Understanding exit pupil and filters in deep sky observing

[Piero]

Premise:

I am not offering a review on this vast topic as I do not have enough experience. My aim is to collect relevant information so that this WHOLE thread becomes an important reference for amateur visual astronomers. 

I decided to start this new thread after reading a couple of very interesting (and recent) posts by YKSE and jetstream (thread: http://stargazerslounge.com/topic/236689-5mm-eyepiece-for-planatary-views/), where they kindly shared these two links:

1) http://starizona.com/acb/basics/observing_theory.aspx (thanks YKSE!)

2) http://www.lumicon.com/pdf/3filterspec_prnt.pdf (thanks Gerry (jetstream)!)

At the same time, I was quite surprised by my UHC filter. To summarise, under light-polluted sky (stars up to 4-4.5 mag) I achieved better results on M42-M43 if I decreased the exit pupil from 4.0mm (24Pan) to 3.3mm(20Plossl) or even to 1.2mm(7Nag) in combination with an UHC filter. In more detail, I found that the contrast was largely increased and the borders, particularly in the central regions, arose clearly. Somehow, the feeling was that the decrease in exit pupil simulated the view from a dark sky place and favoured the work of the UHC. Removing the UHC filter and using low exit pupil did not improve the view so much, indicating that the UHC filter was actually of help. In a slightly off-topic post that you can find in this thread http://stargazerslounge.com/topic/236749-rosette-nebula/, I asked Gerry a potential explanation for this, as I expected that the UHC filter worked better at around 4.0mm exit pupil, and that the view became too dim if the exit pupil was reduced in combination with the filter. In his precious comment (thanks a lot!), he highlighted the differences in contrast change between extended and point source objects, when increasing/decreasing the exit pupil. In particular he reported how both the sky and extended objects becomes dimmer by decreasing the exit pupil, maintaining the same contrast.

I suspect M42 is a quite specific object to view with a filter. I think the whole nebula works as extended object, whereas the core works more similar to a point source object. With this in mind, I believe that for viewing the whole nebula, large exit pupils and an OIII/UHC filter are suitable, whereas for viewing the core, smaller exit pupils and an UHC filter can be of benefit. Interestingly, these differences in M42 seem to exist in imaging too. I came across this thread (http://stargazerslounge.com/topic/227537-different-exposure-times-vs-same-exposure-times/), where combining long and short exposures, for M42 nebula and the core respectively, seems the right thing to do in order to detect the best contrast and detail for both (thanks Olly).

Questions:

1. How does sky transparency affect the optimal exit pupil+filter?

2. How much of these improvements is actually due to our eyes?

3. Is it really true that decreasing the exit pupil simulates a dark sky and that the UHC filter “takes advantage” of this, or is this just an artefact of our eyes to perceive contrast differences a bit better when an object is magnified? What are your experiences?

4. Do you use different combinations of exit pupils + filter, if you use a small or a large telescope?

5. Please feel free to ask other questions

I do not want to give a structure to the answers, but I think it would be worth stating some of the following conditions in your answers:

- sky: dark vs light polluted

- telescope size: small vs large

- filter used: deep sky, UHC, OIII

I also added the links to the important and extensive work done by David Knisely, as related to this topic:

http://www.prairieas...ep-sky-objects/

http://www.prairieas...common-nebulae/

Thanks for reading and for your contribution,   

Piero

[ronin]

In the several years of doing astronomy and the several years of optical engineering I can honestly say I have never given a thought or the slightest concern to the exit pupil when observing. Absolutely none.

I have read that 2mm is the optimum, I have read that 1mm is the optimum.

Someone on CN supplied the maths behind the 1mm one about 3 years ago, looked good, may or may not have been right, but looked good.

When looking through a scope I am not thinking of the exit pupil, I am thinking of what I can see.

If you get to a 2mm exit pupil are you going to stop viewing?

Exactly the same at a 1mm exit pupil?

Or are you going to ignore the "exit pupil" rule and go for a bit more magnification and see if you can get a bit more out of it?

I strongly suspect the latter and if so forget exit pupil.

One reasonably good rule in science is you will get told what you cannot do, then someone will go out and do it anyway, and you have to start all over again. The worst thing is the person that said you cannot do it will likely be the person that goes out and does it.

You must have read that 200x is the maximum realistic magnification?

If so go search out the people that were looking at Mars the last time it was convenient and using 300x and 350x and getting good results.

Filters are simply a subtractive item and for understanding those you really need an accurate set of filter characteristics for that item. Different UHC filter will/may have different transmission curves, although the same name. Others are the same. It is surprising by have much the "same" item differs.

One thing I have learnt is not to get bogged down in numbers and rules.

Just about every thing in astronomy appears to get rethought every 10 years.

Best I can say is listen to it, learn it, understand it and get prepared to unlearn it and relearn something else pretty soon.

Imaging is a good example.

SBIG say 1 arc second per pixel, others say 2 or 3 arc seconds per pixel and I have read 5 and 6 arc seconds per pixel somewhere. All astro ccd manufacturers, all slightly (widely?) different. Which one, who's right?

[John]

I don't have any years in optical engineering and only my 30+ years in the hobby of astronomy to draw on but I do now place quite a high priority on using an effective and not excessively large exit pupil. I do this simply because I have found that it's been a contributary factor in getting the best visual results rather than though having a strong grasp or adherence to theory.

I think there are many guidelines out there but no rules really so trying stuff out and seeing how it goes and how the results compare to other combinations is the way I've found what works best for me, under my skies, with my eyes, scopes etc.

As Ronin says though, it's good not to get "stuck in our ways" and trying different combinations from time to time can help discover something that works better or can confirm the your current aproach is the most effective.

YMMV

[Piero]

mm.. I am a bit confused.. I didn't ask for rules but simply your experience. 

I think the description of how I observed M42 clearly shows that I didn't respect "the rules", but I simply tried out alternatives. It seems to me that the use of the word 'exit pupil' labelled my OP as theoretical, whereas I simply used it as a reference for image brightness and to understand how the differences between small and large telescopes.

Thus, the reason why I asked this was to understand more about it and to hear other people's opinions.

Apologise for the misunderstanding. 

[John]

Sorry if I misunderstood what you were asking for Piero.

In my case, I don't keep records of my observing sessions so I can't look back and answer your questions.

I guess what I was saying in my previous post was that I've tried quite a lot of different combinations of equipment out and decided what I felt worked best for me and thats how I've arrived at my current set of equipment.

I can see now that you were looking for something more systematic perhaps ?.

Perhaps others who keep detailed observing records can help more than I can.

[jetstream]

I don't understand the hesitation of some members to use the optical spec of exit pupil, when magnification specs are commonly thrown around, quoting eyepiece focal length's etc. Using the properties of magnification and eyepiece focal length people are inadvertently quoting the exit pupil in their own scope, whether they realize it or not.

Having a closed mind is not the path to enlightenment or better views with the equipment we have or want to buy IMHO. Using "exit pupil" may provide an easier way to discuss what eyepiece works in any telescope, regardless of the magnification it provides.

I would love to hear of any exit pupil doubter try and use a 10mm exit pupil or a .3mm exit pupil in their scope and then talk about what happens using magnification as the standard of comparison in their different telescopes.....

pdp asks some very good questions and has already made some good observations and will continue to excel.

I also have a question for any members- what has been re thought in visual astronomy in the past 10 years? the concept of exit pupil/entrance pupil has been used for many decades....

[YKSE]

Piero, thanks for bringing up the topic.

What I've understanded so far (much more to learn, of course), knowing the impact of the exit pupil has general underated, if not ignored. I see it's of great help to know more about it. Many understand 0.5mm as lower limit, and around 7mm as upper limit, what do we know inbetween these limits?

R.N. Clark's "VISUAL ASTRONOMY OF THE DEEP SKY" is a great book for explainning these things, among many other aspects, some free online pages can be read, like this one:

http://www.clarkvision.com/visastro/omva1/index.html

And Nils-Olof Carlin's interpretations gives a simplied guideline:

http://w1.411.telia.com/~u41105032/visual/blackwel.htm

As to my personal experience, the monkey head nebula(NGC2174) was a clear example, with UHC filter, 4mm exit pupil showed the whole nebula, while 3mm exit pupil, only the top halv, and it was under reasonably dark sky, NELM6.3(SQM 21.4).

[John]

In an effort to contribute something at least slightly useful to this topic, I'll offer the link below. It's nowhere near as sophisticated as most of the sources included above but at least I can understand it !

https://www.astronomics.com/eyepiece-exit-pupils_t.aspx

I agree that exit pupil is well worth consideration and I know that some do base their eyepiece selection solely on it. I reckon it is the reason that my longest focal length eyepiece remains 31mm and also why under my moderately light polluted skies I find 21mm more effective than the 31mm on DSO's when being used with my F/5.3 12" dobsonian.

[YKSE]

That's an excellent link, John.

I'd think that statement about the Veil is exactly the same as your choice of viewing, 4"f6.5 Vixen with 31T5, nearly 5mm exit pupil, better than 31T5 in 120ED in just over 4mm exit pupil, in addition to framing the Veil better. My 2cent, based on exit pupil, is that a 41Panoptic in 120ED will most likely show the veil better than 31T in the Vixen.

[Piero]

Thank you for your answers. I am glad that this thread turned into a positive and constructive conversation. 

Although I think this thread is quite distant from being scientific (and this is not its purpose either!), I think that one reasonably good attitude in a hobby (and in science), is to listen to other people opinions, ask new questions, express your doubts and being proactive when investigating your own solutions. 

With 0 years of experience in optical engineering, a few years of experience in amateur astronomy, which could be largely condensed to, let me check, less than 1 year, basically since I joined SGL, where I can happily say I started learning more about this hobby, I can say that I know very little or more correctly nothing in the subject.

As I found, and I believe that I am not the only one, that one of the criteria to perceive the quality of a view is image brightness rather than magnification, and this can be used for comparing the performance of different telescopes more effectively. For instance on my TV60, 50x correspond to 1.2mm e.p., whereas on a Dob 200F6, 50x correspond to 4.0mm. Same magnification, but well different view! With this, I don't think that magnification should be discarded completely as measure. I think it is important and as the two documents provided by YKSE and jetstream show. The fact that I perceived an increase in contrast in an extended object while decreasing the exit pupil seems (and is) a contradiction with how the exit pupil works on those objects. 

However, this effect can be explained if we consider that our eye perceives an increase in contrast when we magnify the object. In conclusion, to me, the concepts of exit pupil and magnification are both important and should be both reported when a session outside is described. 

From the way I see, the concept of exit pupil is even more important when filters are used. Not casually there is a myth saying that OIII filters should be used on telescopes above 8". This does not make sense if we think in terms of exit pupil and my TV60 should be able to show the Veil nebula with a 24Pan+OIII (15x, 4.0mm ep) with a contrast sky-object proportional to the one using a dobson 200F6 at 50x, 4.0mm e.p. . (looking forward to trying this as well as many others!  )

Although I haven't had a chance to try my new OIII, considering that it filters more than an UHC, I would suspect that the reason why the OIII is less flexible/universal than the UHC on extended objects is because the UHC works well under dark skies, but can also be beneficial under light polluted skies if decreasing the exit pupil (and therefore increasing magnification), whereas the OIII generally works well with large exit pupil under both dark and light-polluted skies.

(This considering filters such as Lumicon/Astronomik OIII/UHC, and objects suitable for both these two filters). 

[Piero]

Errata: 

"with a contrast sky-object proportional to" => "with an image brightness proportional to"

[jetstream]

I want to report an interesting finding that I have noticed with 2 very good, but vastly different eyepieces concerning contrast and exit pupil. I purchased a 12.5mm Tak ortho with the hope that it would excel on galaxies, even with a larger exit pupil than I consider optimum for myself (2.6mm). I compared the Tak to some other good eyepieces that have excellent and proven contrast on faint galaxies.

The 17mm Ethos offers great contrast on everything I point it at and is a mainstay of my collection. So here is my finding.... at an exit pupil of 3.5mm in my 10" the 17E gave equal or better contrast compared to the excellent Tak 12.5mm at a much lower ( and appropriate) exit pupil of 2.6mm on galaxies. This 1mm difference may not seem like much but it actually is when looking at these objects (Clark etc OMVA). To put it another way, the apparent brightness was the same, when it should have been better in the Tak. The Tak ortho is a high transmission, sharp eyepiece that works very well in general.

To conclude I'll say that individual eyepieces selected can give much better contrast on some objects than others... I have no idea why. My 10mm Ethos is unbeaten so far on the faint galaxies, but I am trying a 50 pound EP coming up that may best it, we'll see.

Gerry

ps another EP gave the same contrast as the 12.5 Tak....... the 18mmBCO

[Merlin66]

Just remember and take into account (maybe the reason for the 8" telescope quote?) that all filters reduce the amount of light getting through to the eye - by design.

They affect the wavelengths transmitted, a UHC type filter will only transmit probably 10%-15%(!) of the total light due to the very select pass bandwidths ( consider the response curve of the eye) and transmission efficiencies can vary from 60 to 90%. Say on average 20%, that means an 8" telescope is only "passing" as much light as a 7" and then the limited transmitted light may be in wavelengths where the eye is less sensitive.

Just my 2c

[Piero]

I want to report an interesting finding that I have noticed with 2 very good, but vastly different eyepieces concerning contrast and exit pupil. I purchased a 12.5mm Tak ortho with the hope that it would excel on galaxies, even with a larger exit pupil than I consider optimum for myself (2.6mm). I compared the Tak to some other good eyepieces that have excellent and proven contrast on faint galaxies.

The 17mm Ethos offers great contrast on everything I point it at and is a mainstay of my collection. So here is my finding.... at an exit pupil of 3.5mm in my 10" the 17E gave equal or better contrast compared to the excellent Tak 12.5mm at a much lower ( and appropriate) exit pupil of 2.6mm on galaxies. This 1mm difference may not seem like much but it actually is when looking at these objects (Clark etc OMVA). To put it another way, the apparent brightness was the same, when it should have been better in the Tak. The Tak ortho is a high transmission, sharp eyepiece that works very well in general.

To conclude I'll say that individual eyepieces selected can give much better contrast on some objects than others... I have no idea why. My 10mm Ethos is unbeaten so far on the faint galaxies, but I am trying a 50 pound EP coming up that may best it, we'll see.

Gerry

ps another EP gave the same contrast as the 12.5 Tak....... the 18mmBCO

Gerry, this is very interesting.

Have you tried the same test using your SW 120ED, despite the changes in ep to 2.3mm and 1.7mm, respectively?

If so, did you find the same result? 

(of course on reasonable galaxy targets for your 120ED.. dunno, possibly M81-M82).

Piero

[Moonshane]

I have some experience but not anywhere near as much as others on SGL.My view is that it is always worth trying a few different options with filters as a small difference in exit pupil, magnification or even apparent field can make a difference.

On the point of apparent field, I have maintained for some time that it seems to me that at the same magnification, a narrow field means that a bright object covers more of the field than when using a wide afov and that this may create a smaller eye pupil and sharpen the view. Conversely I think a wider afov might have the opposite effect and be more suited to faint objects. This may be tripe but seems to work for me

[John]

Another little piece of information that I've picked up is that some planetary observers deliberately reduce their exit pupil by staring at a piece of illuminated white card immediately before putting the eye to the eyepiece. Apparently this helps to pick out subtle planetary features more easily early on in observing.

Quite the opposite of how one would prepare for viewing faint deep sky objects of course.

[jetstream]

Gerry, this is very interesting.

Have you tried the same test using your SW 120ED, despite the changes in ep to 2.3mm and 1.7mm, respectively?

If so, did you find the same result? 

(of course on reasonable galaxy targets for your 120ED.. dunno, possibly M81-M82).

Piero

I have not tried this as a test in the 120mm, but I have tried the different EP's in it and noticed the same thing in general, some eyepieces give more contrast than others, no matter what scope is used (mine). Dark, transparent skies highlight these differences to a greater extent. After a while you just know which EP is better for what and use it- this can really help on some things when trying to seriously observe. Some eyepieces also work better than others with filters too....

All my eyepieces have been chosen to give reasonable specs in my 10" dob as this is my most used telescope, and it works very well. Having a zoom EP of good quality can help define the boundaries for magnification and exit pupil, just zoom away! It will give hints as to whether seeing is the limiting factor, or exit pupil or mag. Of course fixed fl's can do the same if you have a pile of them and a pile of timelol!

One thing not mentioned yet is that exit pupil can be used to pick the telescope that will give the best views on your favorite objects.....

[Piero]

Another little piece of information that I've picked up is that some planetary observers deliberately reduce their exit pupil by staring at a piece of illuminated white card immediately before putting the eye to the eyepiece. Apparently this helps to pick out subtle planetary features more easily early on in observing.

Quite the opposite of how one would prepare for viewing faint deep sky objects of course.

Wow, never heard this! Have you tried it, too?

It seems a quite clever mode of eye adaptation!

[YKSE]

Here is a link of an ongoing discussion about filter on the other site, where the well-regarded optical engineering Glenn Ledrew gave some very interesting comments about filters and apertures.

http://www.cloudynights.com/topic/491178-recommended-filters-for-venus-in-small-scopes/#entry6449513

[John]

Wow, never heard this! Have you tried it, too?

It seems a quite clever mode of eye adaptation!

I've not tried the white card method but I have recently tried swiching back and forth between observing the Moon and Jupiter when the two were quite close together in the sky and it certainly did not seem to harm the views of Jupiter. If I have been viewing a relatively faint deep sky object and move to viewing Jupiter my eye seems to need more time to adapt, which is probably not surprising at all.

A few months back the Moon and Uranus were very close in the sky and I can recall observing the bright lunar limb and Uranus in the same field of view at 199x with my 12" dobsonian. I remember being surprised how strongly blue coloured the disk of Uranus seemed compared to viewing it isolated against the blackness the sky when it's tint is more subtle. This was one of the most memorable views I had over the past 12 months

[Piero]

A few months back the Moon and Uranus were very close in the sky and I can recall observing the bright lunar limb and Uranus in the same field of view at 199x with my 12" dobsonian. I remember being surprised how strongly blue coloured the disk of Uranus seemed compared to viewing it isolated against the blackness the sky when it's tint is more subtle. This was one of the most memorable views I had over the past 12 months

Your clear description let me imagine what you saw very well.. it is always a pleasure to read these experiences from other members here in SGL. Thanks John for sharing it. 

After a few nights of bad weather, it seems that tonight is going to be clear! 

Com'on little TV60! Let's go to catch some big sister out there!    

[Piero]

Another little piece of information that I've picked up is that some planetary observers deliberately reduce their exit pupil by staring at a piece of illuminated white card immediately before putting the eye to the eyepiece. Apparently this helps to pick out subtle planetary features more easily early on in observing.

Quite the opposite of how one would prepare for viewing faint deep sky objects of course.

Here is a link of an ongoing discussion about filter on the other site, where the well-regarded optical engineering Glenn Ledrew gave some very interesting comments about filters and apertures.

http://www.cloudynights.com/topic/491178-recommended-filters-for-venus-in-small-scopes/#entry6449513

Am I wrong or there is a connection between John's information and the first post by Glenn in which he reported that "The best 'filter' (for Venus) is daylight or bright twilight"? 

I wonder whether this is due to the fact that being planets surface brightness quite high, our eyes are able to capture a better contrast and detail when the difference between sky and object is not too much.. do you this this is makes sense? 

What I am trying to say is whether our eyes reach an optimum when the difference between object surface brightness and sky brightness is neither too much or too little. Any ideas? 

[John]

Venus is definiately best viewed when there is still a fair amount of light in the sky in my opinion. With Mercury you don't usually get much choice but to observe under such conditions !

The eye is pretty amazing though and will adapt to make the best of the prevailing light conditions.

[jetstream]

The best filter is daylight for Jupiter too IMHO, my very best views have been at dawn or dusk, with a high light haze.... I also view in daylight, using my Baader single polarizer which works very well.  I've been known to shrink my pupil with bright light to obs Jupiter at times.....depends on the conditions.

[YKSE]

Piero,

I think it all make perfect sense. It just  decribes both size of a coin.

The key point is that our eyes have about 1 arcmin resolution. details smaller than that will look lika point. So, for bright planets, we need to have smaller exit pupil, to get those details bigger than 1 arcmin or more, sacrificing surface brightness (which is much higher extended DSO), the same is true with bright M42 or the stellar like nebulae; while for faint extended DSO, many details are already bigger than 1 arcmin, what we need to have bigger exit pupil to maintain the brightness. M33 is a good example here, being missed in many bigger scopes simply because astronomers were using too small exit pupils to dim it too much.