Does atmospheric transparency affect filter offsets?

[markse68]

I’ve noticed a strange phenomena over my past few nights trying to image the Crescent nebula. The first nights imaging came out fine but i only managed a couple of hours total so need more integration time. The following 2 sessions have been a disaster and i think it’s down to the transparency which has got progressively worse, and it seems to be affecting the focus offsets of my filters.

I don’t have an automated focus setup yet so get precise focus in red and then rely on my filters supposed parfocality for the other colours. For the first nights imaging this worked out ok- sure the green isn’t sharp but it’s passable. But last nights and the previous session the blue and green are unacceptably out of focus- unusable yet the red is still in focus.

The moon last night was severely dimmed by the haze in the atmosphere but viewing it was pretty sharp if a bit wobbly.

So is this a thing? I think it must be that moisture in the atmosphere changes it’s refractive index and that in turn spreads the focus point for the different colours? but if so then using dumb filter offsets and a robo focus motor wont ever be perfect either, though should be better than nothing 

Mark

[Sp@ce_d]

As you’ve found out filter offsets are relying on the conditions not changing since the original focussing. I found I lost more subs than it was worth it for the time gained.. certainly with my ever changing sky conditions! So soon gave up using them & refocused with a bahtinov mask every time I changed filter. I moved on pretty quickly to automated focus with all my imaging kit though years ago & never looked back. It’s the only reliable way of leaving imaging runs unattended.

You do need to keep checking your subs on each filter change really to see how acceptable it is & refocus as necessary. Otherwise move to autofocus as soon as you can.

[vlaiv]

1 hour ago, markse68 said:

So is this a thing? I think it must be that moisture in the atmosphere changes it’s refractive index and that in turn spreads the focus point for the different colours? but if so then using dumb filter offsets and a robo focus motor wont ever be perfect either, though should be better than nothing 

Could be due to that, but it can also be due to temperature and maybe even due to atmospheric pressure.

Different atmospheric conditions happen with different temperature and both lens cell and telescope body are made out of metal that shrinks / expands with temperature.

I think that atmospheric pressure and temperature affect refractive index even more than amount of moisture (changing air density).

Edited June 18, 2022 by vlaiv
typo

[markse68]

32 minutes ago, Sp@ce_d said:

You do need to keep checking your subs on each filter change really to see how acceptable it is & refocus as necessary. Otherwise move to autofocus as soon as you can.

Thanks @Sp@ce_d, but alas that’s not really an option for me- my camera (QSI583) takes ~20secs to download each image so i don’t think autofocus would be practical and I’ve taken to cycling through the filters one by one each iteration of a plan to try and minimise weird gradients caused by sky changes over time (if I bunch subs together by filter there’s also the risk of losing all of one filter should eg clouds roll in) I think i must prioritise fitting a motor to the focuser to at least minimise these shifts!

25 minutes ago, vlaiv said:

and both lens cell and telescope body are made out of metal that shrinks / expands with temperature.

thanks @vlaiv, my scope has a “hard paper” tube that doesn’t change much at all with temperature and my red is still focussed in all sessions so I don’t think it’s that (plus i’m cycling filters per iteration of the plan so they get similar conditions) A motorised focuser will at least allow me to set rough filter offsets that should improve things- I was just a bit surprised how much difference there was between sessions!

Mark

[markse68]

I’m coming up with a plan. Barring active auto focussing on the fly which i don’t think will work practically, I should be able to get good accuracy once i have a motorised focuser sorted out if i do my usual Bahtinov focus on red for the baseline, then do a second focus for the green and scale the other offsets between (well it’s only the blue anyway). This is only one additional step to what i’m doing now and should ensure much more accuracy based on current conditions.

Mark

Edited June 18, 2022 by markse68

[CraigT82]

Just FYI Auto focus routines can be set to run on binned images which speeds up the image download time. I used a Lakeside Astro kit with my old Atik 383L (same sensor as your camera I believe?) and with binned images for focusing it was a couple mins at most to focus between filters. 

[markse68]

I’ll give that a try too then Craig- thanks Just need to sort out the motor and figure out how to do that automatically

Mark

[Sp@ce_d]

With my QSI683 the focus routine averages around a minute. I use Focusmax (under ACP/Maxim control) & refocus every filter change. I tried sequential RGB but it adds too much time with refocus, plate solving & stop/start guiding to consider. I tend to do half hour runs of 6 subs per filter for RGB & repeat as required. Of course having a fully automated Obsy makes the difference! You didn’t mention what scope you are using, for instance a doublet frac would really need refocus on blue anyway. Maybe also think about prioritising red & blue. If you can’t get green try creating a synthetic green.

[newbie alert]

Certain parts of your questions don't make sense, can you clarify that you're using a motorized focuser or not?  , you can't use offsets if not

My LRG HA O111 and S11 are pretty similar but my blue is off.. I wouldn't say that they're exactly parfocal

If you are using a motorized focuser and the skies degrade it won't accurately record the hfd or fwhm as haze will bloat the stars.. you will get outliners which will affect the final focus point or measurements 

[markse68]

Hi Newbie Alert, no i don’t have a motorised focuser yet. I always relied on the RGB filters being close enough to parfocal which they are on a good night. My scope is a newt with MPCC. My observation based on looking at subs from recent sessions is that as the transparency reduced, the parfocality also reduced based on the width of the diffraction spikes on brighter stars. I guess it could be that they are just blurred which is spreading the width of the spikes but i wondered if the changing refractive index of the atmosphere might actually be causing more dispersion and thus causing a larger offset between the different filters focus.

3 hours ago, Sp@ce_d said:

With my QSI683 the focus routine averages around a minute

Thanks @Sp@ce_d that’s good to know. I think the 683 has significantly faster download speed but i will definitely try this when i get a motor sorted out- this is much higher in my priorities now!

Mark

Edited June 19, 2022 by markse68

[newbie alert]

7 minutes ago, markse68 said:

Hi Newbie Alert, no i don’t have a motorised focuser yet. I always relied on the RGB filters being close enough to parfocal which they are on a good night. My scope is a newt with MPCC. My observation based on looking at subs from recent sessions is that as the transparency reduced, the parfocality also reduced based on the width of the diffraction spikes on brighter stars. I guess it could be that they are just blurred which is spreading the width of the spikes but i wondered if the changing refractive index of the atmosphere might actually be causing more dispersion and thus causing a larger offset between the different filters focus.

Thanks @Sp@ce_d

Reduced transparency will affect all filters and the green filter spike isn't going to be as strong as say the luminance or the red.. if you have a Ha filter it's not going to be as bright as most of the others and that's on a good night... Just have a longer loop on the camera or a longer exposure time if using a dslr