Achievable FWHM for different filters - help please

[Adreneline]

Hi Everyone,

Whilst imaging last week, under unusual and exceptionally clear conditions, I was using RGB, L and Ha filters (all Baader 1.25") on a WO-ZS71+Atik428ex combination.

I was using Artemis Capture and a trial version of APT to check focus - I really can't get on with HFR on SGPro.

I was achieving FWHM fiures of typically 1.2 for each filter except Blue which refused to go below 3.5 - there is always some fluctuation I find and my experience to date is the fluctuation is often much more than +/- 0.5 - I think the unusually good seeing was helping. I checked the Blue filter and it appeared exactly like all the others - no visible condensation, misting or big fingerprints! I also used the Bahtinov Mask utility in APT which confirmed the 'Far' focus on Blue.

Combining the LRGB in PI has resulted in some nice blue halos around the stars. The stars in the Blue component also appear somewhat bloated.

This is a crop of the centre of the image showing the problem; I hasten to add this is only part processed 

Is this a typical problem with Blue filters? Does it explain why there are so many macros/actions to remove blue halos? Or do I have a suspect Blue filter?

CCD focussing on the ED80 and the ZS71 has always been an issue for me; I find focussing the dslr with ByEoS FWHM is never a problem so I am sure it is not the scope.

Any thoughts or advice would be much appreciated.

Thanks in advance.

Adrian

[DaveS]

I think "blue bloat" (Sounds like a horrid disease) afflicts most 'fracs short of eg Taks or good triplets.

[MartinB]

Blue light is dispersed more readily than the rest of the spectrum which is why the sky is blue, but not to this extent!!  Also, you shouldn't get much of a halo if you are adjusting the focus for your blue filter.  I wonder if there is some reflection although this normally the halo is then slightly offset.  What make of filter are you using, seems a bit dodgy.  

[Adreneline]

1 hour ago, MartinB said:

What make of filter are you using, seems a bit dodgy.  

Hello Martin.

i'm using a set of Baader 1.25" filters from FLO. I cannot understand why I couldn't get the same level of FWHM with the Blue filter. Conditions hadn't changed because I checked the other filters and was still getting figures around 1.2 or less. With the Blue filter I was still up around 3.0 or more. They are supposedly parafocal but  this one seems quite different. That's why I was wondering if I had a dodgy filter. The image looks a whole lot better if I don't include the blue data.

I presume I should expect comparable FWHM for R,G and B. I assumed L and Ha might be different but not sure why I think that would/might be the case.

Since I started AP it has been obtaining good focus and processing noise that have been my nightmares! I think I'm making progress with noise but focus continues to blight my images.

Perish the thought that one day I will have solved all the problems!

If it is ....

1 hour ago, DaveS said:

"blue bloat"

then I'm doomed because a Tak is never going to grace my observatory/extension!

Thanks to you and DaveS for your thoughts and advice.

Adrian

P.S. A thought occurs! I use an IDAS UV/IR filter on the ED80 with the 414ex - should I also use one on the ZS71 with the 428ex and filters?

[MartinB]

Your scopes are good quality doublets, the original SW ED80, whilst not truly apochromatic, was a real game changer for affordable, well corrected optics.  The mistier the sky the greater the atmospheric dispersion but you said is was a very clear night, I don't know if you have an objective way of checking this such as counting the number of stars you can see in the square of pegasus but it doesn't seem that sky conditions were to blame.  The other blue halo creator is dew and  a fine misting on your objective will blow out the blues much more than the other colours.  You checked with a bahtinov so the issue clearly wasn't focus related.  

If this has happened once my guess would be dew somewhere in the optical path (including the possibility of your chip) however if it is a constant occurrence there may be an issue with your filter.  It would be worth seeing whether the problem is still present before turning on your camera cooling.

[Adreneline]

19 minutes ago, MartinB said:

If this has happened once my guess would be dew somewhere in the optical path (including the possibility of your chip) however if it is a constant occurrence there may be an issue with your filter.  It would be worth seeing whether the problem is still present before turning on your camera cooling.

Hi Martin,

Thanks for your response.

Next time the clouds clear I'll have another go and start with the Blue and see how I get on. I am tempted to say it is the filter because I tried focusing with the Red and Green and got low (<1.3) FWHM figures but going back to the Blue pushed them up above 3 again. The stars are obviously bloated on the blue lights despite my best efforts to sharpen the focus.

Sorry to ask but should I add an IDAS UV/IR to the ZS71 at the front end of the reducer anyway? Would it make a difference?

Thanks again.

Adrian

[MartinB]

The increase in FWHM is exactly what you would expect to see with dew somewhere along the image train.  

I can't recommend the IDAS enough.  It needs to be the IR blocking variety.  Even with fairly dark skies it will help if there is any hint of sodium light pollution around.  It won't help  reduce your problems with the blue channel though!

[ollypenrice]

If the scope has given clean blue stars in the past then perhaps you do have a bad filter. (If I read your post correctly you've used this scope with a DSLR?) The filter might not be blocking correctly at the short wavelength end.

Olly

[Adreneline]

34 minutes ago, ollypenrice said:

If the scope has given clean blue stars in the past then perhaps you do have a bad filter. (If I read your post correctly you've used this scope with a DSLR?) The filter might not be blocking correctly at the short wavelength end.

Thank you for your response Olly.

Focussing has always been my nemesis - but I think I am getting better at it - albeit slowly. It was the fact that it was just the blue filter I was struggling to focus. Going back to the red and green gave me low figures again. Not impossible but odd that only the blue should mist up.

Next time the clouds clear I'll give it another go. One thing that I have gleaned is that I should expect to achieve the same FWHM from all the filters, albeit with an adjusted focus position.

And yes, I've used the WO-ZS71 with a dslr focussing with ByEoS and achieved very low FWHM figures so I'm pretty sure it's not a scope problem.

Thanks again.

Adrian

[ollypenrice]

3 hours ago, Adreneline said:

Thank you for your response Olly.

Focussing has always been my nemesis - but I think I am getting better at it - albeit slowly. It was the fact that it was just the blue filter I was struggling to focus. Going back to the red and green gave me low figures again. Not impossible but odd that only the blue should mist up.

Next time the clouds clear I'll give it another go. One thing that I have gleaned is that I should expect to achieve the same FWHM from all the filters, albeit with an adjusted focus position.

And yes, I've used the WO-ZS71 with a dslr focussing with ByEoS and achieved very low FWHM figures so I'm pretty sure it's not a scope problem.

Thanks again.

Adrian

Don't forget that when you focus with a one shot colour camera you are focusing on a 'compromise spectrum' which might help contain the blue bloat (if it exists) of the scope. If you could borrow an alternative blue filter, that would be best.

Olly

[Uranium235]

On 21/12/2017 at 12:48, Adreneline said:

They are supposedly parafocal but  this one seems quite different.

There is part of your problem, no matter what telescope you are using you should always refocus after changing filters.

Also, (IMO) there is no substitue for a simple B-mask, your hand, and your eyes - rather than relying on software to tell you its in focus (you never know, it might be fibbing!). Additionally, the standard ZS71 (not the Star71) will suffer in the blue channel as its an FPL51 doublet. The SW 80ED is FPL53 and a slightly slower FR, so it controls colour much better.

But that blue bloat is suspiciously bad, almost as bad as what you would get from an achromat. Might be worth seeing if you can mount a UV/IR blocker in the imaging train somewhere - just to see if it helps.

 

Edit: Here is a reference image from Cloudynights on the subject of CA with the ZS71, it does seem fairly consistent - or contributary to your issue:

Lots of blue/violet going on there.

 

[Adreneline]

1 hour ago, Uranium235 said:

Might be worth seeing if you can mount a UV/IR blocker in the imaging train somewhere - just to see if it helps.

Hi. Thank you for the advice. I've got an IDAS UV/IR 2" filter on the ED80 so I will transfer it into the ZS71 and see what it does.

Focussing has always been an issue. Sounds like a pathetic excuse but wearing varifocals doesn't always help when it comes to using the eyeballs!

I have found HFR in SGPro to be very trying to use. I find FWHM in ByEoS very good and always get pin-sharp stars using the dslr. Using the CCDs has proven more difficult which is why I was giving APT a go and seeing how I got on with the FWHM and B-Mask utilities. They were working out really well except for the Blue filter - the L,R,G and Ha subs are very good and all from using the FWHM utility, confirmed by the B-Mask. Hence this Post.

I focus using the hand-controller associated with the SW focussing system. I've 3D printed a mount so I can mount the SW dc-motor on the ZS71. It gives me very fine control with the 1:1 gearing ratio - far better than I can achieve by hand. I'm hoping Father Christmas is bringing the USB controller!

I'm not rulling out a slightly suspect Blue filter at this stage - I need to see if I can borrow one from somewhere.

Thank you again for your help.

Have a good Christmas!

Adrian