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Evident tilt only when using Ha filter??

ZWO ASIStudio Equipment Filter Wheel H-alpha Tilt
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Rostokko avatar
Rostokko avatar
I have been observing a strange situation where I can notice some tilt when taking exposures through the Ha filter, but not through other filters.

I finally decided that that was annoying enough to use a tilt analyzer (I used ASIStudio in this case - but I guess the others would give me similar results), and it did confirm an evident tilt when shooting Ha, but not when using other filters.

Ha (600s):


Oiii (600s):


Red (180s):


That trend is visible in all subframes, across multiple nights. All filters are about the same area in the sky. While Oiii, Red and others seem to be somewhat in the range of "analysis noise", Ha seems to be a bit out of whack. All filters are in the same EFW, and no changes to the optical train have happened over the time these subframes have been acquired.

What do you think could cause this? Defective Ha filter? Ha filter come loose in the EFW (I would hate to have to disassemble all that for nothing)? Tilt analyzer disliking Ha?
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Himanshu Pandey avatar
Himanshu Pandey avatar
If you are using mounted filters the H filter glass could be mounted incorrectly, perhaps a burr or debris on the frame kept the glass from being flat.  The glass in the mounted filters I have are held in place by two very fine o ring that screw in place on both sides.
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Rostokko avatar
Rostokko avatar
They are mounted filters, yes. I'll take that as a vote in favor to open up the efw and take a close look at the Ha filter.
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
Are you refocusing with filter changes?
Rostokko avatar
Rostokko avatar
> Are you refocusing with filter changes?
Yes - I use offsets.
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
> Are you refocusing with filter changes?
Yes - I use offsets.



If you find the filter installed correctly, then id recommend focusing with the HA filter to see if your offset is wrong. 

focus error amplifies the tilt effect, and since all your images above show tilt, the ha could be worse with a little focus error.
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Rostokko avatar
Rostokko avatar
Chris White- Overcast Observatory:
> Are you refocusing with filter changes?
Yes - I use offsets.



If you find the filter installed correctly, then id recommend focusing with the HA filter to see if your offset is wrong. 

focus error amplifies the tilt effect, and since all your images above show tilt, the ha could be worse with a little focus error.

Thanks; of course I did that test when offsets were first computed; but it is indeed an empirical approach, and things may always change a bit in time. I'll try explicitly focusing on Ha for a couple of nights and see if anything changes.
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
You might find this helpful: Guide To Tilt Analysis

Part III touches on how focus can impact tilt.
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Rostokko avatar
Rostokko avatar
Chris White- Overcast Observatory:
You might find this helpful: Guide To Tilt Analysis

Part III touches on how focus can impact tilt.

Thanks for the link, a very useful read.
Considering I am already using Hocus Focus in Nina, I will spend some time using it to analyze things. I have actually meant to do that for quite some time now, but whenever I end up with a night good enough to do that - also considering how such nights haven't been too frequent lately - I always decide to take exposures of my active target(s) before they disappear from my view for the rest of the year...
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory:
You might find this helpful: Guide To Tilt Analysis

Part III touches on how focus can impact tilt.

Thanks for the link, a very useful read.
Considering I am already using Hocus Focus in Nina, I will spend some time using it to analyze things. I have actually meant to do that for quite some time now, but whenever I end up with a night good enough to do that - also considering how such nights haven't been too frequent lately - I always decide to take exposures of my active target(s) before they disappear from my view for the rest of the year...



Many people get stuck in the weeds worrying about tilt, so it's smart to not forget about imaging.  You can fix a lot of issues in post processing, so definitely dont spend all your time making a perfect tilt graph.  You can always pick away at it and make it better when the moon is bright or clouds are coming in.  I would not be surprised if what you are seeing above is an issue with your focus offsets.  FWIW, I used to be a user of offsets with SGP. however I have found over time that I get better results by simply using the actual filter for focusing.  This doesn't allow for interleaving filters when capturing, but does tend to lead to better data.
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Tony Gondola avatar
Tony Gondola avatar
I agree with the above. I just don't implicitly trust offsets myself. It's a nice trick that works for many but it depends on your hardware (focuser, backlash and focus motor) to be absolutely repeatable and I'm sure that with absolutely the best hardware ($) it is, just not with mine. It doesn't take long to run a focus and that way I am absolutely sure it's right.
Rostokko avatar
Rostokko avatar
Chris White- Overcast Observatory:
Many people get stuck in the weeds worrying about tilt, so it's smart to not forget about imaging.  You can fix a lot of issues in post processing, so definitely dont spend all your time making a perfect tilt graph.  You can always pick away at it and make it better when the moon is bright or clouds are coming in.  I would not be surprised if what you are seeing above is an issue with your focus offsets.  FWIW, I used to be a user of offsets with SGP. however I have found over time that I get better results by simply using the actual filter for focusing.  This doesn't allow for interleaving filters when capturing, but does tend to lead to better data.

Forcing an Ha-based autofocus on the next usable night is easy enough to do; I will definitely do that and see if anything changes. I can definitely live with the small tilt across all other filters; if I can convince Ha to behave, I'll be a happy camper.
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Rostokko avatar
Rostokko avatar
To close the loop on this: thanks so much for the suggestion @Chris White- Overcast Observatory ; what I was noticing was indeed a wrong focus offset on the Ha filter (for whatever reason that came to be).
I corrected that, and the reported tilt in Ha exposures now is in line with the other filters, and perfectly acceptable (to me; I am not going down the rathole of fighting a small tilt with my current equipment and existing seeing conditions).
CS.
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Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
To close the loop on this: thanks so much for the suggestion @Chris White- Overcast Observatory ; what I was noticing was indeed a wrong focus offset on the Ha filter (for whatever reason that came to be).
I corrected that, and the reported tilt in Ha exposures now is in line with the other filters, and perfectly acceptable (to me; I am not going down the rathole of fighting a small tilt with my current equipment and existing seeing conditions).
CS.



Hey thats great!  Always nice when its an easy fix.  A rare experience in this hobby.... lol.