Quote:
Originally Posted by Startrek
My guide scope specs say you can use it for focal lengths up to 1500mm
My scope is 1000mm and has an image scale 5.4 times the guide scope image scale
At 1500mm focal length you would have an image scale 7.4 times the guide scope image scale
So the magical < 4x image scale is probably a figure in the “ideal world” I suppose
|
The 4x is just a guideline just as the 1500mm is. The 1500mm assumes that you have the same pixel size on both guide and imaging cameras and is made on the basis that autoguiding software can potentially resolve down to about 0.1 pixel movement. Your 50mm guidescope has a focal length of around 160mm which is about 0.1x the imaging focal length. It ignores the differential flexure that often becomes evident at long focal lengths and the mechanical issues of being able to point your mount at that 0.1 pixel accuracy.
The 4x guideline is also based on the autoguider being able to resolve down to 0.1 pixel but given that is an ideal figure it provides a bit of margin for deviation from that. Both guidelines aim for the smallest detectable movement to equate to less than 1 pixel on the imaging camera.
Another guideline is that above 1000mm focal length you should consider using an OAG due to differential flexure in the guide scope.
Astrophotography is very complex due to the large number of variables involved. That's why people come up with guidelines to try and simplify things. But the guidelines are only as good as the assumptions underpinning them.