View Single Post
Old 08-10-2021, 12:24 PM
Wackyguiding (Bernie)
Registered User

Wackyguiding is offline
Join Date: Sep 2021
Location: Australia
Posts: 4
Back Focus Issue ED80?

Hi all,

I have had my current set up for the past 6 months and have started to try to fix the star shapes in the corners. It normally wouldn't bother me but I'd like to do mosaics down the track.

Current equipment set up:

Skywatcher ED80-Original Focuser-SW 0.85x Reducer/Flattener-16.5mm spacer-21mm spacer-11mm Spacer-ZWO UV/IR Cut filter 1.25"-ASI294MC Pro (6.5mm).

Back focus required is 55mm.

This totals to 55mm spacing from the reducer to sensor. Using the 1/3 rule for the filter, the glass thickness is 1.1m (could only find this from a supplier website). This comes to 55.37mm. However, the star shapes don't seem to change regardless if I put 0.5-2mm spacing.

First two photos are both top left of frame and different targets due to obstacles during the night.

Click image for larger version

Name:	UV Filter No Spacers ps.jpg
Views:	25
Size:	32.1 KB
ID:	281827
No spacers, 55mm reached with zwo spacers

Click image for larger version

Name:	2mm metal spacers ps.jpg
Views:	22
Size:	36.8 KB
ID:	281828
2mm metal spacers

Next two are top right

Click image for larger version

Name:	no spacers top right.jpg
Views:	16
Size:	62.9 KB
ID:	281829
No spacers

Click image for larger version

Name:	2mm metal spacers top right.jpg
Views:	10
Size:	42.0 KB
ID:	281830
2mm metal spacers

If anything 2mm looks worse.

Could someone please let me know if it looks to be a spacing problem or a reducer/flattener quality issue or something else entirely? I will try two 0.2mm spacers to get as close as to the .37mm extra required but it may be a while with the weather. I was surprised to see no real change with the different spacing, and would be surprised if it required another 5mm of spacing as that's a fair way out from the specification.


Last edited by Wackyguiding; 08-10-2021 at 01:14 PM.
Reply With Quote