Filters and exit pupils

[bomberbaz]

There has been a lot of information around around the use of visual filters, some people like to use them, some people not so and I fall into the former camp. It depends what you are viewing, scope size has a bearing and some objects are pretty much invisible without a filter. So I decided to stoke the filter fire a little myself by carrying out some of my own research. 

The equipment used is my Taurus 14" with DSC, a range of fixed focal length and zoom eyepieces and filters used are Astronomik.  I made good use of my observing hood, I never really appreciated the benefit of a hood until I got one 18 months or so since. Being able to relax your eyes and get to the eyepiece in absolute dark is something all astronomers should experience and it sits in my bits n bobs bag which accompanies me to every nights observing. (Unlike my observing chair)

These findings are all kind of personal but I believe certain aspects are more universal such as exit pupil to sky brightness correlation as an example. More on this later.

I know of information going around regarding rules of thumb or theories for filters, when to use them, what type of eyepiece to use with them and on what object to use them as well. I wanted to see if some of these hold water or are simply off the cuff ways of making your  opinion known and/or closing a discussion.

So my main concern that i wasn't getting the best out of my equipment (no changing room jokes please) but in particular with regard to nebula viewing. After my most recent trip to Galloway (visit B ) in Bortle 2 skies I was reviewing some of my findings, comparing with my previous visit A about 4 years previous and found that for all my planning, I had neglected my exit pupil size planning for OIII sensitive nebula. In visit B I had simply used my Nikon 17mm, more for the lovely wide FOV and completely overlooked exit pupil. The Nikon gives me 3.5mm exit pupil which I personally think is the lower end size for use in more light polluted skies, certainly not Bortle 2. Previously during visit A to Galloway I had used a smaller dob, larger 22mm eyepiece with an exit pupil of 4.8mm. My previous notes (from visit A) showed some objects viewed had been a better resolved than from my visit B which made me think and it was this that prompted me to look into filters in more depth. 

I do think that all visual astronomy is more complicated than at first people think. In particular viewing nebula which adds an extra layer of complication to this aspect of astronomy as you are throwing another layer into the mix, namely a filter.

For those who are unsure as to what I am driving at the brightness of a nebula and thus the ability to see it via a filter has many variables but given a scenario where certain aspects are equal. And we all use say and 12" dob and a identical 8-24 zoom there are limited variables. These are sky quality/location, quality of your filter, magnification/exit pupil (these are directly correlated) and finally experience.  The latter is a big thing as I recently proved to myself for the umpteenth time but that's another story. Primarily i am focusing on the third variable, magnification/exit pupil but this does bring into it the quality of filter variable.

Anyway, my last few trips out I have been experimenting with different exit pupil sizes for line filters (OIII/HB) and to a lesser extent, UHC narrowband.

I am not going to go through every single test or object but in broad terms I selected all emission type nebula with a mix of diffuse & planetary. I didn't bother with reflection type objects as these tend to behave differently.

I did lean towards more faint nebula because a lot of planetary are seen relatively easy due to their size and relative high surface brightness.

I tested out at three sites of varying darkness (SQM 20.25-21.15). My HB/OIII system was to start with a 24mm APM (It's my goto line filter) and vary between other ranges from 17mm to 28mm. The exit pupil range goes from 3.5mm to 5.8mm. (magnification variance 100-61) For UHC I varied all the way from 5.8 to 1.6mm exit pupils. That's 28 to 8mm eyepiece or mag 61-214 😱

My finding at all sites were identical but in the darkest skies, the results were slightly less conclusive for line filters  

HB filter targets were a mix of sharpless, NGC and IC catalogue. Performance at exit pupil size of lower than 4mm (baader zoom at 20mm) had a low responsiveness of nebula. I saw very little at these low exit pupil sizes and given the low actual brightness of these objects in general and tight bandpass of HB it is not surprising to me to find this. I did manage a very feint NGC 1499 and campbell's H star but nothing else verified. The most responsive eyepiece in my scope was the 24mm APM UFF which gave a 4.9mm Ex pupil. On campbell's H star I found a slightly larger exit pupil may have been an advantage but this was inconclusive.  I would also point out that this Ep (24mm apm) and filter combination gave me my only successful view of the horsehead nebula. 

I had a similar result with my OIII filter, however at the darkest sites I found that a 26mm eyepiece might have offered slightly more than the 24mm, it was hard to tell, possibly equal view at times, on occasion I simply called it a draw.  To be fair the 26mm eyepiece didn't help as the eye relief was awful so I simply gave up trying and called it a draw.  For information purposes the 26mm gives a 5.35mm exit pupil vs the 4.9mm. A difference in area of light collected of 15% so something to think about for me here.

Staying on the OIII I did find it more responsive to higher magnification/lower exit pupil than the HB although at my good to very good skies where tests took place the benefit of this were less on diffuse objects than on brighter planetary. Pushing the magnification on diffuse nebula (EG Pacman) allowed me to tease out a little better detail in the brighter areas but the overall image was less resolved than at lower magnification/larger exit pupil. For comparison purposes the 17mm Nikon gives an exit pupil that is nearly 60% less in area than a 24mm eyepiece. (0.243 vs 0.096 sq cm)

Regarding UHC. I did, if I am honest become too pre-occupied with line filter testing and only did limited testing on this filter class.  I find that UHC and OIII are more closely aligned than UHC/HB filters.  I don't know what the full technicalities of this are other than I find OIII seems to be greater light emitting than HB.

UHC filters have the greatest range of variation of exit pupil response, have the greatest overall range of responsive objects (arguably) of the three filters and certainly should be the first filter you buy if you're on a limited budget.  I can happily pop the UHC/zoom combination and take it up to x140 on planetary. Beyond this and no filter vs filter becomes the question, I am then pushing the exit pupil below 2mm, the skies are pitch black and many nebula readily pop into view anyway without a filter. Even on diffuse nebula (of which I only did a couple) I was able to push it to the same x140 but the benefit was limited, if any and I doubt I would make a habit of doing this.  I think optimum for UHC is much more varied, ranging an exit pupil anything from 3.5-6mm (at dark skies) with the ability to still be effective all the way down to 2mm. As mentioned beyond this your maybe as well without a filter.

When you compare a nebula viewed via a UHC against a line filter your liable to find two major differences, the UHC will probably be brighter but the line filter will yield more detail due to it's much narrower bandpass.  I found the nebula that fits this comparable best was the crescent (NGC6888). Very bright and apparent viewed in UHC and easy to observe, notably darker in OIII but the detail was superb, much more of a 3D effect at the expense of brightness. For the record the OIII view easily wins imho.

Based upon these results my possible search for another eyepiece for OIII has been shelved. If I can pick up the right EP at 26mm I might bite but I am in no rush.  I did make a post querying a 20mm EP as a potential suitor for this position some time back, I am glad I didn't bite before looking into this matter more closely.

The HB filters long used rule of thumb 5mm exit pupil seems to hold true to me. This is the rule that a 5mm exit pupil holds the perfect combination of line reduced light and what hits the retina. (think that's right) Anyway, it was this rule that helped me bag the Horsehead last year

The fact OIII seems to have the same was not unsurprising although my caveat ref 26mm possibly being better in ultra dark skies is interesting and something I will look into when opportunity arises. 

The above is very subjective because my eyes are not the same as yours, your equipment is not the same as mine and so results may vary. 

Other research (reading webpages from various sources mainly) I have carried out suggests that filters used under light polluted conditions should have a smaller exit pupil/more magnification to darken the skies more  to improve the contrast. I do know this works but I also know from experience that under severe light pollution (like my garden), you should simply stick to planets or other bright objects and save your filters for darker skies. The darker, the better

I feel I still have unsolved questions regarding both OIII and HB. But for now I am happy that my current set of available tools as it were are up to the job in hand.

 

 

[Don Pensack]

In general, how a nebula filter works is to dim the background sky without dimming the nebula.

So figure that all you are seeing is the nebula.

As is the case with any extended object, it will appear to dim as the magnification goes up.

You will see more details in the nebula, however, as magnification increases.

So there is always a compromise between brightness and magnification when looking at any nebula.  Otherwise, we'd all use the largest exit pupil possible with every nebula.

 

The fainter the nebula, the lower the maximum usable magnification will be, with or without a filter.

Nebulae that perform best with an H-ß filter are usually large and quite faint, which leads to better results with a large exit pupil, like 4-7mm.

Nebulae that perform best with a narrowband (UHC-type) filter tend to be much brighter, so smaller exit pupils will be fine, down to perhaps 2.5mm at a minimum.

Nebulae that perform best with an O-III filter fall into the large and faint (Crescent Nebula, Veil) or small and bright (many small planetaries).

     With the large and faint, exit pupils of 3-7mm seem to work best, while the small and bright might be suitable for as small as a 2mm exit pupil. though I would point out

     that many small bright planetaries show the most detail with exit pupils under 1mm, which is a magnification not suitable for filters.

 

So the brightness of the object and its emission profile will determine how small an exit pupil you can use, and the emission lines will determine which filter will work best.

Larger H-II star forming regions (M8, M20, M17, M16, M42 will work best with the narrowband filters passing both H-ß and O-III

Planetaries, supernova remnants, Wolf-Rayet excitation nebulae will work best with an O-III filter.

Large, faint, nebulae with almost exclusively hydrogen emission, will work great with an H-ß filter (example: NGC1499, IC434 behind the Horsehead)

 

 

 

 

[bomberbaz]

6 minutes ago, Don Pensack said:

 

Large, faint, nebulae with almost exclusively hydrogen emission, will work great with an H-ß filter (example: NGC1499, IC434 behind the Horsehead)

 

I bought the APM 24 mentioned in this thread off the back of something you put into another thread I wrote some while back, it framed IC434 / HH perfectly in my scope with a shade under 1 degree FOV.