collimation beginner

[cranefly1973]

Hello,

I am new to Stargazers lounge and relatively new to astronomy.

I have a Sky-Watcher 200P DS and have taken the plunge to collimate it myself. I am currently using Astrobaby's excellent guide to collimation but was unsure whether the focuser should be fully extended while collimating?. Does this make a difference? In fact, should the views through the Cheshire collimator and the collimation cap be consistent whether the focuser is fully extended or not? Apologies if this has been answered in another thread or is stupidly obvious! Any advice would be appreciated.

[Ally8446]

Hi Cranefly, welcome to SGL.

The common consensus is that when looking through your collimation cap, focus in or out, whichever is necessary, in order to see all the primary mirror clips. In your case I believe there are three. Hope this helps.

Ally

[Charic]

cranefly1973..........Hi, not certain if it make any difference, but to be sure, the picture of the focuser in the guide shows the focuser  fully in. Hope this helps.

[Charic]

..........that said, the picture defies the text? which says.......Use a collimation cap in the focuser to force your eye central to the focus tube and rack the focuser tube as far out as possible!   

[Charic]

.........if your cheshire tube is too long, it could impede and strike the secondary mirror if fully racked in, and  longer tubes shorten the field of view if racked in, so you may not see the primary mirror clips.(Think that's the theory) I see the primary clips well enough just using the 35mm film casec. (soon i`ll be able to edit, instead of multiple replies Lol!

[ronl]

Try thishttp://www.andysshotglass.com/Collimating.html it should help

[ScottS]

Try thishttp://www.andysshotglass.com/Collimating.html it should help

Good video there. Major uglage!

[cranefly1973]

Thank you all for your comments and advice. The video on Andy's shot glass is very helpful.

regards, Michael

[astronymonkey]

Ive read before about adapting the cheshire to the f ratio of the scope ..... so an f4 scope needs a cheshire with a 5 inch tube , presuming its a 1.25inch diameter fit. I can see the point of this and you rack the focusser in until you can see the whole of the secondary. I think in general all cheshires are the same length though, but this gives the optimum Cheshire position.

cheers

[Moonshane]

the focuser (and the cheshire) need to be far enough in or out to see the whole of the secondary mirror. here's my slant on collimation

http://stargazerslounge.com/topic/197640-collimation-and-star-hopping/

[bambuko]

Ive read before about adapting the cheshire to the f ratio of the scope ..... so an f4 scope needs a cheshire with a 5 inch tube , presuming its a 1.25inch diameter fit. I can see the point of this and you rack the focusser in until you can see the whole of the secondary. I think in general all cheshires are the same length though, but this gives the optimum Cheshire position.

cheers

almost but not quite

have a read here

it's the internal diameter that matters not external so it is not 1.25" times f  but whatever the internal diameter of sight tube times f

As I have found when I added cross-hair piece that made the internal diameter smaller - had to shorten my sight tube to see what I wanted to see here