A tour through flexure, guiding and sag

[Paul Haese]

Man what a busy couple of weeks sorting out problems with the QSI on the GSO RC.

Just a quick run down. The stars were oval in shape and well looked really bad. I had this all sorted when I was imaging with the 40D. Once the QSI came along I had to change the imaging train which instantly produced oval shaped images. So first step was sorting out the guiding or what I thought was flexure (no I am not confusing flexure with guiding). Good news is that it was not flexure. I got the guiding sorted thanks to Strongman Mikey suggesting that his minimum pixel movement setting was 0.1 When using the 40D I had mine set to 0.15 but for some reason I read somewhere here that the correct setting was 1.15 and I had set my guiding to that. No wonder the stars were oval every where. Once I set it back to 0.15 the guiding looks very smooth. However the stars still look elongated on the bottom of my images.

I was thinking this was camera tilt. How wrong can you be?

I have been doing some more investigating and working with CCD Inspector. Sunday night I took a heap of data after collimating visually with the camera in the telescope. I collimated on Achernar at about 45 degrees. It looked collimated or so close to being perfectly collimated. I then slewed the scope to NGC1365. Took all my RGB while it was close to the horizon and then luminance past the 45 degree point.

Now having gone through the images with CCD inspector I have noted one common theme. My collimation is quite a way out, but that is not the real issue. The images gain less and less collimation as it rises from the horizontal position. That indicates that sag is at work here and my thought that camera tilt being the problem is wrong.

What does it all really mean? Well it means I found all of the causes of my elongated stars. Guiding, collimation and sag. The shifting collimation means that there is sag in the imaging train and I know how to fix that. I need the extensions from GSO to stiffen up the rear of the telescope and Andrews are supposed to be getting a shipment this week or early next. Once I have them I can finally collimate and hope that the train holds the camera in position.

I thought it would be good just highlighting the thought process one should take to sort out elongated stars. It took me a while (must be a bit thick) but this is how I should have gone about sorting the problem.

1. check for flexure.
2. sort guiding (understand what the graphs really mean in the guide program)
3. Make sure your collimation is spot on. (mine has been out by as much as 46" and even at the zenith it is 12" out. That has to impact your images.
4. While doing the above stop sag. This kills images too.

Rant over.

[Dennis]

Sounds like you’re having fun Paul!!!

In my own investigations, I found another couple of sources for my particular set up:

1. Moonlite motorized focuser had, over the years, come slightly loose and required me to re-tighten the socket head cap screws that “locked” it in place on the OTA.

2. Brass compression ring was just biting on the edge of a recessed 2” camera adapter and this was causing a small amount of “rocking” as the OTA orientation changed with altitude.

As the temperature dropped from the start of a session where I hand tightened locking screws and clamps, I found that it made these two “faults” worse as the contraction of metal upon metal made things fractionally looser too.

I’m glad you’ve got to the bottom of your issues.

Cheers

Dennis

[Paul Haese]

Dennis, the difficulty I found was not so much the problems themselves; it was identifying what parts of the problem existed. It is easy to get caught up thinking one thing is the primary fault such as differential flexure and it being more like a combination of errors and at this focal minor errors shown up nicely in the images. And this is not to say that the very problems you faced are not present also in the conundrum I face. Good suggestions for things I need to check out of thoroughness.

Perhaps a screwed on focusor and screwed on camera too???

[leinad (Dan)]

Good to hear you've nailed the possible problem Paul.

I went through a similar scenario a week back trying to sort out the elongated stars in my images, and by process of elimination found the side by side bar was causing real bad flexure which was identified by slight diagonal star trailing in my images.

I changed it to a piggyback setup and that trailing was eliminated, except I now had trailing running vertical. Changing PHD settings made little difference and this ended up being an RA counterweight issue.

Thats now solved with additional weight, now I believe I have focuser tilt issue to deal with, which is identified by elongated stars in bottom-left and top-right corners but not at center or the other corners.

With breezes about each session I always thought it was PHD and the gusts of wind causing the problem.

Thanks for sharing your troubleshooting results. Once I've got my scope sorted I'll most likely also post up my troubleshooting results as originally most people mentioned that my images could have been the result of a pinched-mirror

A couple of guys have mentioned to rotate the camera at 10degree intervals during your imaging session, and watch for the changes that may highlight a focuser tilt/weight issue or sag problem.

[Paul Haese]

Isn't it intriguing how this stuff can be caused by multiple issues. I had a side by side set up too for a while until I found that this was causing flexure. I promptly got rid of that. The bars were great, just not what I needed.

Yeah rotating the camera is a sure way to see what is happening. Several images at 90 degrees from each other will tell you what is happening. Good idea.

[multiweb (Marc)]

I thought I'd share something I've just figured out last night. I know you guys have gone through a bit of trials and errors with those new GSO RC scopes. I believe they come fitted with a GSO 10:1 micro focuser standard. I have one fitted on my C11 as well. This week-end I put the C11 in pieces again and I did a mechanical aligment of the Primary and the Baffle tube. I had a collimated laser directly bolted in the baffle tube projecting a dot at 3m on a wall on a cardboard marked with a cross. The scope was held on a flat bench. Rotating the laser in the baffle tube showed absolutely no circular motion on the laser dot. Then I did the same thing for the primary optical axis. Finaly I put the GSO focuser back on the back of the mirror cell and stuck the laser in the focuser this time. I was absolutely shocked at the offset I was getting with the focuser mounted . It wasn't a problem with the adapter rings at the back of the SCT or the focuser barrel itself because I checked that, but it is an issue with the fastening mechanism that holds the camera/nose piece in the barrel of those focusers.

[Paul Haese]

Marc,

I am not using the standard focusor though. I bought a feather touch and had everything sorted when I was imaging with the 40D. The extension setup was quite different to how I have it at present with the QSI. Certainly check for optical alignment is a must and I will be putting a catseye collimation cheshire over the back of the scope to see if everything is concentric. You raise an interest idea though that everything can be a little out and create a large problem. The compression ring on the feather touch is not likely to be out, but still worth checking though.

[multiweb (Marc)]

Hey Paul, I'm like you. Wondering where the hell in the imaging train things go wrong. And I always though it had to be with the mirror alignments, etc... but as it turns out these things are pretty easy to fix and most of the problems looking back at it happen way closer to home. The focusers and particularly the way things are attached to the back of the imaging train. I'm seriously rethinking that part as I always assumed focuser were squared and aligned. Last night was a revelation. After nailing all the assembly spot on and checking through the focuser once mounted I thought wtf? What was the point?

Originally Posted by Paul Haese

Marc,

I am not using the standard focusor though. I bought a feather touch and had everything sorted when I was imaging with the 40D. The extension setup was quite different to how I have it at present with the QSI. Certainly check for optical alignment is a must and I will be putting a catseye collimation cheshire over the back of the scope to see if everything is concentric. You raise an interest idea though that everything can be a little out and create a large problem. The compression ring on the feather touch is not likely to be out, but still worth checking though.