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New Laser collimation method?

sbooder

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sbooder

sbooder

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Because I have been involved in a few threads of late discussing the pros & cons of lasers to collimate the secondary and I being one who thinks they can lie, I decided to discover why they can lie and this is what I have found.

I first collimated the whole scope using just a laser and checked it with my Cheshire and as suspected it was off.  So I wanted to know why?

I thought hard, then it hit me...Where is the laser hitting the secondary? 

We tend not to see this for real because the laser being in the focuser blocks the view and the view in the reflection of the Primary is not reliable because it is curved.

So I took the primary out of the OTA and looked up the tube and sure enough the laser was hitting the secondary off towards the far edge, about 1cm from the centre of the secondary.

So how to fix? This is what I came up with (others may have a better idea, like building a cover for the secondary with a elliptical hole in the centre so it appears a small circle when at 45 degrees).

I decided to make a cardboard cover for the end with a small hole in the centre and a small peep hole just off centre for viewing. 

I then collimated the secondary until I got the laser to come through the hole in the centre and viewing through the peep hole to make sure it was in the centre of the secondary and then checked the secondary with my Cheshire until they both agreed.  It took some time but I finally got there.

I then replace the Primary and the laser was dead centre and the Cheshire still agreed.  I then collimated the Primary with the laser and guess what?  Yes the Cheshire agreed with the laser for the first time.

I have yet to do the final test of using the scope under the sky but hopefully tonight will be good.

I will let you know the outcome.

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sbooder

sbooder

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Just don't look at the laser :-)

Your are correct of course, hence the slightly offset peep hole.  Although it is a very close peephole only 3mm off from the centre whole for as much accuracy as possible, that is close as I dare go.

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CumbrianGadgey

CumbrianGadgey

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That's interesting,

I've been put off buying a laser collimator for the same reason. I think people reckon it's a quick fix, but as you say, it has its drawbacks.

I'm new to astronomy and so far, ever since I got my Skywatcher Explorer 130, it just never seems to go out for some reason, not that I'm worried of course!

When I do have to attempt it, I was going to go the more old fashioned route, but after your post, I might consider a laser.

Thanks.

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sbooder

sbooder

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That's interesting,

I've been put off buying a laser collimator for the same reason. I think people reckon it's a quick fix, but as you say, it has its drawbacks.

I'm new to astronomy and so far, ever since I got my Skywatcher Explorer 130, it just never seems to go out for some reason, not that I'm worried of course!

When I do have to attempt it, I was going to go the more old fashioned route, but after your post, I might consider a laser.

Thanks.

Although it worked, the funny thing was that to get the laser to hit the centre hole in the cardboard at the primary end I used the Cheshire to get it so close it was nearly there by just doing what you do with a Cheshire to collimate the secondary.  Which as far as I am concerned show the superiority of the Cheshire.

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John

John

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I've been through 3 different laser collimators from the low cost Revelation, a Baader to a Hotech. I found them all somewhat frustrating in varying degrees. Currently I use a low cost cheshire / sight tube and it works really well - confirmed by star testing and scope performance "in the field" (garden). I've held on to the cheap revelation laser as it's well collimated itself now and I'm going to try barlowed laser collimation again which I did find effective last time I tried it. But the simple cheshire really does work very well.

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Jason D

Jason D

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Because I have been involved in a few threads of late discussing the pros & cons of lasers to collimate the secondary and I being one who thinks they can lie, I decided to discover why they can lie and this is what I have found.

I first collimated the whole scope using just a laser and checked it with my Cheshire and as suspected it was off.  So I wanted to know why?

I thought hard, then it hit me...Where is the laser hitting the secondary? 

Lasers don't "lie". Quality laser collimators do a great job for what they are meant to do. Lasers are meant to achieve axial alignment. Axial alignment is defined as ensuring both the eyepiece and primary mirror focal points and focal planes coincide. They are NOT meant for centering/rounding the secondary mirror under the focuser. After all, lasers interact with a tiny area of the secondary -- not with the edge of the secondary mirror. Only lasers with holographic attachment can be used to optimally position the secondary mirror under the focuser.

I recall we had this discussion before. To avoid repeating the same arguments, I am including links:

http://stargazerslounge.com/topic/200934-barlow-lazer-method/page-1

The following posts come from the above link

http://stargazerslounge.com/topic/200934-barlow-lazer-method/?p=2124976

http://stargazerslounge.com/topic/200934-barlow-lazer-method/?p=2124981

http://stargazerslounge.com/topic/200934-barlow-lazer-method/?p=2125748

Jason

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sbooder

sbooder

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Lasers don't "lie". Quality laser collimators do a great job for what they are meant to do. Lasers are meant to achieve axial alignment. Axial alignment is defined as ensuring both the eyepiece and primary mirror focal points and focal planes coincide. They are NOT meant for centering/rounding the secondary mirror under the focuser. After all, lasers interact with a tiny area of the secondary -- not with the edge of the secondary mirror. Only lasers with holographic attachment can be used to optimally position the secondary mirror under the focuser.

I recall we had this discussion before. To avoid repeating the same arguments, I am including links:

http://stargazerslounge.com/topic/200934-barlow-lazer-method/page-1

The following posts come from the above link

http://stargazerslounge.com/topic/200934-barlow-lazer-method/?p=2124976

http://stargazerslounge.com/topic/200934-barlow-lazer-method/?p=2124981

http://stargazerslounge.com/topic/200934-barlow-lazer-method/?p=2125748

Jason

Yes they do lie!!

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sbooder

sbooder

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I think that is what I have been saying.  I have a good laser but if you start from the premise that your secondary is being hit in the middle and it is not then you will go wrong from that point.  That is what I mean by lie.  Of course lasers do not actually lie, they just put a beam in a straight line (if they are decent and collimated well themselves.

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Jason D

Jason D

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I am glad that we are in agreement but the use of the term "lie" to convey the point is unfortunate since the term has a negative connotation. Just state the point clearly: Laser collimators can't be used to center the secondary mirror under the focuser unless a holographic attachment is used. This point has been well-known among experienced users for a long time.

Jason

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sbooder

sbooder

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You and sbooder are just using different writing styles. Sbooder's is more journaliste (yes, I know it shouldn't have an 'e') whereas your's is more succinct :smiley:

Steve 

Oooh!  I feel quite flattered.  Typing is very subjective when discussing points of view, you try to connect in an pleasant way but it can come across as short but it is not meant to be.  The long and short of it is...we all want to help each other and are passionate about our love of astronomy.  Only good can come from beating it out in this forum.  I love it.

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sbooder

sbooder

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Anyway, I have the scope tracking M42 at the moment and I always think that the Trapezium is a good test of collimation?  If you can see the individual stars and they are pin point then you have good collimation...and I have!   Happy bunny me.

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Moonshane

Moonshane

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to me this sums up the way to collimate. if things look great in the eyepiece just enjoy it and stop worrying about the collimation. if they don't then blame the seeing LOL

seriously though, get it just about right and then sit back and enjoy.

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