Thanks guys for the contributions. Individual response below with more after that.
Quote:
Originally Posted by Russj
Hello Paul, I've been reading this thread with interest, sounds like ur working thru the problems on the new scope and its starting to come together for you.
Could you answer a newbie reflector owner question? what are the indicators on miscollimation in your photos, I cant see a problem with my newbie unpracticed eye. Nice image by the way.
Regards russ
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Russ the star shapes on the left corners and the top right indicate collimation issues but I also had out of focus images which confirmed this to me. The position of the secondary becomes increasingly important the fast the f ratio of the Newtonian.
Quote:
Originally Posted by alpal
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Hi Allan, I have been a long time user of CCDInspector. It gives good general analysis which helps to resolve these types of issues. Getting to the site and doing the tweaking has been generally possible during the day at present. Last night I was there but could not complete the final tweaks, which I hope can be done next week.
Quote:
Originally Posted by gregbradley
Don't you love having to rebuild a scope you paid big bucks for. Wow, if this were a new car there would be hell to pay.
Luckily you are patient.
It does look a tad soft in the centre and the left hand side stars look fatter than the right hand stars.
I have a question though. With miscollimation you are looking for stars anywhere in the field that are eggy (not from tracking) as opposed to tilt which would be one side or one or more corners?
Greg.
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Adjusting a folded optical design is part of the situation I suppose but I never expected to remove silicone and have to actually move the secondary into the correct position.
The left side has collimation error and perhaps some tilt or more probably error from the corrector making corrections to the collimation error.
The softness is the collimation error too. I expect it will be quite sharp once properly adjusted.
Quote:
Originally Posted by ericwbenson
Hi Russ,
Miscollimation causes aberrations (other than spherical) to appear. Usually coma is the first and worst, but astigmatism can also be generated.
A simple way to think about it: the 'corrected' field is only a finite size, collimation shifts it around, when it is perfectly centered you see no aberrations, or the same aberration in each corner, depending on the size of the sensor relative to the 'corrected' field. Symmetry is the goal.
Image tilt will only induce defocus (inwards in one corner and outwards in the opposite). Defocus means the stars stay round but get bigger. NOT the same as coma or astig which makes the stars non-round. So when you hear someone blaming their eggy stars on image tilt it usually is not the case (but can be, read on...)
In reality separating the two factors (aberrations and tilt defocus) is really hard unless you have very good seeing to be able to examine the shape of the non-round stars. What can really be confounding is a little bit of inherent astigmatism in the optics (from mounting or built-in), which is not detectable in focus, but is revealed by slightly going out of focus. So now is that astig in the corners from image tilt or field curvature or collimation?!?
It takes practice and patience to iterate the setup, which is what Paul is doing now...
and then throw in a camera rotator and it becomes a really tough nut to crack!
HTH,
EB
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Perfectly put Eric. Fast optics make this task not so easy.
Quote:
Originally Posted by Joshua Bunn
All the best Paul with ironing out technical issues. What was the reason to move the secondary forward?
Nice write up Eric. As I'm sure you are aware... one way to find out if it's tilt or colimation is to rotate the camera relative to the scope. If the star shapes, whatever they be, follow the chip (stay in the same place) then you have tilt. If they change place from rotating the camera, then it's likely you have an optical issue. The next issue to be aware of is, is tilt being introduced when rotating the camera  .
Josh. |
Hi Josh,
the secondary when it arrived was 25mm away from the primary and looked like an ellipse when viewed through the sight tube. I could not see the edge of the secondary closest to the spider either. About a third of the secondary was not visible.
So now to the latest developments. I finally nailed down the correct position of the secondary toward the primary mirror. It took a few hours yesterday to get it perfectly positioned. The problem stemmed from not using a white sheet of paper behind the secondary and then viewing the reflection of the darkened walls in the observatory. I had the secondary skewed a little and not centred correctly. The secondary is enormous too at 110mm, it put deceived me a little too. So I am now happy that I have this correct, but by the time I did a star test it was getting late and I wanted to get home. As you can see in the first image, this is how the out of focus stars in the centre looked before yesterday. The second image is how an out of focus star looks now (still a very small tweak needed but enough to still cause problems with this fast scope). The third images is the corners and centre as seen now and a single sub is the last image which show the vignetting which confirms the collimation error. So another trip down next week at night to give it that final tweak. Next time I take my laptop instead of my iPad.