Paul Haese
23-10-2009, 06:42 PM
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.
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.