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Viewing galaxies, eyepieces and exit pupils.


bomberbaz

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Hello all.

I am sure this question has been put up before but it's always good to get up to date perspectives and also with galaxy season just around the corner, it's quite topical. 

I read people saying they use a big eyepiece as the finder which is fairly standard practice before cranking it up if required and also that rule of thumb is that exit pupils of between 4 & 2mm is optimum for galaxies. 

Last time out I used the Nikon 17mm all night which, with a near 1.5 degree fov is perfectly acceptable to use as the finder eyepiece and the exit pupil is 3.6mm. Given the above it fits well into this rule of thumb formulae.

In simple terms, how far do any of you manage to get in terms of exit pupil/magnification and does it fit within the parameters above.

I realise there are always differing views and it is this I am trying to gauge.

Scope is a 10"/F4.7

TIA

Steve

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To be honest I rarely if ever think of exit pupil. In terms of magnification I generally expect or hope for a magnification equal to the scope mm diameter, which by definition means an exit pupil of 1mm and an eyepiece equal to the f number. After that I just take higher magnification (= smaller exit pupil) as a bonus.

Let's ask it this way, your scope is f/4.7, if you dropped in a 2.5mm TMP planetary and obtained clear sharp views of M1, at an exit pupil of about 0.6mm would you take it out and opt for lower magnification to keep within the ""rule" of 2-4mm ? Or would you carry on with the clear sharp view you had managed.

Will also say that as I have read around assorted posts about exit pupil I have seen 0.5mm, 1mm, 2mm and 4mm given as the best or optimum. They cannot all be right.

 

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Hi Steve,

I find the 21mm Ethos an excellent finder eyepiece for galaxies and often that stays in the focuser for viewing as well. With my 12" F/5.3 dob the 21mm gives an exit pupil of a little under 4mm. Before I had the big Ethos I used a 20mm T5 Nagler in the same role so something around 20mm obviously does a good job for me under my skies.

When I wan't to tease out a small and faint galaxy I find the 8mm Ethos works very well (exit pupil 1.5mm). I often find I shuttle between these two on galaxy nights. The poor 13mm Ethos must feel unloved at times !

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8 minutes ago, ronin said:

To be honest I rarely if ever think of exit pupil. In terms of magnification I generally expect or hope for a magnification equal to the scope mm diameter, which by definition means an exit pupil of 1mm and an eyepiece equal to the f number. After that I just take higher magnification (= smaller exit pupil) as a bonus.

Let's ask it this way, your scope is f/4.7, if you dropped in a 2.5mm TMP planetary and obtained clear sharp views of M1, at an exit pupil of about 0.6mm would you take it out and opt for lower magnification to keep within the ""rule" of 2-4mm ? Or would you carry on with the clear sharp view you had managed.

Will also say that as I have read around assorted posts about exit pupil I have seen 0.5mm, 1mm, 2mm and 4mm given as the best or optimum. They cannot all be right.

 

I was referring to exit pupils for galaxies specifically ronin, obvs you can go a lot smaller on planets and stars due to thier brightness but I see where you are coming from.

 

6 minutes ago, John said:

Hi Steve,

I find the 21mm Ethos an excellent finder eyepiece for galaxies and often that stays in the focuser for viewing as well. With my 12" F/5.3 dob the 21mm gives an exit pupil of a little under 4mm. Before I had the big Ethos I used a 20mm T5 Nagler in the same role so something around 20mm obviously does a good job for me under my skies.

When I wan't to tease out a small and faint galaxy I find the 8mm Ethos works very well (exit pupil 1.5mm). I often find I shuttle between these two on galaxy nights. The poor 13mm Ethos must feel unloved at times !

I can see me using the 17mm and powermate here John going off what you said and the 12.5mm being left in the case, although the 12.5mm Nikon does get more use on nebula :laugh2:

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Steve,

These are my simple rules of thumb for galaxy hunting:

1. Starting with 31mm in C8, that's about 3mm exit pupil. Partly it's close to max TFOV, also that Acey has mentioned a couple of times, IIRC, that Willam Heschel has done mostly of his NGC objects with EPs of 3mm exit pupil.

2. I would have checked the sizes of galaxies concerned, then keep magnification to get the galaxy to around 2° to 3°, e.g. a galaxy of 2' size, my 31mm gives about 70x in C8, so it's 140' in retina, that's about  2°20', the 31mm should be good enough to see it. 

3. If a galaxy is of clearly larger size than 2', and I still cant see it in 3mm exit pupil, then I'll dropped to 4mm or even 5mm exit pupil, conversely, if the galaxies is about 1' size or smaller, then I'll go up in mag to get the 2° to 3° size, also in the cases when the galaxy is bright 3mm and I'd like to see more detail of it.

4. If the sky transparency is not as good, then I may need more mag (even though the galaxy gets fainter) to detect galaxy in averted vision(It wouldn't help though if the sky is too bright).

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16 hours ago, Paul73 said:

3 to 3.5 gives the best contrast for me. On galaxies.

For some reason PN's seem happy down to less than two. 

Paul

I read on cloudynights I think that PN's can take a lot of mag especially with a filter on. I have had the ring, which is very bright in fairness at x180 and it took it very well although thats the only one. Typically I go to x90-120 with the Nikon 12.5 also with the EIC.

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3 hours ago, bomberbaz said:

I read on cloudynights I think that PN's can take a lot of mag especially with a filter on. I have had the ring, which is very bright in fairness at x180 and it took it very well although thats the only one. Typically I go to x90-120 with the Nikon 12.5 also with the EIC.

I've used 300x plus on the Ring when trying to spot it's central star. The Blue Snowball is another than seems to do well under high power.

Some PN's are really tiny - just a few arc seconds in diameter. You need high power just to separate them from stars.

 

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Steve, Johns VX12L 1600mm fl will show galaxies bigger than yours or mine @ 1200mm fl using the 21E increasing the visibility. Your 17mm will be a vg finder as you know, but the 10mm-12.5mm will increase many faint galaxies apparent object size enough to increase their perceived "brightness" in the scope. The increase in visibility using this exit pupil range (about 2mm) can give dramatic results in a 10" f4.7-f4.8 scope. I use more mag than this to seek out galaxy cores, but not much more.

For me PN's mean "up the mag", not only in finding them, but in viewing them. Obviously some are much easier to find than others.

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2 hours ago, John said:

I've used 300x plus on the Ring when trying to spot it's central star. The Blue Snowball is another than seems to do well under high power.

Some PN's are really tiny - just a few arc seconds in diameter. You need high power just to separate them from stars.

 

Did you ever see it yet John?

 

2 hours ago, jetstream said:

Steve, Johns VX12L 1600mm fl will show galaxies bigger than yours or mine @ 1200mm fl using the 21E increasing the visibility. Your 17mm will be a vg finder as you know, but the 10mm-12.5mm will increase many faint galaxies apparent object size enough to increase their perceived "brightness" in the scope. The increase in visibility using this exit pupil range (about 2mm) can give dramatic results in a 10" f4.7-f4.8 scope. I use more mag than this to seek out galaxy cores, but not much more.

For me PN's mean "up the mag", not only in finding them, but in viewing them. Obviously some are much easier to find than others.

Think this might be something I have not paid enough attention too despite my earlier comments somewhat to the contrary. Thinkking back, there have been times when I have thought I could not see things when i felt sure i should be able too. Given what you and John say, maybe I should look a little harder and longer with even more magnification at some of the DSO's. 

Looking forward to next session when I will try out a few different things. Again thanks guys.

Steve

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I tend to use an exit pupil around 2 for many galaxies (Naglers 22T4 and 17T4 for 2.2 and 1.7mm exit pupil), except the largest, most diffuse ones, where the LVW42mm and Nagler 31T4 come into play (4.2 and 3.1mm). For planetaries I sometimes go down to 1mm. For separating near stellar planetaries from stars, nothing beats a filter-switch system, like my Denkmeier Filter-Switch Diagonal, to quickly swap between O-III and no filter. With O-III in the light path, the planetary brightens with respect to the stars. Very easy to spot

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14 hours ago, michael.h.f.wilkinson said:

I tend to use an exit pupil around 2 for many galaxies (Naglers 22T4 and 17T4 for 2.2 and 1.7mm exit pupil), except the largest, most diffuse ones, where the LVW42mm and Nagler 31T4 come into play (4.2 and 3.1mm). For planetaries I sometimes go down to 1mm. For separating near stellar planetaries from stars, nothing beats a filter-switch system, like my Denkmeier Filter-Switch Diagonal, to quickly swap between O-III and no filter. With O-III in the light path, the planetary brightens with respect to the stars. Very easy to spot

wish they made one of these for dobs

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