I recently purchased a GSO RC8 and its basically the first time I have had to do any colimation, so not sure what to expect.
I was out tonight testing out a new focuser upgrade and rebalancing after some setup changes and did a couple of tests.
I checked collimation and indications are its pretty close (image uploaded, if its way off please shout out) but when I did a capture, I noticed the stars are seriously soft, though they do look focused. They look like stars all across the field of view.(bit of a first that..)
I did a slew back to my focus star at the zenith and double checked focus and all seemed ok with a Batimov mask.
Popped out of the obsy for a break and realised theres a lot of seriously warm air blowing about out there tonight so wondering if the seeing is totally messed up and this is the first time I have seen the impact or is it possible I have other colimation related issues.
Images were taken 2 to 2,5 hours after the roof was rolled off and cool down started so tube should have been ok?
Any guidance appreciated. Am hoping its just seeing related.
Looks like tube currents , but after over 2 hours they will be gone especially with an open ended tube like yours , so I would say its 'seeing' eg. atmospheric refraction .
I had the same sort of problems last Saturday night up here in Darwin with my Takahashi M210 , it looked like out of collimation? , then tube currents , then the seeing , David , it was horrible ! , I was only able to get to 150x max , when normally 300x on doubles is easy with my Tak and it holds collimation very well .
Oh yes a star test was out of the question , the 'Seeing ' was so bad last Saturday .
I just put my 12" dob back in the shed and had the same sort of seeing issues tonight. I too did a star test and my collimation was great its just all that hot air hanging around, even straight up to the zenith it wasn't much better. Things I can normally see fine like NGC55, or the Grus Quartet, looked pretty soft tonight. I'd suggest you try again on a cool night.
Seeing can really play havoc with the quality of your views, and your mind for that matter!
An easy way to tell if it is the seeing, is to defocus the star so that you see the "donut". Don't de-focus too far though, only about a turn or two of the focus knob. If you can see rapidly moving waves of turbulence and shimmering, it is the seeing. If you are seeing slow moving "worms" or a heat-plume type effect, it is tube currents.
If the seeing is poor, an in-focus star will appear to shimmer and will appear fuzzy and hairy, sort of like a "koosh ball".
One of the easiest ways to diagnose poor seeing is to slightly de-focus Jupiter or Saturn. If you see a boiling effect, seeing is poor.
I agree with marc, still a way to go with the collimation, especially if this was taken from the exact centre of the frame. Check concentricity with Al Sheeny's collimating aid. See thread here: http://www.iceinspace.com.au/forum/s...ad.php?t=62696
That is what I thought, a little bit off in collimation.
It can be surprising how little it takes too. My C925 was just a sniff off though I did not initially think it was enough to make any significant difference to viewing. At a star party I had a look at someone elses BIG dob and thought it made my star images look ordinary. A tiny tweak later and it was like looking through a different scope.
I actually bought Bobs Knobs collimation screws for mine as the throught of waving a small and sharp pointed screwdriver around the corrector plate in the dark did not appeal to me much at all.
Collimation of those scope is pretty finicky. You need to spend a fair bit of time at it to get it right. That said this is pretty close to collimation but some extra tweeks are still required but not much.
The 8" scope cools really quickly and would be ready after 1.5 hours. The mirror is a thin conical and does not exhibit tube currents after 1 hour. The tube is carbon fibre and will cool within minutes of exposure to the night air. It is thermally stable and its expansion and contraction is minimal compared to metal tubes.
Once you get it collimated with the secondary you need to take another shot and see how that presents. Be methodical with your elimination of what is going on. If you are convinced the seeing is ok on any given night, then you need to eliminate secondary collimation. If that is ok, then primary collimation needs to be looked at. I doubt this is in issue based on the image.
I am thinking it is just a little out of collimation and some bad seeing. Feel free to post a focused shot of the entire field of your camera over 3 minutes (ensure guiding is working). That will help us more to help you.
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I am thinking it is just a little out of collimation and some bad seeing. Feel free to post a focused shot of the entire field of your camera over 3 minutes (ensure guiding is working). That will help us more to help you.
Will do and thanks for the tips. Am pretty happy with the scope, just now shaking out the last bits and pieces.
That's how you do the final very small tweek on a Takahashi Mewlon . Use the 1st diffraction ring around the 'Aiery' disc.
Brian.
Yep, then just to take it one step further, you can look at the corners of the chip when in focus and adjust the collimation according to the star shapes. If you do this enough, you get an idea of which knob to turn for a specific star movement.
AHHHH! , we speak the same language , Josh .
Perfection .
At this point you ain't going to get it any better , just take your time , small steps .
Brian.
Quote:
Originally Posted by Joshua Bunn
Yep, then just to take it one step further, you can look at the corners of the chip when in focus and adjust the collimation according to the star shapes. If you do this enough, you get an idea of which knob to turn for a specific star movement.
So I have had far too much fun tonight..
Have spent a few hours on collimation tonight and I think its an improvement on last time.
I believe I still have to faff with the primary as it looks like the right hand side is a little bit off. But I will get to that when the moon is in play as after tonights fun I'd like to get an image so have framed one a tad over to the left ;-) Bit nervous with the primary as it is going to shift a lot with the weight thats on it if I want to try collimating this with the imaging setup attached and I keep seeing threads saying you should not try this at home..paah.
Attached are the following
1) Out of focus star at center of frame collimated with the help of Als widget ( I think there is a little bit more to go here but when I have caught breath and am ready to play with the primary I will go back to secondary)
2) Composite of the corners and centre of FOV after collimating with a focus mask in place to show adjustments to primary I think are still needed. Star at each corner and one dead centre.
3) Full frame NGC104 showing what I think is Tilt off to the right.
NGC104 is a 360sec single sub at ISO400 taken with a Canon 60a so this should help assess if there are any other issues (thanks Paul)
Per advice in this thread, collimating this RC8 takes some patience but once I sort out the primary I think its going to be a good little scope
Yep looks like tilt to me too. Star shapes are pretty consistent in the globular image. Just sort that tilt out and you will have winner. Primary could be causing the slight off center of the star in the out of focus image, so a tak collimation scope will help you there.
Looks like tube currents , but after over 2 hours they will be gone especially with an open ended tube like yours , so I would say its 'seeing' eg. atmospheric refraction .
AHHHH! , we speak the same language , Josh .
