Pinched mirror?

[Scott]

hi all,

just after a bit of advice from more knowlegable people than me (just about everyone on here).

i have a 200p which i.ve always been a bit scared of doing too much tinkering for fear of messing things up, however i'm on hols in australia at the moment so i bought a small dob for while i'm here and thought it would be a good chance to strip it down to get a better understanding of how it all fits together. all pretty easy really but i was wondering..... i've heard/read that the primary mirror should be almost loose but the one in my little dob seems quite tight in its mounting plate (don't know what its called). am i right in thinking that to loosen this i just "back off" the 2 screws in each of the mirror clips until it rattles a bit? sorry for the use of technical terms such as rattles a bit and mounting plate

any advice would be much appreciated

Scott

[Stephen]

The highly technical measurement is you should be able to slide a business card between the primary and each of those three clips

[Scott]

The highly technical measurement is you should be able to slide a business card between the primary and each of those three clips

thanks stephen, now thats the type of feeler gauge i've got

[neil phillips]

Yes rattles is a good indication that things are not over tight. but of cause theres rattles and LOOSE. a buisness card is good advice.

i used to slip paper between the clips to see if it was over tight. But be careful even paper can cause slight scratching of the primary when checking this gap.

To be honest even a gap of a 2 or 3 mm should still be ok, because the only time the mirror could really fall slightly foward ( if the mirror clip gap is quite large ) throwing off collimation a touch. is when elevation is very very low. We dont really look at objects that low very often ( other than mercury perhaps ) so as soon as the scope is lifted up a bit in elevation the mirror will sit on the primary cell base, flat anyway.

The problem about too loose, isnt really a problem unless youve got like 5mm gaps. and even then as mentioned only only very low elevation targets.

I dont think ive gone that low in recent years. so you likely wont either.

Its better to be too loose than too tight i belive. Also make sure the 3 side clips that hold the mirror horizontaly are also not too tight, they will also distort the mirror shape. A tiny amount of movment from side to side is ok. Just tighten the 3 side clips untill they touch the side of the primary. Then back off one side clip just a mm or 2. And all should be fine. try to aim for the 1 mm gap if poss, And collimation will be rock solid. But even 2mm should have very little effect. So i wouldnt stress too much. But you can if a perfectionist set up with the buisness card and 1 mm side gap tolerance. For textbook primary cell setting

[Scott]

thanks neil, i'll have a go this afternoon.

[Mr Q]

Though by now you probably have fixed the clip issue, you can go to the below link and scroll down to example #7 and see how a pinched mirror effects a star test. The site is also good for lots of optics problems:

http://www.telescope-optics.net/diffraction_pattern_and_aberrations.htm

[assasincz]

The highly technical measurement is you should be able to slide a business card between the primary and each of those three clips

I would prefer using a piece of cigarette rolling paper for this

[Stephen]

I would prefer using a piece of cigarette rolling paper for this

Maybe somewhere between the two then. My personal feeling is that a rizzla won't give enough space for expansion / movement

[Scott]

Maybe somewhere between the two then. My personal feeling is that a rizzla won't give enough space for expansion / movement

yeah, a ciggy paper seems a bit thin unless you mean the cardboard on the packet? in the end i used the "that looks about right" method.

thanks for the help everyone.

[Steve Ward]

I just tighten the screws gently until the blocks just touch the mirror and then back them off 1/4 turn , no need to worry about touching the surface with paper , card or fingers..............