Jump to content

Focus offset - Measure and setup?

Recommended Posts

Hi, I am having some trouble with my focus offset and thought you guys might have som pointers :)

Equipment: 10" f4 1000mm,  with 3" TSNTRED corrector reducer f3,4 850mm
Astrodon 36mm unmounted filters

So, first of, theese filters are stated to be parfocal, but in my experience, they are somewhat off, either that or it has to do with collimation / altered conditions during the night.

How ever, i took three readings (Autofocus in sequence) and avreged them out per filter, and the autofocus routine says the offset for RGB compared to L is around 0.02 to 0.03mm (if the autofocus routine measures in mm)

The narrowband differs from L like this;
Ha    0.11mm
O3  -0.15mm
S2  -0.13mm

I think theese figures sound a lot to me, specially when astrodon is supposed to be parfocal.

Is there someone here who uses Astrodon, what is your figures?

I do have some more questions i need to get out of my head.
- Can the offset be set as a negative number?
- Is the focus routine measuring in mm? ( at f3.4 is looks like the numbers are to big to hit the CFZ )

Thanks / Daniel

Link to post
Share on other sites

Hi Daniel,

I can't comment on the Astrodon filters. I use Baader filters and I also experience differences:

The RGB-filters are basically parfocal but about 0,04mm off compared to the L-filter.

The narrowband filters are about 0,33mm off compared to the L-filter, as the narrowband filters are 2mm thick whereas the LRGB filters are 3mm thick.

The differences that you measure are substantial if the filters are claimed to be parfocal as expecially the difference between L and the narrowband filters is way bigger than your critical focus zone.

To compensate for that, I put the differences into MaximDL - not in sequence. You can use negative values (my narrowband filters need negative values whereas the RGB filters are positive). In Maxim, I had to enter the differences in steps, so I entered e.g. -330 for h-alpha instead of 0,330mm (I use a ASA N8 with the OK3Z focuser).

After I measured the differnces and stored them in MaximDL, I verified the correctness by doing an autofocus with the L filter, then put a bahtinov-mask on the telescope and started bahtinov-grabber (a tool that gives you very precise information whether your focus is perfect) and then steped through all other filters to check, whether the focus compensation worked correct.

Hope this helps,

clear skies,



Link to post
Share on other sites

Thank you for the input, much appreciated!

I did another round of autofocus and this time, I got a totally different result.
I perhaps got my scope a bit better collimated before.
I will also try a third time with new V-slope measurements.

This time it wen't much better and it looks like Ha and O3 is spot on, compared to my reference filter which is L.
RGB is just a hundred of a millimeter off compared to the L so that is good to go.

I will also calculate the distance in steps to have that in hand if i need to try the Maxim DL route.

(Do i need FocusMax plugin for maxim to get that working?)


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...