Cheshire or laser - discussions?

[earth titan]

Well I've just ordered a cheshire collimator as the best guide I can find (thanks Astro Babe) deals with the cheshire.

However, I've also been told I'll probably make any collimation errors worse rather than better if I use the Cheshire.

Price was one of the governing factors with selecting a cheshire and the fact you can't adjust your laser colimator (unless you spent megga bucks on one) so it could also be out.

So what do you think?

Will I totally screw it up?

[kniclander]

who on earth said that a cheshire would make collimation errors worse?? Whilst you will get lots of views on what is the best way, I'd be surpriesd if you got many people agreeing with that. Having said that, I'll retreat and watch the sparks fly......

[pvaz]

Cheshire works great. The laser would make it easier and faster but you need a good one (i.e: hotech). If you buy a cheap laser they often come with errors.

You probably read "a cheap laser can causer errors".

[Doc]

A cheshire collimator once mastered is one of the most accurate forms of collimating your scope. The problem is it can be a bit daunting using one. I found you can get different results depending on where you place your line of sight.

A good quality laser such as the Hotech is very quick and easy to use and very accurate.

The best way is a star test which doesn't cost you anything to do and is very accurate.

[sgazer]

A cheshire if you can reach the primary adjusting bolts whilst looking into the eyepiece, or have someone to help you. I've never read a cheshire can make it worse, but plenty saying a laser can if the initial stages are wrong, or the laser out of cal.

ps. sorry, don't know why I had a quotation in there...deleted now.

[WayBig]

yesterday I was flicking through a sky at night issue and they were reviewing cheshires and lasers.

They preferred the cheshires, with an Orion cheshire coming top. Apparently cheshires have the advantage that you can use them in the daytime.

They also said that these were fine for scopes of f6 or slower, but faster scopes required more precise collimation (though they didnt say how to achieve that)

I've not used either so cant vouch for how accurate their review is

[earth titan]

So far this is as I had suspected.

Sounds like both have their drawbacks and strengths.

Gonna check, check, check and double check before I start fiddling about with things.

[pvaz]

Lasers can be used at day time too. Problem is a good laser colli costs money.

The hotech 2" (regarded by many as the best) costs 127£. I got one and it makes the process accurate and fast. But with a bit more patience you don't need one. I often use it in daytime without a problem.

[SlipperySquid]

I use both..I have the Orion Cheshire and an Antares laser. I use the cheshire for getting a good acurate collimation and then use the laser when I just need to quickly check that things are aligned.

Best thing about the laser is that I can adjust the primary and see the effect of the adjustment at the same time....Instead of adjust a bit...look through chesire...adjust a bit more....look through the chesire..

Yeah Collimation is a pain in the backside but it gives us something to do whilst the clouds are in!

[acey]

A Cheshire is well known to be the best collimating tool. A laser risks amplifying errors because of focuser slop; one way around this is Nils Olof Carlin's "barlowed laser" method. But even without a barlow, a laser is useful for adjusting the secondary. Carlin is an acknowledged expert on collimation and his website has a lot of info.

FAQ about Collimating a Newtonian telescope

I use laser for secondary and cheshire for primary. I invariably find that once collimated, the laser tells me that I'm not! The reason is focuser slop - I trust the cheshire (and a star test).

[themos]

The other night I used someone else's laser for primary collimation and then checked with my Cheshire. It was spot on.

The laser also indicated that secondary collimation was a bit off, the spot was hitting the marked ring on the primary instead of falling in its centre.

[acey]

I find that when using the laser for the secondary, the merest touch on the laser, or turning it in the focuser, can be enough to make the spot go off-centre on the primary. But this limited degree of accuracy is still good enough to get the secondary satisfactorily positioned (and a very convenient way to do it in the dark).

The problem with using the return beam in order to collimate the primary is that the error now gets magnified: this is the point of Carlin's barlowed method. If a cheshire is used for the primary, the error at stage one is effectively cancelled. As often as not, you end up seeing that the return beam, visible as a spot on the laser's angled face, is off-centre, though you know from the cheshire that you're perfectly collimated.

[Astronut]

Cheshire or laser? Cheshire every time. Lasers are ok as a check out in the field but you need a good one and a consistent fit in the focuser. If there's any slop in the focuser then forget about lasers. Also a laser is useless for the all important step of getting the secondary aligned and concentric with the drawtube.

The ultimate system (especially for a 'fast' dob) is the 'Catseye' tools (cheshire+ and then some) IMHO.

[TopHouse]

I agree with Astronut 100% lasers lie, whether it be hotech or any other, I'm still waiting for my V2 prototype to be engineered of a piece of kit i've been playing around with and will post full details as and when it's sorted.

The glatter laser is now the most accurate laser method with regards secondary, but you can NOT accurately align a secondary mirror with a single point or barlowed laser. I've spent a lot of time with this now and I can adjust a secondary to many different positions that will look fine to the eye and to a laser collimater but are actually out. And I can prove it! All will be revealed when my engineer gets my part finished.

[Moonshane]

I collimate my dob every session and use a laser all the time and find it is both quick and accurate, certainly good enough for visual. you need to ensure that it is collimated itself though before you collimate your scope with it. I use the laser for the secondary and the barlowed laser method already mentioned for the primary; this is essential to remove the effects of the slop in the focuser; you are collimating to the shadow of your primary mark, not the dot of the laser. that said, I also have a cheshire and usually check with this after doing the collimation with the laser - I never have to change the collimation after checking with the cheshire. my laser is a cheap (£30) Revelation Astro version.

having a 1600mm focal length makes collimating the primary quite tedious with the Cheshire as you are back and forth constantly and a laser is also much easier in the dark if required (unless you have three hands!)

I have tried a start test too and it always looks good.

[beamish]

So guys what would be recommendations regarding the best Cheshire available ???

[Jason D]

Collimation consists of three alignments:

1- Rounding/centering the secondary mirror under the focuser which is meant to center the 100% illumination area within the FOV. This step is the least critical especially for visual observation. It is performed by using either a sight-tube or a holographic laser collimator.

2- Aligning the eyepiece optical axis is the second alignment. It is typically performed indirectly by using a laser or a sight tube with cross-hairs in place of the eyepiece. This alignment is responsible for eliminating the tilt between the focal plane of the eyepiece and the focal plane of the primary mirror. The error tolerance of the tilt is somewhat generous for visual observation. That is, if the forward laser beam hits off-center by few mm’s, the image at the eyepiece will hardly be impacted for visual observation.

3- Aligning the primary optical axis is the third and last alignment. This is by far the most important alignment which is typically performed by laser collimators (including barlowed laser) and cheshires. If you use unbarlowed laser then the error of the eyepiece optical axis alignment will carry over and impact the accuracy of aligning the more critical primary axis alignment. If you use barlowed laser then you do not carry over the error – this is the power of the barlowed laser collimator method. But the barlowed laser collimator does not eliminate the focal plane tilt errors introduced while performing the second alignment.

Jason

[DarkerSky]

This is a good thread! A good balanced argument for both cheshires and lasers, which is good to see.

I use a cheshire, and very happy with the results. I would add though that whatever device is used that a proper collimation process is followed, starting with centring the secondary in the draw tube visually.

One scope I recently looked through looked perfectly aligned through a cheshire, but visually it was obvious that the secondary wasnt far enough down the tube.

I also agree with Doc that there is no substitute for a star test to fine tune what you have

[Doc]

A star test is a must for that final tweak and normally it is only a very small tweak on the primary screws.

I use my 7mm Uwan and defocus on polaris, this gives me a pretty large image and basically just position the secondary image bang in the middle of the defocused star.

You can also use the coma inherent in the mirror to your adavantage see illustration for details.

[Jason D]

I agree with Astronut 100% lasers lie, whether it be hotech or any other, I'm still waiting for my V2 prototype to be engineered of a piece of kit i've been playing around with and will post full details as and when it's sorted.

The glatter laser is now the most accurate laser method with regards secondary, but you can NOT accurately align a secondary mirror with a single point or barlowed laser. I've spent a lot of time with this now and I can adjust a secondary to many different positions that will look fine to the eye and to a laser collimater but are actually out. And I can prove it! All will be revealed when my engineer gets my part finished.

Lasers are not meant to optimize the secondary mirror alignment under the focuser. The only exception is when using the holographic attachment that comes with Glatter lasers.

We strive to center/round the secondary mirror appearance under the focuser to optimize the placement of the 100% illumination area within the FOV field. Centering/rounding the secondary mirror under the focuser does not impact the resolution and sharpness of the image – unless the secondary mirror is grossly off. Small errors will not be noticed visually especially for typical oversized seocndaries that come with mass produced scopes. Secondary centering/roundness alignment is more critical for imagers.

There are infinite positions for the secondary mirror which will allow the forward laser beam to strike the center of the primary mirror then retrace its path all the way to the source.

Few days ago I included the following animation in this thread

"http://stargazerslounge.com/equipment-help/102722-got-my-chesire-i-think-collimation-out-slightly.html"

Each frame in the animation will yield perfect laser collimation (unbarlowed and barlowed) yet the secondary mirror rotation/tilt position changes. Only one frame represents the optimal placement.

Jason

[Jason D]

Doc,

I noticed you are showing the spider vanes shadows in your photo. It is typically recommended to perform star collimation at high magnification with little defocus. That is, just enough defocus to show few rings without showing any of the spider vanes.

Few more points about star collimation:

1- Star collimation can't be used to adjust the secondary mirror which means it can't be used to correct any focal plane tilt errors between the eyepiece and the primary mirror -- granted this is less critical for visual observation.

2- Beware of the eyepiece used. Some show astigmatism around the edges of the FOV which could be confused as coma.

Jason

[DarkerSky]

That's an excellent animation Jason. Makes the point about the secondary much better than my feeble attempt

[Doc]

Doc,

I noticed you are showing the spider vanes shadows in your photo. It is typically recommended to perform star collimation at high magnification with little defocus. That is, just enough defocus to show few rings without showing any of the spider vanes.

Few more points about star collimation:

1- Star collimation can't be used to adjust the secondary mirror which means it can't be used to correct any focal plane tilt errors between the eyepiece and the primary mirror -- granted this is less critical for visual observation.

2- Beware of the eyepiece used. Some show astigmatism around the edges of the FOV which could be confused as coma.

Jason

Cheers Jason I never knew that it was advisable to star test without the spider vanes showing. Also I never touch the secondary on a star test I only adjust the primary and it only requires the smallest of adjustments.

[Moonshane]

I agree this is a great thread. it's really great that viewpoints can be put across without any of the sniping and arguing that I have seen on other forums (fora?).

Reading between the lines give me confidence that assuming I centralise the secondary under the focuser with a sight tube, then use the bare laser on the secondary tilt and the barlowed laser on the primary then I'll be fine for visual. this is supported by a) what I can see through the scope and what the Cheshire confirms when I double check with this.

I must try star testing sometime. I think I have done this but have not really read up enough to ensure I am doing it right.

[TopHouse]

@shane

Yes, with what's readily available, that will give you decent usable colli. Once the colli OCD kicks in though you'll get fussier and fussier lol.