Help with star test images please

[davidpretorius]

Hi,

The new big fan sucking air from the surface of an icepack onto the back of the mirror, got my temps within 1.5 degree, so I thought I would get some collimation pics.

I have defocussed outwards and inwards on Alnitak mag 1.71. I have stacked say 80 to 100 frames.

I would love some opinions on these images. I have my own thoughts on adjustments, but would appreciate your input as well.

First image is the defocussing inwards and the second is the defocussing outwards.

Magnification is 208x

[janoskiss]

I think you've defocused too much. I can even see the mirror clips in the second shot. I find that the defocus test is more sensitive closer to focus.

[gbeal]

I'm with Steve, but also add that the star needs to be exactly central (not sure if this is the case here), and maybe not a double.
Good plan though. Be also interesting to see if the image changes with and without the fan.

[Satchmo]

David

Your star test results seem to indicate that your morror has an overcorrected edge ( at last at theat point in time if your mirror is plate/BK7 ) . Note the bright ring at the edge of the inside focus slice.

Mark

[davidpretorius]

Thanks mark, it is bk7.

Nothing i can do i suppose about that... Houghy was suggesting i could use some sandpaper to straighten it out a bit to fix the overcorrection!:nerd:

according to http://legault.club.fr/collim.html

Tonight, i will try a single star in the belt of orion around mag 1 @ 208x and then try and centre it.

Then a mag 2 star at 1000x and do the intra focal and extra focal

[Satchmo]

Um, probably a bit earlier in the day for figuring mirrors for me :confuse3:
, Thats actually a high edge you got there , which is focussing short, so technically the edge is undercorrected ( far more likely scenario too ).

Mark

[davidpretorius]

thanks, i will post some more results tonight!

[Robert_T]

Hey DP, yeh as the guys say, as well as centering bring the images a little closer to focus. 1000x would only be useful if the seeing was exceptional. Look forward to seeing some starry rings :)

[ving]

what! you cant even split rigel!!!!! :P