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wolfpaw

Someone else with collimation woes (with photos)

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I collimated my telescope for the first time last week, used a normal Cheshire with no lasers. Yes it was a bit of a hassle going from one side of the telescope to the other to check your progress, but got it right at the end. I have to say I watched/read around 30 mins of videos and articles about collimation. Didn't touch the secondary mirror as I've been told you rarely need to adjust it, I think that saved a lot of time.  

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I collimated my telescope for the first time last week, used a normal Cheshire with no lasers. Yes it was a bit of a hassle going from one side of the telescope to the other to check your progress, but got it right at the end. I have to say I watched/read around 30 mins of videos and articles about collimation. Didn't touch the secondary mirror as I've been told you rarely need to adjust it, I think that saved a lot of time.  

Wish I'd done that when I first got my telescope! I ended up fiddling with the secondary as I didn't know what I was up to, despite reading endless stuff about collimation online. I like to think I've progressed a lot since then. Collimation is really something that can only be understood with a hands-on approach. IMO it's very hard to explain without actually doing it yourself and working out exactly what does what.

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Wish I'd done that when I first got my telescope! I ended up fiddling with the secondary as I didn't know what I was up to, despite reading endless stuff about collimation online. I like to think I've progressed a lot since then. Collimation is really something that can only be understood with a hands-on approach. IMO it's very hard to explain without actually doing it yourself and working out exactly what does what.

You don't need to worry, your collimation is almost right judging by the pics you posted, plus you know now how to adjust the secondary :p. I'm sure that the experienced members will guide you well, all I can do is recommend reading this if you haven't already, one of the best articles in collimation. http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Good luck buddy

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I go outside and notice it's clear.' Yippee!' I think, 'now I can try out the recollimation and get a look at the SN in M82'. I set the thing up, go back inside, get a cup of tea and a snack while the mirror cools, go back out and it's starting to drizzle! Much swearing and I'm back inside. How frustrating.

:sad:

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Hi Wolfpaw,

For the secondary there is a good explanation here, at around 6:10

Though the whole video is pretty cool.

Agreed.  I think there are several in a series of "newtonian collimation for the rectally retentive" videos there and they're definitely worth a watch.

If you really want to know it's right you have to go back to the beginning, which is making sure the focuser is properly square to the tube and the optical axis.  Then you can work on getting the secondary in the right place and then on collimating the mirrors.  In my experience there is absolutely no reason to believe that as delivered the focuser will be spot on.  Put a laser collimator in it with the spider removed and you may well find that it doesn't run at right angles to the optical axis or even wanders around as the focuser is moved in and out.  If that's happening you're never going to get reliable collimation.

James

Edited by JamesF

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Agreed.  I think there are several in a series of "newtonian collimation for the rectally retentive" videos there and they're definitely worth a watch.

If you really want to know it's right you have to go back to the beginning, which is making sure the focuser is properly square to the tube and the optical axis.  Then you can work on getting the secondary in the right place and then on collimating the mirrors.  In my experience there is absolutely no reason to believe that as delivered the focuser will be spot on.  Put a laser collimator in it with the spider removed and you may well find that it doesn't run at right angles to the optical axis or even wanders around as the focuser is moved in and out.  If that's happening you're never going to get reliable collimation.

James

this video by astronomy shed is great and i have watched it a few times, but how do you no your camera is square to the tube

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I'm not sure you needed any help or advice, more like reassurance and confirmation of a job well done!

Enjoy the clear skies.

Cheers

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this video by astronomy shed is great and i have watched it a few times, but how do you no your camera is square to the tube

It's a good question.  And perhaps there's no way to be absolutely certain that the camera sensor is square to the focuser tube.  I adjusted mine so I could see the end of the focuser tube in the image and made sure that looked round by comparing it to the alignment rings in the Mire de Collimation program.

James

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with a fast scope there is an off set

I know this is the case with some scopes, but is there any definite source for this as regards the Skywatcher scopes, and any list of scopes to which it applies?  Certainly looking at the spider for the 250PX, it doesn't look as though it's intended to have an offset.

James

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I can only find these and comment on my own and what others have said on here about there scopes having an offset,

http://www.lcas-astronomy.org/articles/display.php?filename=newtonian_secondary_offset&category=telescopes

http://www.skyandtelescope.com/howto/diy/3306996.html

That suggests you'd need a 3.4mm offset towards the primary for a 250PX.  I think that might be a bit marginal with the supplied fittings in the 250PX.  I think you'd need a new set of collimation bolts and possibly a longer secondary retaining bolt too.

James

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Secondary offset concept intimidates many. It should not. Below are two facts about secondary mirror offset:

1- All properly collimated Newtonians will have a  secondary mirror offset regardless of F-ratio. The amount of offset dependent on the F-ratio and the secondary mirror size. Faster scopes will have larger offset and therefore it is more detectable compared to slower scopes.

2- There are two types of offset: Away-from-focuser offset and towards-the-primary offset. The latter is automatically taken care of when the secondary mirror is centered under the focuser. There is no need to calculate the offset or use a measurement tape. The first type of offset can be ignored if your scope does not have DSC. Even if your scope has DSC, lack of away-from-focuser offset will contribute just a tiny error to DSC calculation.

Jason

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I have taken my secondary out a couple of times and Shane a fellow SGL member took it out to flock the scope once i put it back in and collimated the scope i had the offset and i didn't intentionally do it the scope performs really well. I think as others have said once you centre the secondary under the focuser you by no fault put the offset in so you dont need to measure anything.

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I'm not sure you needed any help or advice, more like reassurance and confirmation of a job well done!

Enjoy the clear skies.

Cheers

Thanks! I think the problem for me, and others I'm sure, is that I don't have anyone around with a lot experience who can actually look through the scope and give advice. The telescope turns up on the doorstep and you're into collimating, especially with a 10''/1200 scope. It's why forums like this are invaluable :)

I've not had chance to try out the barlowed laser technique. I just wish the skies would clear up a little.

One last thing: Jason said that the white circle highlighted below is the reflection of the 1.25 to 2-inch adaptor. Should the primary mirror spot be in the centre of this white circle? As can be seen, at the moment it's a bit off. If it does have to be in the centre what do I need to alter on the scope in order to correct it?

vg5xj9.jpg

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One last thing: Jason said that the white circle highlighted below is the reflection of the 1.25 to 2-inch adaptor. Should the primary mirror spot be in the centre of this white circle? As can be seen, at the moment it's a bit off. If it does have to be in the centre what do I need to alter on the scope in order to correct it?

vg5xj9.jpg

In theory, both should be aligned; however, I will not place too much importance on this alignment.

The following might sound confusing but you asked the question. All reflections seen within the secondary mirror in your photo are virtually located at infinity including the 2"->1.25" adapter white ring. The only except is the primary center spot which is located at a distance equals the focal length of your scope. That means both the primary center spot reflection and the white ring are highly susceptible to parallax.

Try to move your eye's placement. Do you see parallax? That is, do you see the positional relationship between the white ring and the center spot change?

Jason

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Hi ,

As I have just bought a 150 PDS ( my first Newt ) and by heck it was out of collimation, I have followed this thread with great  interest. In the end I had to take everything out and put it back together including the focuser. I think that your collimation is spot on. The problem that I have with mine is that no matter what I do the secondary does not center under the focus tube. I have a good circle when I look through the collimation cap so the angle is correct  but the circle is always offset towards the primary. The other problem that I discovered while I took the spider and the secondary holder out was that the threaded hole for the central Phillips screw is not true to the face of the of the secondary's holder, it has been drilled at a slight angle so the secondary is  at an angle with respect to the spider and the tilt screws, this is not a big problem as theis adjustable to some extent.  As you have mentioned I have not had single clear period long enough to star test the collimation in the last week or so since got the scope so I share you frustration.  

Regards,

A.G

Edited by lensman57

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this video by astronomy shed is great and i have watched it a few times, but how do you no your camera is square to the tube

I used a point and click camera and the lens fits down the tube, but its not exact. So its a best guess. Now i have the ASI camera so can try it with that, it fits just like an EP  so "should" be centred. However as James says, focuser has to be perfectly aligned and the stock one on my PL is somewhat wanting in that department. So best guess, star test and crossed fingers.

Secondary offset concept intimidates many. 

Just when i thought i had collimation figured out this comes along :) But seems that the towards-the-primary offset is taken care of by the method im using.

But im wondering... 

Ive collimated as best as i can and a star test looks fine.

What would be the difference in a perfect system ( focuser squared, correct offset, primary perfectly centred ) but under typical UK seeing conditions?

Given that i use a 6" F8, should I be getting "retentive" over this? Or is it a case of a lot of work for no real gain?

Cheers

Mark

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thanks for that wookie

Not a problem i have got some pdf documents as well pm me if you want them.

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As a way of taking my mind off the ever-present rain I've spent the afternoon fiddling around with the collimation. I think I'm hooked on it, sadly, partly because I finally understand what it actually involves and it's still a novelty.

Anyway I deliberately knocked the secondary and primary mirrors out of collimation and went about setting it back up again (I said it was sad!). I used the collimator cap and Cheshire to get it back into what I think is the best collimation it's been - although only taking it outside will tell.

I've collimated the laser pretty accurately but didn't use it this time around. When I test the laser at a distance of about 4 metres the red dot only moves about 2mm, so I'm thinking it's fairly good. I put the laser in the focuser after I'd finished with the Cheshire and, according to the laser, both the secondary and the primary are some way out!! This just isn't the case. I'm sure of it.

The laser moves around in the focuser too much for it ever to be accurate, which I've read is a common problem. I can manually move the laser around in the focuser so that the red dot appears both in the primary's centre spot when looking down the tube and in the centre of silver circle on the collimator itself. Even wrapping tape around the laser doesn't seem to guarantee that the red dot always appears in the same place. Masking tape is too thick and the white plumbing tape I was using doesn't go on evenly enough.

I know/hope that the telescope is now well-collimated using the Cheshire so is it worth trying to collimate the laser off the current set-up? If I put the laser in the focuser and use the allen screws to centre the red dot on the collimator would that make the laser more usable? It would mean knocking the laser out of a fairly accurate alignment but I don't know how else to make the laser work.

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Barlow the laser to collimate the primary. It eliminates any issues with the laser moving. You can even waggle it and you'll see the reflected donut image on the collimator face does not move :)

You'll also get an idea how well the secondary is aligned as the strip of diffuse laser light should sit nicely central across the donut on the primary.

Edited by mitchelln
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Barlow the laser to collimate the primary. It eliminates any issues with the laser moving. You can even waggle it and you'll see the reflected donut image on the collimator face does not move :)

You'll also get an idea how well the secondary is aligned as the strip of diffuse laser light should sit nicely central across the donut on the primary.

I don't think I can do the barlowed laser collimation :( I've read about it and apparently I need to cut a small circular piece of paper with a central hole in it and afix it to the bottom of the barlow. I've got a Baader Q-Turret barlow and the lens is right at the bottom (not sure if that is common or not). I'm not sure how I can get the circular piece of paper onto the end of the barlow without touching the lens.

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