One more fixPosted By Roland Christen |
The reason for the scaling difference could be due to the focal length of an SCT being variable (quite variable, in fact, changing from F10 to almost F14 when the mirror is moved from min to max focus position). This is caused by the moving mirror focusing system. If you can fix the position of the mirror by locking it down and then focus via an external focuser, the focal length will not change.
The other thing that can change the focal length is if the tube is made of aluminum. Then the mirror to secondary distance will change with temperature, and even an external focuser will not compensate for this change.
Finally, the mirror is made of Pyrex, which does have a small expansion co-efficient. Because of this, the radius of curvature of the main mirror will shrink slightly with falling temperature. This causes the focal length to shorten slightly, and is another variable due to temperature.
You are discoving some of the reasons why a large aperture telescope does not always produce better results when it comes to deep sky photography. Here is an example of the same object that you are trying to image, but taken with only 10 inches of aperture: http://www.bigcigarastronomy.com/PageMill_Resources/M82.jpg
A smaller imaging scope is definitely easier to mount and guide accurately than a large heavy scope, plus it is less affected by the variables of the night sky. While a large scope does give you more bang visually on deep sky, sometimes the opposite is true for imaging.
Rolando
The other thing that can change the focal length is if the tube is made of aluminum. Then the mirror to secondary distance will change with temperature, and even an external focuser will not compensate for this change.
Finally, the mirror is made of Pyrex, which does have a small expansion co-efficient. Because of this, the radius of curvature of the main mirror will shrink slightly with falling temperature. This causes the focal length to shorten slightly, and is another variable due to temperature.
You are discoving some of the reasons why a large aperture telescope does not always produce better results when it comes to deep sky photography. Here is an example of the same object that you are trying to image, but taken with only 10 inches of aperture: http://www.bigcigarastronomy.com/PageMill_Resources/M82.jpg
A smaller imaging scope is definitely easier to mount and guide accurately than a large heavy scope, plus it is less affected by the variables of the night sky. While a large scope does give you more bang visually on deep sky, sometimes the opposite is true for imaging.
Rolando
