6 replies to this topic

[archiebald]

I've been shooting a few galaxies with my new mono imaging train (P1Uranus-M, P1 7x36 wheel, Antlia Pro LRGBHSO) using my Askar 120 APO under NINA.  I had calibrated filter offsets within NINA, and I'm quite happy with the results I've been getting.

 

Now I'm switching this mono train to my WO Z73 for some wider FOV shooting in the coming weeks.  I have a cloudy few nights this week so I have no opportunity to shoot an all-nighter right now.

 

QUESTION

In theory, 

a) should the offsets recorded through the 120APO be close enough to use the Z73 straight off the bat, or 

b) should I expect the need to re-calibrate everything according to the change of OTA.

 

If opinions go with b), then I'll try to squeeze in a 1-2 hour calibration session between expected clouds this week.

[happylimpet]

If its the same focuser/EAF setup too, itll work fine.

[joshman]

Copy your current NINA profile, rename it to something meaningful and unique to this setup and re-run the Filter offset calculator for the different OTA.

 

unless your transferring the physical focuser mechanism in it's entirety between scopes, there will always be differences.

[archiebald]

Thanks for the feedback.  Yeah, I had forgotten that I am uses two different focusers on the OTA's, so I guess I'll just have to do a filters offset routine as soon as I get a couple of hours of cloud free time.

 

I have the plugin Filter Offset Calculator so it doesn't take too long.

[AstroFromHome]

If it is the same focuser make, so both ZWO or QHY, etc then you can re-use the offsets if your filters are parfocal. 

My set of Touptek filters have a total distance in focuser steps of 29 with my ZWO EAFs. Any air flicker has the same effect on the focus point. In theory I could run my camera with these filter without any offset calculation or re-focus after filter change.

If your offsets are as low or even lower just don't bother and enjoy imaging. 

[Der_Pit]

Well, filter corrections also implement remaining chromacy of the OTA.  You have to change focus, because the focal length depends on wavelength.  At least for refracting optics.  On top of that is the physical displacement of the focus due to the changed refractive index.  This part is independent of the OTA.

Both are absolute changes (i.e., in μm).  How much that is in focuser steps depends on the whole mechanics and gear ratios involved.

 

In short, I would not expect them to be the same when switching from a large refractor to a much smaller one with a different focuser...

Edited by Der_Pit, 14 May 2025 - 02:09 PM.

[archiebald]

Thanks for all the responses and idea.  Finally had a clear couple of hours last night so I set up the rig just to run the offset calculator.

 

As expected, very different results, but the most surprising for me was the difference in error direction of the Blue filter and the relatively greater shift of the Green filter..

 

For the 120 APO I had only gotten L R G B and H done, last night on the Z73 I did the full set, here are my results all in step counts - each had been averaged through three iterations.

 

   120APO   Z73    Comment

L     0          0

R    25        12   (difference could simply be due to different step size of the focuser)

G    6          22   (Relatively much larger difference considering the R result)

B    -5         38   (Vastly different value and in the opposite direction)

H    25        8     (difference could simply be due to different step size of the focuser)

S    -           25

O    -          15

 

The step count values are all different, to be expected, but the % change and the direction of change became vastly different too.  I guess that relates to what Der_Pit said above.

 

Anyhow, a useful practical lesson has been learned!