Skywatcher 0.9x coma corrector - Forums

Topic starter

Oct 4, 2023

Hello

Just a quick question. I`m using a 150 PDS (FL 750 mm). According to my calculations using this corrector should come out at 675 mm. I took an image and it plate solved at 698 mm. Does this mean the spacing is too close or too far away from the sensor?

Any help appreciated.

CS
Alan

Lee

Oct 5, 2023 · Edited: Oct 5, 2023

Alan Hancox:
Hello

Just a quick question. I`m using a 150 PDS (FL 750 mm). According to my calculations using this corrector should come out at 675 mm. I took an image and it plate solved at 698 mm. Does this mean the spacing is too close or too far away from the sensor?

Any help appreciated.

CS
Alan

Hi Alan, it probably still means you're suffering a hang over from your previous issues/queries around the spacing. The backspace is fixed at 55mm + filters, there's no getting away from that and I don't think anything should correctly be pushing the plate solve it to 698mm except the aforementioned "issue" which caused you to run out of in focus.

My 0.9x + 150PDS plate solves to 678 or 679.

Apologies, I don't have a reason for this except I'm fairly certain it's related to your first issue from last week.

andrea tasselli

Frequent contributor

Oct 5, 2023

It could be either way or none at all. The focal length reduction factor isn't necessarily a constant across the board of focal ratios. Neither is a constant across different specimens, so in your case is 0.93x against the advertised 0.9x. Assuming that the original focal length was 750mm, which isn't a given. Have you measured it (the original focal length)?

Alan Hancox

Topic starter

Oct 5, 2023

Actually no i haven`t. I think i need to check that out…..