Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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Hey everyone,
I have been reading quite a lot about different methods of collimation for several different scope designs.
On my 8" lx90 SCT i use:
http://legault.club.fr/collim.html
I never truly understood how a laser collimator or any other artificial light producer can be as accurate as a star test method, but nonetheless it seems to be a very popular method here at cloudy nights.
From what i understand so far, it seems that for compound telescopes like SCT or Maks, the star method i linked above is superior.
However for Reflector types like newts or dobs, they say that a laser collimator can be used at home during dark skies, and that this method of this particular telescope design is most accurate.
I would like for someone to help clarify this issue for those of us that do not quite fully understand it.
What are the advantages and disadvantages of using the traditional star method, vs artificial "collimation tools", and how do these aspects effect the different telescope designs?
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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Achernar
Post Laureate
Reged: 02/25/06
Posts: 3690
Loc: Alabama, USA
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With fast Newtonians, you can't just collimate with a star test alone, especially if you just assembled the telescope or otherwise removed and replaced one or both mirrors. It's necessary to both mechanically align the mirrors as well as collimate them. A sight tube is a must for aligning the secondary mirror with the focuser as well as adjusting the tilt of the secondary mirror, a Cheshire eyepiece or laser collimator is necessary to at least get the primary at or close to proper collimation. A star test can be used for verifying that the telescope is in fact collimated. I found with my Cheshire eyepiece and the center spot on the primary mirror, I get exact collimation on my 10 and 4.25-inch reflectors without a star test at all.
Taras
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10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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ahh i see.
So collimation for sct telescopes only requires the latter portion of the process.
Laser collimation will be needed for reflectors and dobsonians since they have an OTA which is pieced together manually at each site relocation.
My next telescope will be a dobsonian of large aperture (at least 12"), so i have a feeling ill be needing a laser collimator regardless.
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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Mike K
sage
Reged: 04/01/07
Posts: 460
Loc: Central Texas
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Hi Brooklyn,
The lasers make it a bit easier: you can use them during daylight and you can stay at the rear of the tube without having to keep going back and forth to the eyepiece. However, I tend to fine tune the collimation on a star afterward. The laser seems to only get things close, but not perfect.
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Clear skies,
Mike K.
30°31" N 97°44" W, LP: Red
Observe: Once or twice a week back yard, once a month under dark skies
Favorites: Globulars, planets, face-on spirals
Equipment: CPC925/XT10i/TMB-92SS/Lunt LS60THaDS
Eyepieces: Naglers, Ethoi, UO HDs, Hyperion Zoom
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d1anderson
sage
Reged: 10/11/07
Posts: 252
Loc: North Central Texas
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Quote:
ahh i see.
So collimation for sct telescopes only requires the latter portion of the process.
Laser collimation will be needed for reflectors and dobsonians since they have an OTA which is pieced together manually at each site relocation.
My next telescope will be a dobsonian of large aperture (at least 12"), so i have a feeling ill be needing a laser collimator regardless.
From what I understand, a laser will emit a perfectly straight line of light but it isn't necessarily concentric to the body of the laser and therefore to the focuser tube. I think the site tube and cheshire collamation tool are a much more reliable method.
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Dewayne
-------
Z12 dob
MEADE DS2130AT NEWT 494goto
MEADE NG60 REFRACTOR
A Bunch of supplied Eye Pieces
The more you look the more you'll see! 
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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Quote:
With fast Newtonians, you can't just collimate with a star test alone, especially if you just assembled the telescope or otherwise removed and replaced one or both mirrors. It's necessary to both mechanically align the mirrors as well as collimate them. A sight tube is a must for aligning the secondary mirror with the focuser as well as adjusting the tilt of the secondary mirror, a Cheshire eyepiece or laser collimator is necessary to at least get the primary at or close to proper collimation. A star test can be used for verifying that the telescope is in fact collimated. I found with my Cheshire eyepiece and the center spot on the primary mirror, I get exact collimation on my 10 and 4.25-inch reflectors without a star test at all.
Taras
Well don't tell some of the old timers around here that. they are amazing at collimation with only a star test.
I generally agree with you though. Those guys put me to shame on collimation.
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nitroexpress
professor emeritus
Reged: 12/16/06
Posts: 534
Loc: Hemet, calif
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Book to read "Star Testing Astronomical Telescopes" (A Manual for Optical Evaluation and Adjustment)..by Harold Richard Suiter....You will learn more about real star testing and optical evaluation than a cheshire or a laser can show you.....
clear skies...........(and big mirrors)............gary 
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"Life is not measured by the number of breaths we take but by the moments that take our breath away."
10in Zhumell-$15/ push to circles-flocked tube
8in-c8-on a Meade GoTo EQ mount with a cg-5 Tripod
5in Meade with goto...
2in Gilbert reflector (1955 or so)
7x35 binos
Orion 100mm X 600mm Achromatic Refractor
Sac8-2 ccd camera
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1907
Loc: California
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Quote:
You will learn more about real star testing and optical evaluation than a cheshire or a laser can show you.....
Are you implying that some use cheshires and laser collimaors to evaluate opics? 
That would be an impossible task...
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1907
Loc: California
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Quote:
Well don't tell some of the old timers around here that. they are amazing at collimation with only a star test.
Using only a star test to collimate a way out-of-collimation Newtonian reflector is a cumbersome task. The star test will help reach axial alignment (EP+Primary focal points coincide and EP+Primary optical axes coincide) which is critical for collimation; however, it will not help much to ensure your secondary placement is optimized against the reflected light cone (optimize illumination).
Here are some of the collimation difficulties using only a star test:
1- Any adjustment of the secondary or the primary will move the star test image off-axis or even off-FOV. Therefore, you will have to nudge the OTA each time you make a minor tweak. I am not even accounting for stars drifting – I presume you can use Polaris
2- The human eye/brain has limitation to discern which is considered to be “perfectly” concentric circles. This is especially true for fast Newtonian which end up with a larger secondary offset. In this case, the central shadow in the star image will be shifted. That will make it more difficult to discern which is considered to be perfectly concentric circles.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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rickertk
sage
Reged: 06/14/07
Posts: 217
Loc: Outer Philadelphia suburbs
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There are also some of the simple practicalities of the matter: Using only a star test means that you can't collimate until and unless you have a clear night, with seeing that allows a decent degree of magnification, and your scope is already well cooled. On the flip side, many of the collimation tools used for Newtonians require a precisely placed center mark on the primary, which can't be done with any of the Cassegrain style scopes (since there's a hole there for the visual back/diagonal/eyepiece train).
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Oberwerk 11x56
Orion 8XTi
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nitroexpress
professor emeritus
Reged: 12/16/06
Posts: 534
Loc: Hemet, calif
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Read the book..........................gary
--------------------
"Life is not measured by the number of breaths we take but by the moments that take our breath away."
10in Zhumell-$15/ push to circles-flocked tube
8in-c8-on a Meade GoTo EQ mount with a cg-5 Tripod
5in Meade with goto...
2in Gilbert reflector (1955 or so)
7x35 binos
Orion 100mm X 600mm Achromatic Refractor
Sac8-2 ccd camera
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10920
Loc: Los Angeles
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Collimation has two parts: optical collimation, and mechanical collimation.
Collimation starts with centering the secondary under the focuser, which is a mechanical operation, and this requires a sight tube.
Next is the adjustment of the tilt of the secondary mirror, and this can be done with a sight tube or a laser (assuming the laser is collimated). This is an optical collimation step.
Third is the alignment of the primary mirror. This requires a cheshire or a barlowed laser. This too is an optical collimation step.
Last, and this is only important at f/5 or below, is to use an autocollimator to eliminate the residual inaccuracies of pointing the mirrors in the earlier steps. This is an optical collimation step, too.
Other mechanical collimation steps that are not critical are offsetting the secondary mirror away from the focuser, centering the primary in the tube, and "squaring" the focuser to the tube. There are a variety of protocols to accomplish these, but I emphasize that they are not really critical to attain collimation. But if your newtonian uses digital setting circles, paying attention to these steps can result in more accurate pointing (note I said "can" and not "will").
I am not a believer in the star test for collimationbecause it is so inherently limited by seeing. Plus, translating what you see to the adjustment of either mirror is difficult.
If you collimate accurately during the light and don't move the scope, you'll be fine all night, and it's a lot easier to collimate when you can see what's going on.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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auriga
sage
Reged: 03/02/06
Posts: 410
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Quote:
Hey everyone,
I have been reading quite a lot about different methods of collimation for several different scope designs.
On my 8" lx90 SCT i use:
http://legault.club.fr/collim.html
I never truly understood how a laser collimator or any other artificial light producer can be as accurate as a star test method, but nonetheless it seems to be a very popular method here at cloudy nights.
From what i understand so far, it seems that for compound telescopes like SCT or Maks, the star method i linked above is superior.
However for Reflector types like newts or dobs, they say that a laser collimator can be used at home during dark skies, and that this method of this particular telescope design is most accurate.
I would like for someone to help clarify this issue for those of us that do not quite fully understand it.
What are the advantages and disadvantages of using the traditional star method, vs artificial "collimation tools", and how do these aspects effect the different telescope designs?
Although there are multiple experts on collimaion, all of them very impressive, they don't all agree. Rod Mollise, who has written several books on SCTs, has very recently posted on "that other forum" that Polaris is the only correct method of collimating an SCT and that the other methods lead to miscolllmation.
I recently looked through an acquaintance's Celestron 8 SE, which had been collimated only on Polaris at 200x, and I found the images of Jupiter to be very sharp indeed. I was very impressed.
I think Howie Glatter has posted somewhere that it is impossible to fully collimate an SCT because there are too few degrees of freedom in adjusting the optics.
So, take your pick, and good luck,
Bill Meyers
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10920
Loc: Los Angeles
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You can get around the seeing problems when collimating an SCt if you collimate using the reflection of the sun off of a Christmas tree ornament at about 200' or more. The image appears just like a star, and the seeing problems through 200' of air will not interfere with collimation. You can use crazy high powers and really take your time.
In my post above, I was referring only to newtonian collimation.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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"I recently looked through an acquaintance's Celestron 8 SE, which had been collimated only on Polaris at 200x, and I found the images of Jupiter to be very sharp indeed. I was very impressed."
Hmmm....but polaris is so high in the sky...wouldn't the collimation of lower degree objects be completely off?
Most people i have talked to suggest that I use ALTAIR, DENEB, or VEGA as collimation stars.
They form a very noticable triangle, although they're not in the same constellations.
Altair is around 45 degrees all night long, while deneb and vega alternate on who is higher and lower around 50-70 deg.
Lets not forget, for most people in the northern hemisphere, ALTAIR VEGA and DENEB are the 3 brightest stars in the night sky. For me in new jersey, the only other stars that are close to these in magnitude are maybe Arcturus and the other bright red giant, whose name escapes me right now.
I think the collimation star should depend on what degree range you are going to observe.
For me, I do all of my observing over 20 degrees in all directions. To the north and east I have a large forest of trees blocking my view all the way up to 40-60 degrees high!
So, for the most part I end up using Vega since its ridiculously bright...which is just what you need to get that near perfect collimation. The brighter the star is, the more noticeable it's diffraction rings will be.
--------------------
Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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FirstSight
Carpal Tunnel
Reged: 12/26/05
Posts: 2514
Loc: Raleigh, NC
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Quote:
You can get around the seeing problems when collimating an SCt if you collimate using the reflection of the sun off of a Christmas tree ornament at about 200' or more. The image appears just like a star, and the seeing problems through 200' of air will not interfere with collimation. You can use crazy high powers and really take your time.
Don:
I assume this same Christmas-tree ornament-generated "artificial star" would work equally well for collimating refractors?
--------------------
Chris M., aka "First Sight"
Orion XT12i Dob with Moonlite CR-2 focuser
WO Megrez 90 refractor on UniStar Light mount
Nikon 10x50 Binoculars
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rickertk
sage
Reged: 06/14/07
Posts: 217
Loc: Outer Philadelphia suburbs
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Quote:
"I recently looked through an acquaintance's Celestron 8 SE, which had been collimated only on Polaris at 200x, and I found the images of Jupiter to be very sharp indeed. I was very impressed."
Hmmm....but polaris is so high in the sky...wouldn't the collimation of lower degree objects be completely off?
Most people i have talked to suggest that I use ALTAIR, DENEB, or VEGA as collimation stars.
They form a very noticable triangle, although they're not in the same constellations.
Altair is around 45 degrees all night long, while deneb and vega alternate on who is higher and lower around 50-70 deg.
Lets not forget, for most people in the northern hemisphere, ALTAIR VEGA and DENEB are the 3 brightest stars in the night sky. For me in new jersey, the only other stars that are close to these in magnitude are maybe Arcturus and the other bright red giant, whose name escapes me right now.
I think the collimation star should depend on what degree range you are going to observe.
For me, I do all of my observing over 20 degrees in all directions. To the north and east I have a large forest of trees blocking my view all the way up to 40-60 degrees high!
So, for the most part I end up using Vega since its ridiculously bright...which is just what you need to get that near perfect collimation. The brighter the star is, the more noticeable it's diffraction rings will be.
Brightness of the star is a good thing for collimation, yes. The altitude of the star shouldn't make a bit of difference, unless your telescope has mechanical shifts at different altitudes. (Not unheard of for larger newtonians, but I would think rare for SCTs). The point of collimation is to get the optics of the scope optimally aligned; the altitude of what you are pointing at is immaterial. Higher would generally be better given that you'll have less atmospheric turbulence. Polaris is often recommended because it won't move during the collimation; with anything else, at best, you'll have to try to collimate with the drives still on.
Keith
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Oberwerk 11x56
Orion 8XTi
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10920
Loc: Los Angeles
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Quote:
Quote:
You can get around the seeing problems when collimating an SCt if you collimate using the reflection of the sun off of a Christmas tree ornament at about 200' or more. The image appears just like a star, and the seeing problems through 200' of air will not interfere with collimation. You can use crazy high powers and really take your time.
Don:
I assume this same Christmas-tree ornament-generated "artificial star" would work equally well for collimating refractors?
Sure.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Protheus
Vaguely offended
Reged: 09/01/07
Posts: 4643
Loc: Illinois, US
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Quote:
Rod Mollise, who has written several books on SCTs, has very recently posted on "that other forum" that Polaris is the only correct method of collimating an SCT and that the other methods lead to miscolllmation.
Well, if Uncle Rod said it, it must be true. That aside, I can't help but wonder if it wasn't his primary concern that the star stay centered during the collimation. If you end up having to track the object during the process, it might indeed cause quite a bit of trouble.
Chris
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"To tread the sharp edge of a sword;
to run on smooth-frozen ice,
one needs no footsteps to follow..."
"Well, people sometimes ask me 'how did you get involved in astronomy?' I said 'I got born, what's your problem?'" -- John Dobson
"In discussing the large-scale structure of the cosmos, astronomers sometimes say that space is curved, or that the universe is finite but unbounded. Whatever are they talking about?" -- Carl Sagan
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