8" scope slowly getting there!

[Garyh]

Hi again,
Well this has been a rather slow project this year which started in March but things are starting to come together and another month should see it ready for some first light imaging and viewing..
Mirror is still uncoated and the tube has only been roughly primed but I know a few of you have asked on its progress..so here is some pics of where I am up too....
I did give it a good star test and a bit of a look through at the moon and Jup a few days back..Showed signs of Astigmatism which gave me a bit of a shock! but on further investigation it looks like its the old secondary I am using as when I rotated the primary 90 degrees the astigmatism didn`t rotate..:doh:and is still in the same orientation. Bumped up the magnifcation to 350x and the focus still looked rather good, 700x things looked on the soft side, mirror gives a star test of slighty overcorrected but I am happy with what I can see...
New secondary ordered today to fix the first problem.....:thumbsup:

Oh and nearly everything is made from secondhand components! from IIS members and Astromart! and a little Ebay thrown in..:)

cheers Gary

[erick]

Four screws on the secondary tilt, Gary? :confuse3:

[Garyh]

Hi Eric,
I checked the collimation twice and through the focuser everything looks concentric except the spider vane and secondary shadow which are shifted slightly away from the focuser....
I allowed 3mm offset down and back from focuser,
Could too much offset give me this affect?
Didn`t bother with any offset in my 6" scope..
cheers

[erick]

Sorry Gary, I was too cryptic. I was wondering why you have four screws rather than the regular three I'm familiar with?

I cannot help you with the offset questions - I only know what I have read and cannot claim to have fully understood that!

[Garyh]

ah yes, it surprised me seeing 4 screws too! makes it a bit more fiddly adjusting it but should hold it position well when collimated...

[Garyh]

here is some images captured with a toucam inside, at focus and outside focus to show what I mean by astigmatism..
I rotated the mirror 90 deg and checked visually the direction of astigmatism and it didn`t change. Also rotated barlow etc and still no change....
cheers Gary

[CoombellKid]

Gary,

Is that the tube and cell you got off me? looking nice mate!

If it is, I found the backing plate that goes with the mirror cell, if you want
it PM your address details again and I'll post it off to you.

Eric, my secondary (and spider) I got from AstroSystem in the US came
with 4 adjustment screws and I thought the same "What The!!" but it
seems to work very well. I thought it was interesting that I could have
AstroSystems in the US build a custom sized spider to my specs built
cheaper than I could buy one in OZ and then would have to modify,
but then the US and AUS dollar was almost at parity.

regards,CS

[CoombellKid]

Just set the one closest to the focuser once you have your offset set. And
just use the sides and back one to align the secondery.

regards,CS

[Satchmo]

You might try remounting the secondary, sometimes they are just glued on sloppily which warps them.

[Garyh]

Thanks Rob,
Yes that`s the tube that was going to be part of your BBQ! Worked out very well, was able to remove some of both ends which left me with a nice clean middle section. I made up a back plate with some aluminum with a 80mm fan attached. Thanks for the offer! Might give it a paint today..
cheers Gary


I think you might be on that one Mark! The silicone is very thin like .5mm thick underneath so that may be the case. It needs recoating so with that in mind it works out just as cheap just to buy a new one already coated and replace it..Looks like the primary is ok with no major faults..
cheers Gary