Strange difference between Ha and OIII filters

[Gina]

I have Astrodon 5nm Ha and 3nm OIII NB filters mounted in an Atik EFW2 filter wheel attached to a 314L+ mono CCD camera. Scope is Evostar 80 ED DS Pro. The problem is that while I can get nice sharp focus with the Ha filter, the OIII shows very much poorer definition. I was particularly careful with clenliness when putting the filters in the filter wheel and there is no sign of any dust. I wondered if anyone had any ideas.

Here's a pair of 30s subs Ha/OIII taken one after the other with no focus change. These are magnified to show the difference and are of the core of M42.

[GlassWalker]

The second one looks out of focus. Are the filters parfocal? Even if they claim to be, it doesn't hurt to check the focus after you switch between filters.

Edit: even if the filters are parfocal, unless the scope is very well corrected for longitudinal chromatic aberration, that could also introduce a shift. I know this is a scope not a camera lens, but on camera lenses I find the deep reds around Ha are usually noticably shifted in focus from OIII.

[davew]

Re focus Gina. And when you think all is well, focus again.

Dave.

[Whirlwind]

Yep definately looks like focus issues to me. Although the Astrodons may be parfocal, I think the scope is a doublet so there is likely to be a focus shift anyway between OIII and Ha as they are in different parts of the spectrum and the scope probably doesn't focus them at the same point - the ED80 definately does not. Ha and NII and slightly less so SII should be closer though.

Ian

[RikM]

I always refocus for each filter.

[Davey-T]

Yes me too, I have Baader NB filters and have to refocus the O111 is the worst offender

Dave

[James]

Yes me too, I have Baader NB filters and have to refocus the O111 is the worst offender

Ditto...

I've found that my Baader narrowbands are all but parfocal in my Borg scope but they weren't on the FLT-98 I used to have, nor my old Megrez 72, I think this just backs up Whirlwind & Glasswalkers points about the scope...

James

[Martin-Devon]

Gina, I'm not sure what focusing method you're using, but the FWMH process in Artemis Capture is usually very good - just double-click on a star (don't chose a huge bright one) and if you are some way in focus a box will pop up showing the FWMH (star size essentially) and the brightness value. Then fine tune the focusing on 2-3 second sub loops until you get the FWMH figure as low as possible, a figure usually around 1.0 or lower is good focus.

[Gina]

I'll check the focus with the OIII filter again. I was hoping I would be able to do alternate filters so that when the clouds came in I would have approximately the same numbers of each.

[Gina]

Well, it's not as bad as I thought. I pointed the scope at a star, set the OIII filter in line and focused using 10x zoom (max) in Artemis. I got the star covering 3 pixels at 50%. I then selected the Ha filter and the star covered 4 pixels. By turning the fine focus knob by something like 40 to 60 degrees I could optimise the Ha. But there was some focus shift tonight due to the atmosphere and I wasn't able to maintain the 2px focus I got last last night.With the elatively small focal shift from Ha to OIII I think my scope can be classified as apochromatic as claimed. Also the filters are basically parfocal. I think there must have been some condensation on the OIII filter last night - it's so much sharper tonight

In conclusion, I think I can consider the equipment is good enough to give good results when the seeing permits.

[davew]

Nice idea but I don't think it's practical. ( Post no 9 )

Treat the word " Parfocal " with caution. Besides which, I'm not aware that Astrodon claim they are parfocal. I have used an APO scope and a friend also has and neither focus at the same point with narrowband. You may find the S11 is close but stil not perfect.

A night of testing is worth a thousand theories.

Dave.

[Gina]

Gina, I'm not sure what focusing method you're using, but the FWMH process in Artemis Capture is usually very good - just double-click on a star (don't chose a huge bright one) and if you are some way in focus a box will pop up showing the FWMH (star size essentially) and the brightness value. Then fine tune the focusing on 2-3 second sub loops until you get the FWMH figure as low as possible, a figure usually around 1.0 or lower is good focus.

Thanks Martin I haven't got round to trying FWHM yet.

ATM Im running one of my my widefield cameras in OSC mode using the 200mm f4 SMC Takumar lens and capturing Orion's Sword - M42 to the Flame nebula. That is easily resolving the trapezium stars in the core of M42. Had LP problem until I moved the CLS CCD clip filter from my scope 1100D to the WF 1100D. That fixed it. Currently running 30s ISO1600 subs at 5C. Something's wrong with the ASCOM connection to my QHY5 and guiding is out. Might try 60s shortly and see if there's any eggy stars.

[swag72]

As everyone as said, it is the focus that needs a tweak - This brings me on to electronic focus wheels - Unless people have automated focussing, then why do they use them? You set a run during the night and tell the PC to change the filters - FAB!!! - But not if you can not also check the focus remotely.

[Gina]

From my testing last night :-

The problem with the OIII filter was only temporary - I think it must have been a bit of condensation as it was fine last night. There is a very small difference in the focus point between the two but if I set the focus to the mid-point between exact focus for Ha and OIII the focus is pretty good and better than the changes in focus caused by poorish seeing last night. OTOH for a lot of DSOs there is much more Ha than anything else and Ha can be used as the luminance and let OIII just add colour. In fact I've seen OIII binned 2x2 in some posts. I think I shall try that to increase the OIII sensitivity.

I might have another go at M42 as that has a lot of variation in luninance and colour - though I think this is one object that might be better in LRGB and also wider field. Anyway, I can get some useful practice on this one. I'll try focussing accurately with Ha and binning 2x2 on the OIII next time we get a clear night. Not sure about tonight but the rest of the week looks like being cloudy and wet

[jjongmans]

See this link:

When filter manufacturers state that their filters are "parfocal", they mean that the filters are made to a specified thickness tolerance. They can provide filters that will not increase the focus shift that is already in your optical system, but cannot guarantee that you will not need to refocus among the filters.

[ollypenrice]

In the Takahashi FSQ85 I find Baader LRGB and Ha to be accurately parfocal at F3.9. However, that's Takahashi colour correction for you. In the TEC triplet they are close to parfocal as well so far as I can see, but F7 is a sight more tolerant.

I'd advocate Artemis's FWHM as well, though I use 3 to 5 second subs to average out the seeing. On the other hand FWM in Nebulosity gives a garbled message, though that's in a big scope. I find it very frustrating to use.

Why do people use motorized focus? Why, to make it more difficult, of course! Using an electric motor you can't feel the difference between backlash and real movement of the drawtube. With your fingers you can.

I hate the things.

Olly

[jjongmans]

Why do people use motorized focus? Why, to make it more difficult, of course! Using an electric motor you can't feel the difference between backlash and real movement of the drawtube. With your fingers you can.

The reason I use a motorized focuser is because I want to refocus after a filter change. Faster ratios have a smaller CFZ, so there's a bigger chance that refocus is necessary. I'm not always attending my setup while imaging, so that's why I automated the refocusing. Backlash in the focuser can easily be compensated with the focusing software (FocusMax), it just needs calibration.

[ollypenrice]

The reason I use a motorized focuser is because I want to refocus after a filter change. Faster ratios have a smaller CFZ, so there's a bigger chance that refocus is necessary. I'm not always attending my setup while imaging, so that's why I automated the refocusing. Backlash in the focuser can easily be compensated with the focusing software (FocusMax), it just needs calibration.

If you're using robitic focus then I entirely see the point. No issues on that score. But I vastly prefer my fingers to a button if I'm on the spot.

Olly