|
Hi Peter,
I wasn't thinking of the difference in focal length as much, but that does also come into it. The finder guider is a focal length of 320mm vs (assuming F5 for the 12 inch) 1250mm. This means 1 pixel of movement in the finder/guide cam relates to nearly 4 pixels of movement in the imaging cam.
I was thinking more of flex between the two scopes. The primary is never bolted down but a small degree of movement is allowed ( or you get astigmatism), that's why mirror clips are never supposed to touch the mirror.
So the movement of the scope across the sky means the primary can move by a few microns.
There is also sag in the focuser from the weight of the imaging chain that will change as the scope moves across the sky ( due to gravity).
Now an OAG will pick up all those movements and compensate for it.
With my old EQ6 and the 8in F5 it made a big difference going from finder/guider to OAG. Before I was limited to 3 min exposures max before I got egg shaped stars, with an OAG I could easily do 15 min subs with good round stars.
Having said all that OAG's are a pain to get right, it took me 4 hours to get the Orion thin-OAG working properly.
Cheers
Bill
|