collimation help

[lhuchison]

Hi all,

Had a great night last night . I managed to see the adromeda galaxy, for the first time.

Today , i thought id re-collimate my 200p using a collimation tool,( non laser)

I,v managed to get the secondary perfectly aligned with the collimation cap but when i use the collimation tool, the blurry( maybe my eyes!) cross piece, that sits in the far end of the device, never seems to sit perfectly with the rest of the image.

Everything else is perfect. Help! clear skies forecast for tonight !

[rory]

sits perfectly ? the crosshairs need to centre on the primary's centre mark. have you a centre mark on the primary leon ?

this is a popular guide . you may already know about , but incase you dont .

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

[lhuchison]

Hi Rory , yes i have a centre mark on the primary which sits perfectly central, but the cross hairs seem off

Could the secondary be tilted?

[WaveSoarer]

Leon,

If you aligned the secondary with the colly cap then I assume that you got all three primary clips in view. With the cheshire you should then see that the centre doughnut on the primary is aligned on the cheshire cross-hair (or very close subject to some minor tweaking) but the cross with a hole in the centre of the cheshire target (that's the shiny slanted bit on the cheshire tube cut-away) will be slightly out as the primary may be slightly tilted. Tweaking the alignment screws for the primary (the big spring loaded guys at the base of the tube) should bring the target cross and hole in register. The issue I have is that the cross-hair on my cheshire is composed of two thin wires quite close to the viewing hole. The wire is slightly too thick really and it makes it difficult to be as accurate as you could actually manage: i.e. you tend to see either side of the cross-hair depending on where you exactly place your eye.

Dave

[umadog]

The "collimation cap" is for adjusting the primary tilt, not the secondary. The cross-hairs are part of a tool known as sight-tube and are for adjusting the secondary tilt. This site tells you how to use the Ceshire/sight-tube combination tool: http://www.propermotion.com/jwreed/ATM/Collimate/Chesire.htm The AB guide tells you how to physically do things and holds your hand a lot. This site here goes through the steps you need to take to get aligned rather than providing a lot of details. I find it easier to understand, but both sites are useful.

[WaveSoarer]

umadog,

Now I'm puzzled. If you look at Astrobaby's guide (http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm) she uses the collimation cap to align the secondary and up to the point of aligning the primary. At step 4 in her guide she switches to a cheshire to adjust the primary. I think that John Reed's guide only uses a cheshire for the whole collimation process. Is it possible to do the whole thing with just a collimation cap?

Dave

[LeeB]

You can do the lot with a chesire, after all a colli cap is just a cheshire with no cross hairs. As to your concern if the cross hairs , primary centre line up and the secondary is concentric in the colli or cheshire. the shadow of the spider will look alittle offset given you F5 scope. if you need a talk through PM and i'll give you a call.

[umadog]

Partly there is a nomenclature confusion here (as LeeB hints). A Cheshire is a tool for adjusting primary tilt. A collimation cap with a silvered inner surface IS a Cheshire. A sight-tube (long tube with cross hairs) is for adjusting secondary tilt. What is often called a Cheshire is, in fact, a combination tool comprising of a Cheshire and a sight-tube.

There are three steps to the collimation process

a. Adjust secondary tilt.

b. Centre and round the secondary in the focuser.

c. Adjust primary tilt

In the AB guide she does step ( with a collimation cap. It doesn't really matter what you use but a sight-tube is easier because the long end of the tube frames the secondary mirror tightly and allows you to centre and round it accurately. You can't do step (a) with a collimation cap. You need the cross-hairs. There are more links here: http://www.physiol.o...tionLinks.shtml

[WaveSoarer]

Thanks to you both LeeB and umadog. There is much to learn and the forum is a wonderful way of learning from the experienced.

[umadog]

Any time! Just spend some time reading those links I posted and it will all start to make sense. Basically the point of the procedure is to get the focal plane of the mirror and the focal plane of the eyepiece to coincide. The theory is shown nicely in this thread about autocollimators: http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=reflectors&Number=3532750

[Helite]

I found this guide a lot easier to understand as a complete beginner http://www.andysshot...ollimating.html , astro-baby's one seemed overly complicated to me for some reason though I do understand it is helpful to a lot of people. A lot of it was not understanding what each "circle" was that I was looking at. I was also getting a bit confused by what seemed to be 2 sets of cross-hairs, one set slightly offset from the others, making me wonder if they should be aligned, even though all my "circles" through the collimator were perfect and lined up on one of the sets of cross-hairs. Now I believe the second set off cross-hairs are actually the reflection of the spider veins now. Im still working on it, and learning, but as I said I found that video helpful.

[umadog]

Be careful with the Andy's Shotglass guide. The illustration of what a collimated scope should look like is wrong. He shows everything concentric. In fact, the secondary shadow is offset: http://www.cloudynights.com/ubbthreads/attachments/3033067-secondary_mirror_explained.JPG

[lhuchison]

Thanks all,

I,ve done the collimation now and all seems ok, thanks to all who took the trouble to help . I,ve found it all very informative.

i ended up adjusting the secondary mirror with the collimation tool /sight tube in place and not the cap. i then moved the secondary about till the cross hairs all lined up.

it was a pain to do, must of taken a couple of hours and plenty of swearing, but all done!

[umadog]

Good stuff. Next time it will take less time!

[LeeB]

Its all about practice, and understanding what you are infact adjusting.

If anyone does'nt understand offset the easiest way is to draw a light ray diagram of an f2 newtonian. To make the secondary collect all the light from the primary it becomes obvious on the drawing how offset works.

Once you get practiced at collimation and understand it, you can get a decent colli with just your eye and the open tube , so keep practing.

Those with quick scopes f6 or less remember taht the collimation needed is more stringent (this fact is a major reason why the comments abound that you need a large f to observe planets), my f4.8 really rewards time spent getting it perfect, planet definition suffers if only alittle out.

And finally if you use a laser make sure you collimate it first, and also learn the barlowed laser method if you have a sloppy focuser.