full of parts and assemble the optics into the housing.

 

If all the optics are on the same centerline, collimation is not usefully different Newt versus SCT (excepting the mirror you are adjusting.)



#4 markb

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Posted 05 December 2021 - 08:23 PM

+1 on Mitch.

 

Centration, or making the mechanical and optical axes coincident, should be an issue never or rarely encountered, but pops up far too frequently after careless disassembly for cleaning or other disassembly of SCTs.

 

Collimation on a properly set up SCT just requires different tools or techniques from a newt as in my post.

 

Refractors might be the easiest to get almost perfect, with just a refractor Cheshire modified to black out half the shiny plug, and a rigid work platform., at least until the final tweak at night.



#5 Vic Menard

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Posted 05 December 2021 - 09:17 PM

I'm contemplating the pros and cons of collimating a Newtonian versus an SCT.  Specifically, the fact that a Newtonian can be collimated with tools, during daylight hours, with less thermal issues, and without relying on above-average seeing seems very appealing to me.  The specific application I am considering is an ~ 8-10" ota with a longish F/6 focal length for visual planetary observing.

An 8- (or 10-) inch f/6 Newtonian will be very easy to collimate to high magnification performance tolerance and given quality optics and an optimized smaller secondary, should be an excellent planetary scope. An 8- (or 10-) inch f/10 SCT is an excellent multipurpose scope that packs good performance in a small package. In my experience, the long focus Newtonian has the visual performance edge, while the SCT is easily handled and has many accessories.

 

It's a tough choice, but collimation shouldn't be the deciding factor. 


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#6 Redbetter

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Posted 06 December 2021 - 12:45 AM

Maks and SCT's should not need collimation very often, so I don't consider it an issue at all after initial collimation is dialed in and fine tuned.  My 8" SCT literally held its collimation for well over a decade, and it took a very long cargo flight beating that damaged parts of the forks to finally throw it out.  My Mak also seems to be holding collimation well.  As long as there isn't some sort of primary mirror shift or wonky misaligned diagonals, the collimation should hold up.  No need for Bob's knobs--and going that route is more likely to require frequent collimation.

 

With a Newt, some sort of collimation tweak is generally required each time it is used, or at least after it is torn down then set up again and/or transported.  A simple laser system can accomplish this well if the laser is well aligned and true.  A Barlowed laser can help fine tune technique, but what I have found with the 10" f/5 is that the Barlow became an unnecessary step once I had technique down.  With the 20" f/5 I use the Glatter laser and Tublug. 


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#7 Asbytec

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Posted 06 December 2021 - 07:24 PM

Collimating a Newt is fun. :)


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#8 Redbetter

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Posted 06 December 2021 - 07:47 PM

Collimating a Newt is fun. smile.gif

That's the spirit!


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#9 scoale

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Posted 06 December 2021 - 09:44 PM

Thanks for responses.  I own two SCT scopes, an 8" and an 11" (I also own a 5" APO).  I use both an artificial star (astrozap) and a real star to collimate.  Donut collimation is easy but fine collimation on an in-focus star is more challenging.  That said, I do NOT have experience with a Newtonian and am trying to do my homework.  I am currently considering a Newtonian for planetary viewing.  Maybe an 8" F/6.  My impression is that Newts need to be collimated more frequently (especially faster ones), but tools are available to make short work of it.

 

All the best, Stephen


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#10 markb

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Posted 07 December 2021 - 12:19 PM

Try the Tri-bhatinov or Duncan masks on your scts, followed by the concentric Poisson Spot diffraction rings. Both work well on artificial stars (Hubble 5 star, and laser/Sun illuminated chromed ball bearing types).

 

You may see the final adjustment process become significantly easier.

 

It yields a far superior and easier collimation result (I'd kind of lean towards a 'fun' label here as well since it is so easy and results are so obvious and satisfying). Donut 'collimation' can yield results so far off, that the final in-focus tweaking is frustrating or nearly impossible. Small turns on the screws seemed not to make the expected changes in the Airy disc, so it obviously did not work for me. But i expect some judge shadow centering better than I do.

 

Once I switched to the masks and concentric diffraction rings my final tweaks made obvious, textbook, changes in the Airy disc pattern, even with tiny adjustment changes. Image improvements were obvious, and I'll never go back.

 

+1 on Redbetters post.

 

Newt collimation just uses different tools, and a bit of experience. Had I not sold my f3.65 SN I would have gone to the Catseye setup, but I had finally gotten the hang of the barlowed laser method. It just took practice, for me.


Edited by markb, 07 December 2021 - 12:20 PM.

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#11 Bomber Bob

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Posted 07 December 2021 - 01:05 PM

I'd favor an 8" F6 Newt for visual / planetary.  More reliable performance vs an 8" SCT.  Depending on how you build your rig, the Newt can be much easier to deal with, too.  Collimating a Newt is about the easiest of the non-refractor scope types.


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#12 havasman

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Posted 07 December 2021 - 01:34 PM

My impression is that Newts need to be collimated more frequently (especially faster ones), but tools are available to make short work of it.

 

You have the idea. One should only collimate a Newtonian system when they want to know it will perform as well as possible. For most that's EVERY time. So you get good tools and learn to use them well. I collimate any of my Dobs in 2 minutes after they travel disassembled > 100 miles to the observation site including 7 or 8 miles of dirt road and reassembled on site. Last week at the site I collimated the 12.5" while talking with Larry, got it done in < 2 minutes easily and then observed NGC604 at 464x and 570x quite successfully during the session.

 

As Vic says above, collimation is not a good criteria on which to base the Dob/SCT decision. It'd be like choosing a car based on how much tire pressure was called for.


Edited by havasman, 07 December 2021 - 01:35 PM.

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#13 scoale

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Posted 07 December 2021 - 01:35 PM

Try the Tri-bhatinov or Duncan masks on your scts, followed by the concentric Poisson Spot diffraction rings. Both work well on artificial stars (Hubble 5 star, and laser/Sun illuminated chromed ball bearing types).

 

You may see the final adjustment process become significantly easier.

 

It yields a far superior and easier collimation result (I'd kind of lean towards a 'fun' label here as well since it is so easy and results are so obvious and satisfying). Donut 'collimation' can yield results so far off, that the final in-focus tweaking is frustrating or nearly impossible. Small turns on the screws seemed not to make the expected changes in the Airy disc, so it obviously did not work for me. But i expect some judge shadow centering better than I do.

 

Once I switched to the masks and concentric diffraction rings my final tweaks made obvious, textbook, changes in the Airy disc pattern, even with tiny adjustment changes. Image improvements were obvious, and I'll never go back.

 

+1 on Redbetters post.

 

Newt collimation just uses different tools, and a bit of experience. Had I not sold my f3.65 SN I would have gone to the Catseye setup, but I had finally gotten the hang of the barlowed laser method. It just took practice, for me.

Ordering mask today.  Thanks for response


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#14 scoale

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Posted 07 December 2021 - 01:40 PM

You have the idea. One should only collimate a Newtonian system when they want to know it will perform as well as possible. For most that's EVERY time. So you get good tools and learn to use them well. I collimate any of my Dobs in 2 minutes after they travel disassembled > 100 miles to the observation site including 7 or 8 miles of dirt road and reassembled on site. Last week at the site I collimated the 12.5" while talking with Larry, got it done in < 2 minutes easily and then observed NGC604 at 464x and 570x quite successfully during the session.

 

As Vic says above, collimation is not a good criteria on which to base the Dob/SCT decision. It'd be like choosing a car based on how much tire pressure was called for.

Thanks havasman.  Definitely not basing decision strictly on collimation but rather trying to ensure I understand all elements. BTW, this would be an additional scope, not looking to get rid of SCT.


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#15 Jon Isaacs

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Posted 08 December 2021 - 03:06 AM

Thanks for responses.  I own two SCT scopes, an 8" and an 11" (I also own a 5" APO).  I use both an artificial star (astrozap) and a real star to collimate.  Donut collimation is easy but fine collimation on an in-focus star is more challenging.  That said, I do NOT have experience with a Newtonian and am trying to do my homework.  I am currently considering a Newtonian for planetary viewing.  Maybe an 8" F/6.  My impression is that Newts need to be collimated more frequently (especially faster ones), but tools are available to make short work of it.

 

All the best, Stephen

That is my experience as well.  

 

One thing about collimating an SCT indoors is that you are probably collimating it horizontally rather than at an angle where you might be observing.  Primary mirrors do flop around a bit.

 

Jon


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