Out of Focus Blue/Oiii Channels

[Jp114]

Hi,

I've been using a mono camera (Atik 314L+) over the Summer (weather permitting) and in my first attempts at LRGB I noticed that the blue channel was a bit soft. Since then I've been shooting in Hydrogen Alpha and Oxygen III (HOO). I've started by focussing on a bright star (using a bahtinov mask) through the Ha filter, grabbing the Ha data and switching to Oiii, but when I'm processing the Oiii images they're definitely slightly out of focus.

Before I tried narrowband, I suspected that the blue filter might not be parfocal, but now I'm thinking that the scope might have a significant amount of chromatic aberration. The Ha and Oiii filters are next to each other in the filter wheel so I don't think something's out of line there. Sometimes I can refocus the Oiii without moving off the target but mostly I can't see stars bright enough to check the diffraction spikes.

I would do some testing (now I've got an idea what to look for) but Clear Outside is just showing clouds for the foreseeable future...

I'm using an Orion ED80 and all of the filters are all Astronomik, so they should be parfocal.  Is it normal for channels to be that far out?  I'm wondering whether it might be time to get a higher end refractor...

Thanks,
Jon

 

 

[vlaiv]

Quite normal for ED80 - as it is ED doublet scope not triplet. ED doublets also have chromatic aberration - only reduced to minute levels by use of very expensive/exotic glass.

You can happily continue to use that scope for imaging - provided that you refocus when switching filters - although filters are parfocal, optics requires it.  Maybe motor focuser can be good solution? Automatic focusing routines let you remember offsets when changing filters so they can easily adjust focus on filter change without refocusing all the way.

[Jp114]

12 hours ago, vlaiv said:

Quite normal for ED80 - as it is ED doublet scope not triplet. ED doublets also have chromatic aberration - only reduced to minute levels by use of very expensive/exotic glass.

You can happily continue to use that scope for imaging - provided that you refocus when switching filters - although filters are parfocal, optics requires it.  Maybe motor focuser can be good solution? Automatic focusing routines let you remember offsets when changing filters so they can easily adjust focus on filter change without refocusing all the way.

Thanks for the reply, motor focusing sounds interesting.  I try to keep setup as simple and fast as possible but this could be a worthwhile step.

I have to say, I've been drooling over a scope upgrade for a long time so I'd better make a decision there first.

 

 

 

[david_taurus83]

I have a full set of Astronomiks and I have to say, when I checked with a mask, there is very little movement of spikes between filters. It's hard to tell if there is enough movement to say one is out of focus. And that's with an ED doublet. That said, I have an autofocuser and would always set it to check focus with a filter change just for the sake of it.

[powerlord]

certainly when I do NB I use my asiair and EAF. And let it refocus with filter changes. When it works, they do tend to be a wee bit different position (position shows as number in asiair). You do need to ensure you have given them sufficient time for autofocus exposures - especially the blue/oiii - I've had a few times now when I see in the morning that my ha with all good, but on the switch to oii it screwed up autofocus and all i've got are blobs.

 

[vlaiv]

If you decide to get auto focuser - then it would be good idea to let it check focus every hour or so (maybe two - depends on how quickly temperature changes)

Tube cools as temperature drops and it shrinks by tiny bit - but enough to throw focus off.

By the way - if there is significant difference between Ha and OIII, part of it might be due to scope cooling - so try to switch order of filters to see if you can make scope cooling work for you and "refocus" in good direction instead of bad one.

[Starflyer]

I've autofocussed for years now and see differences between filters on both my Tak and OO CT10. One of the first upgrades for me after moving to mono was autofocus, it's necessary, unless you want to waste precious imaging time manually focussing.

As vlaiv says, temperature change causes focus drift, even a couple of degrees is enough to make stars bigger and start losing detail.

[Jp114]

Thanks all for taking the time to answer.  I don't think that temperature is the issue here, although it's definitely something I'll bear in mind in future.  I've attached some subs (with crops) that were taken a few minutes apart. Obviously I'm only comparing the stars here and I'd expect the Ha stars to be a bit tighter.  

When I get an opportunity I'll grab an image through the each filter using a Bahtinov mask, that should hopefully show where things are falling over.

Ha
 

OIII

 

 

Ha vs oiii

 

Thanks,
Jon

 

 

[newbie alert]

I have Astronomik lrgb and the narrowband and using a mask there's very little difference between the ha and the o111 on my sct... but using my esprit there's more of a difference , enough to say I need to refocus between filters

 

[vlaiv]

8 hours ago, Jp114 said:

Ha vs oiii

Did you try refocusing for OIII? What sort of subs / stars are you getting if you focus for that filter?

[Jp114]

6 hours ago, vlaiv said:

Did you try refocusing for OIII? What sort of subs / stars are you getting if you focus for that filter?

This was the image that initially revealed the problem.  The thing that prompted this question was when I was imaging M16 (using an inappropriate SCT focal reducer) the issue was worse but I couldn't see this at the time (I didn't have any stars bright enough to see decent diffraction spikes using the Bahtinov mask). Using my quite basic setup, I'd have to slew to a bright star, refocus and slew back to the target and manually recompose the shot.

I haven't tried focussing OIII first yet.  It's a full moon so my only recent chance to get out was just using Ha.

Hopefully I'll get a clear night this week so I'll give it a try. 

[Jp114]

8 hours ago, newbie alert said:

I have Astronomik lrgb and the narrowband and using a mask there's very little difference between the ha and the o111 on my sct... but using my esprit there's more of a difference , enough to say I need to refocus between filters

 

That would definitely point to CA being the culprit then.

[Peter Withey]

Another point to bare in mind - is the blue end  (shorter wavelengths) of the spectrum , including 0iii, will always be inherently softer,   as it is scattered by the atmosphere to a greater extent than the red end (longer wavelengths) ,including Ha.  This is particularly apparent at longer focal lengths eg when planetary imaging.

[ollypenrice]

I think that OIII, being on the green-blue border, should be within the 'safe' performance end of the scope's optical correction but refocus will be necessary.

I'd also mention that manufacturers seem to struggle to make good OIII filters. I have had two dreadful ones from Astronmik and two bad ones from Baader. Also, it took Baader a very long time to bring their tighter bandpass OIII to the market, presumably because of production difficulties. You may have a bad filter.

Olly