I am about to embark on an activity to set up my recently acquired (second hand) GSO RC10 Carbon Fibre OTA (with Moonlite focuser). I will use this thread to chronicle my steps with a view to seeking input from this forum along the way as this will be my first adventure into the delightful world of collimating a GSO RC10.
Achievements thus far have been to mount the OTA (see attached pic), connected Moonlite focuser hand controller to computer, installed software drivers for same and set up Maxim DL to talk to focuser. Have checked out the OTA visually (star test) and (not surprising) the collimation appears to be out. Also, inserting a 2" Astro Systems laser collimator shows that the corresponding beam on the secondary does not line up on the central spot. Before I lurch into and endless spiral of collimation activities though I am going to take some images with my QSI683 so that I have a benchmark to compare against and I know what I am working with. Will post images as soon as I get a chance.
In the mean time I have purchased a Takahashi Collimation scope and started going through the posts on IIS that discuss issues surrounding the collimation of GSO RC OTAs (exciting reading!). I am waiting for the adapter to turn up which will allow me to screw the collimation scope to the M42 thread on the Moonlite focuser.
My first question is to do with the fact that the laser collimator is showing that the focuser is not square by virtue of the fact that the laser beam does not hit the centre spot on the secondary. The Astro Systems laser collimator that I am using is one that I purchased for collimating my dob but I am working on the assumption that it should still be ok for checking that the focuser is square with the secondary. I am interested in thoughts from others here on this assumption.
I will not be touching the collimation until after I take some test images so I can at least baseline my starting point before getting too excited. Any other suggestions on cautionary steps before I commence this adventure will be appreciated.
On a different subject to RC collimation, your observatory looks quite interesting. I am considering building one. Could you post some details on the construction of your observatory.
No problems Rob. Link here to my PBase site. There is a link to "Observatory Construction" on the welcome page. I went for a roll off shed design with an internal wind screen that stays fixed. To open the observatory I fold back the two large doors, clip them to the side of the observatory and push the roll off shed back on to the tracks.
You don't indicate how far outside the spot the laser is? With the typical play and pressure variance in a compression ring eyepiece holder with three screws, it is not unusual for the laser spot to wander within the center area and onto the outer edge of the ring. Try reinserting several times and tightening to see how it changes position. The average position of the laser may indicate it is close to the center. Of course there is also a small risk that the secondary may also not have been spotted accurately!
The moonlite focuser has got collimation ability, however I would be tempted to let sleeping dogs lie. Many GSO RC owners have purchased a tip tilt ring to align the mechanical axis with the optical axis for the purposes of collimation.
Note there is a lot of stiction on the secondary adjustment screws when first making an adjustment. Only make minute changes to screw position and observe changes in the Tak scope
Ted
Hi Ted, sorry for taking a while to respond to your post. Have been side tracked on one of my imaging projects. The laser from the collimating tool landed just outside the outer ring of the central spot doughnut. I did notice that it move some when I rotated the laser tool in the focuser which is one of the reasons for not rushing in to any adjustments. I have also seen reference in other posts to the possibility that the central spot on the secondary may not actually be in the centre though I am unsure as to how you would check this. My setup does have one of the Bintel collimating rings but for now I am holding off until I get a chance to take some test images so that I can see what I am working with.
I've finally managed to get around to trying out some test images on the RC10 and, sure enough, the collimation is not what it should be. I have attached an image which shows the non concentric star shapes with the out of focus image. The bright star in the field is Beta Crucis. The field is not flat as well as the out of focus is more pronounced in one corner of the image. Now for the task of checking and adjusting. Will post more images as they come to hand.
Did you end up getting your Tak scope? That is what I use on my RC10, and it works great. I was using a laser to begin with, but the results were inconsistent, and not accurate.
As Paul H regularly mentions on this forum - for collimating these RCs - Takahashi collimation scope all the way. I love collimating with it. It just makes sense.
Hi Erik,Paul. Yes - I have purchased the Takscope. Haven't run it up yet but will do so in the next couple of days when I get a chance.
I have just come in from tonight's activity which was to line up the focuser. The scope has the moonlite focuser with a bintel collimation ring so I decided to check that first and it was out quite a bit. I used a 2" laser to line up the focuser to the centre dot on the secondary. I checked the laser first and it seems in good collimation. To check it in the scope I rotated it 90 deg. and the spot on the secondary only moved a small amount so I am reasonably confident that it is ok. I have attached some images taken with my 60Da attached to the scope to show the out of focus star image (Bcrux). Whilst the lining up of the focuser has improved things there is still some work to do here.
I have also attached a 120s capture of Bcrux @ ISO200 and it does not look too pretty when moving away from the centre of the image. Lets see how we go with the Takscope.
Last edited by Ryderscope; 26-03-2015 at 09:58 AM.
A friend of mine has just successfully completed collimating an RC 8 using an Orion Cheshire eyepiece. I would suggest you remove the focuser and place the RC on a bench for the initial collimation with a Cheshire eyepiece. Once this done place the focuser back on and use the collimation screws on the focuser to get the focuser orthogonal with the optical path.
Looking at you out of focus star images , I would say that number two ( bottom right had corner ) is closest to collimation. This means that stars in the bottom right hand corner will be rounder than the rest of the image. You have got to get the central out of focus star perfectly collimated. Then the out of focus edge stars may not be perfectly collimated if the optical field is not exactly flat. Also noticed some flat edges around the out of focus stars. I believe this is also a symptom of the RC collimation being out.
I decided to go ahead tonight and give the Tak Scope a go and it looks like I am pretty close, if not maybe close enough for now. I have attached images of Bcrux and NGC4755, both guided 120s @ISO1600 with the 60Da. The look pretty smart to me though I do see some stretching of the stars in the upper corners. I would appreciate some advice as to whether I should persist with collimation at this stage or whether the slight distortion at the extremities may be the non flat field characteristic of the RC scope. If the nasty bits are only in the corners then maybe I can get away with this particularly given that the sensor in my QSI683 is slightly smaller than the 60Da.
For those wanting to take a closer look at the test images I have created a link here to a shared folder on my drop box account.
Thanks for looking and taking the time to comment.
Sorry ...couldn't help but notice the four leveling bolts for the EQ head.
I'm not sure why the notion of leveling bolts has crept in for EQ mounts on piers...but to be blunt...they are rubbish.
They simply are not needed. By using them you have reduced the rigidity of the system down to the diameter of those skinny (compared to the pier) bolts.
If you toss the 4 leveling bolts and drop the EQ head down to be flush with the top of the pier your system's rigidity will likely go up by an order of magnitude.
By the way, even if it was up-side down, you can still polar align the mount...and sure, if it is less than level, there may be some polar axis/azimuth cross-talk...but the system will be seriously more *rigid*...which is what you want.
Sorry ...couldn't help but notice the four leveling bolts for the EQ head.
I'm not sure why the notion of leveling bolts has crept in for EQ mounts on piers...but to be blunt...they are rubbish.
They simply are not needed. By using them you have reduced the rigidity of the system down to the diameter of those skinny (compared to the pier) bolts.
If you toss the 4 leveling bolts and drop the EQ head down to be flush with the top of the pier your system's rigidity will likely go up by an order of magnitude.
By the way, even if it was up-side down, you can still polar align the mount...and sure, if it is less than level, there may be some polar axis/azimuth cross-talk...but the system will be seriously more *rigid*...which is what you want.
...Maybe give it a try?
Thanks for the observation from left field Peter. I must admit that the thought of mounting the EQ8 without the levelling bolts never occurred to me. The two plate system does make it easy to get access to the main mounting bolt under the equatorial head. Admittedly though this could be achieved by cutting an access hole in the side of the pier. Can't say that I've noticed any issues with rigidity to date though we may have to see how things go with the weight of the RC10 and the 2 metre focal length.
Those star shapes look alright to me, though the smaller resolution images can be deceiving. I prefer a Tak scope for sorting collimation in a night. I recently helped a mate with his new 10" truss and then did a final tweak on a star and it is near perfect. So much so it has me thinking about checking my own again.
Those star shapes look alright to me, though the smaller resolution images can be deceiving. I prefer a Tak scope for sorting collimation in a night. I recently helped a mate with his new 10" truss and then did a final tweak on a star and it is near perfect. So much so it has me thinking about checking my own again.
Thanks Paul, the Tak scope certainly did the trick. I am still fiddling though because having the collimation close I can now see some small tracking errors. Or at least I did until it started raining. Will resume activities once the weather clears.
