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Mirror issues with Discovery 15" PDHQ -Help!

Started by dj123, 04/10/2005 12:57PM
Posted 04/10/2005 12:57PM Opening Post
I have a 15"pdhq with possible mirror issues. My issues are soft images and dim dso’s the planets lack detail and look like I'm
viewing through a yellow haze as there is a glow around them anything over 200x is mostly unusable. I assumed that it was my secondary not property aligned. I purchased a Glatter holographic collimator and centered the secondary and then used a cheshire to final collimate,
and also upgraded to Pentax eyepieces. This did not help much. I've
now re-aligned the secondary many times and I'm sure that its tweaked
as best as can be done and still not much improvement.
Now the point where I need help. As I'm sure that the secondary is
aligned I have now started checking the primary, It’s not perfectly
centered in the tube but its close. I also noted that my mirror holder
has 3 legs all equally spaced so that the holder could be rotated and
the tube/bolt holes would still match. There are 3 corresponding bolt
holes in the tube, one on the opposite side of the focuser, one that
would be on top of the tube when it’s tilted level and the 3rd on the
bottom. The hole on top is marked with a green stick-on as is one of
the legs of the mirror holder, I assume that this is an alignment mark
and installed the mirror green to green. No matter where I've aligned
the secondary, in order to collimate, I've always had to tighten the
collimation wing nut fully on the leg with the green mark while he
wing nut on the leg opposite the focuser was always fully loosened.
This did not seem correct but it was the only way to
collimate it, I just assumed that this was just a correction of improper bolt holes or mirror set and that
the mirror was not really tilted, well I measured and the mirror is
tilted about 1/4" between the 2 legs. This shows that the mirror
is tilted towards the focuser, can this be correct? Is it suppose to
be tilted?
I then loosened all of the wing nuts so as to level the mirror and
looked through the focuser w/o an eyepiece and it can be seen that the
mirror tilts away from the focuser enough to show a cresent of the
top of the tube reflected in the mirror. This accounts for having to re-tilt the mirror when
collimating. Now the tricky part where I become totally lost. I
rotated the mirror with the leg with the green dot counter clockwise
so that the green dot leg is now matched with the bolt hole opposite
the foucser. I again looked through the focuser and now the mirror
looks level, no longer tilted. Why? I then collimated and noted that the collimating wing nuts are much closer to being the same only about 1 turn difference
and the viewing looks to be better but I’m not sure how much as it was
poor seeing last night.
I tried a star test and inside focus did show center obstacle
(centered) and rings, poor seeing so not clear. But outside focus
could not see center obstacle, looked like rings just got smaller and
smaller. What to do now? I'm thinking I should send the mirror out for
testing. Any suggestions, recommended mirror testers?
Thanks for the help, I'm really frustrated.
Dave




Posted 04/10/2005 01:40PM #1
Hi David,
A couple of questions.
Is the scope stock, or has it been modified as far as coatings on the primary are concerned?
How about the secondary, is it original?
The yellow haze sounds like scatter of the light by a rough surface, but some caotings do empart a yellow or gold tinge to the light. But if the optics are not aligned properly you could be gettiing a loss of light to the exit pupil.
You say you have the scondary aligned and the primary but the mirror seems to be tilted by quite a lot. This wouldnot be normal, it should be fairly straight without tilt. I suspect that the mirror is not properly mounted or that it is not aligned to the secondary. It could be that the mounting bolts are not the exact same hieght from the bottom of the tube. The mirror may also be glued to the cell at an angle, it is one of the glued to a wooded cell right?
Is the secondary located in the center of the tube, measured using the spider vanes as a guide from the inner edge of the tube to the center of the secondary mounting stud or bolt? If not, then the primary mirror would have to be tilted to align with
it.
Aligning of the optical train is frustrating when things are off and can take some time to figure out.
Try looking at these links for a little explanation with pictures of what you should see in the focuser and sight tube.

http://www.fpi-protostar.com/collim.htm
http://www.efn.org/~mbartels/tm/collimat.html

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 04/10/2005 03:48PM | Edited 04/10/2005 03:49PM #2
Hey David,

I don't know about the tilting issue, usually a slight tilting of the primary can be accommodated. As long as the optical axis is not obstructed by the mechanical axis, it shouldn't be a problem.

Have you checked to see if the primary mirror is pinched?

Take care,
john
Posted 04/10/2005 04:22PM #3
If objects have a haze around them, and the seeing is good, and the scope is cooled, and collimated, and planets or DSO's lack detail and contrast, then it sounds like rough optics.

Hi end Newt optics in good seeing will show snappy hi contrast images!

Chas