32mm plossl for DSO's?

[Jimtheslim]

So then,

As some of you might be bored of reading about already, I recently acquired a 10" dobsonian reflector (Skywatcher). I bought this with the intention of looking for some DSO's having had limited luck with my 90mm refractor. 

I already own a 32mm Meade plossl which I bought for looking at the moon and searching for DSO's as well as Barlowing it into the mid-range magnification gap that I have. I haven't been able to use the new scope on any really good dark nights yet, however when they do come along I was wondering if the 32mm plossl will start showing me some DSO's, or is it worth investing in a good 2" eyepiece in something like a 40mm? I assume a  40mm 2" eyepiece with a wider FOV will give me a better chance of seeing more galaxies etc? I have moderate LP in my garden, the East horizon being a bit washed out and also to the West until well after dark, overhead the skies are dark enough to see the Milky Way. I also intend to travel to dark sky spots. 

Any suggestions folks?

[Stu]

I wouldn't suggest going for a 40mm, the exit pupil will be very large and the sky background will be washed out.

Actually most galaxies are not that big, so you don't need widefield eyepieces to see them, you need the correct exit pupil to get the best contrast.  EDIT This should read percieved contrast which is largely due to the increased image scale making the object easier to see.

Around 2mm is normally optimum which, assuming your scope is f4.8, would be something like a 10mm. By all means use the 32mm for finding, but observing is likely to be better with the 10.

[nightfisher]

The answer is pretty simple, give it a try, should be pretty good

[rockystar]

I also think the exit pupil will be too large - probably pushing 8mm, which means this is more light than your eye can take in.

your 32 is probably better as a finder, unless you want more FOV.

[Starwiz]

13 minutes ago, Stu said:

I wouldn't suggest going for a 40mm, the exit pupil will be very large and the sky background will be washed out.

Actually most galaxies are not that big, so you don't need widefield eyepieces to see them, you need the correct exit pupil to get the best contrast. Around 2mm is normally optimum which, assuming your scope is f4.8, would be something like a 10mm. By all means use the 32mm for finding, but observing is likely to be better with the 10.

 

7 minutes ago, rockystar said:

I also think the exit pupil will be too large - probably pushing 8mm, which means this is more light than your eye can take in.

your 32 is probably better as a finder, unless you want more FOV.

Agreed.  If the exit pupil is larger than the diameter the eye's pupil can expand to, then some of the light from the distant object will not go into the eye.  It's also worth noting that as we get older, the ability of the eye's pupil to expand deteriorates.  

John

[Jimtheslim]

I see (I think). I had no idea that you would get a good view with a 10mm. I always thought the lower the magnification, the sharper the image and the more contrast you get in the colours and the better chance you have of getting the whole of the 'faint fuzzy' in the eyepiece. 

I really need to do some proper reading on exit pupil, I have never understood it. I can never understand why you need your eye so close for some eyepieces and not for others, why you can't just build an eye cup/guard on to it. 

I'm not a spectacle user, in fact I recently went to the opticians for an eye test as I constantly get migraines behind my left eye. I was sort of hoping it might have something to do with my eyes so I could get to the bottom of the problem, but apparently I still have top quality vision, so the whole glasses thing doesn't apply. 

I knew the answer would probably be to use what I already have, but I can't resist looking to buy new stuff! It begs the question, why spend a lot of money on a 40 or 36mm if you would only really use it for finding and not observing? And yes my scope is the f/4.7.

[John]

Eyepieces with a focal length longer than around 30mm create an over large exit pupil when used with an F/4.7 scope. With a slower scope, eg: F/8, F/10 etc, the exit pupil is smaller and more effective so 40mm and even longer focal length eyepieces are much more useful.

To work out the exit pupil in mm you divide the focal length of the eyepiece by the focal ratio of the scope so 40mm divided by 4.7 = 8.5mm which is too large to be effective because the average fully dilated eye pupil is somewhat less than this - not all of the light from the scope gets into the eye.

Another factor is that higher magnifications tend to darken the background sky a bit. This makes faint deep sky objects (eg: most galaxies) a little easier to see.

Most of the time when I'm galaxy hunting with my 12" F/5.3 dobsonian I have a 21mm ultra wide eyepiece in the focuser. I do sometimes use a 13mm or even 8mm to examine galaxies in more detail. Messier 82, for example, looks wonderful with the 8mm eyepiece (199x) extending across most of the field of view and showing dark rifts across it. To see M82 together in the sky with M81 though I go back to the 21mm eyepiece for another splendid perspective within the context of the starfield that these galaxies are located in.

 

[Piero]

Hi Jim, 

In line with John's post, generally users look for targets with the eyepiece showing the largest field of view. This is easier particularly with star hopping. There are targets which benefit from a large field of view. Typically extended nebulae (e.g. the Veil, the North America, the Rosette nebula, etc) and some large open clusters. This is why a few people opt for a 2 inches low power eyepiece: to frame the target completely or almost. Therefore a low power eyepiece can work as a finder together with the real finder and as the largest view one can have. Of course there are limits for this. If your sky is moderately light polluted, it is sensible not to use an eyepiece giving an exit pupil larger than 5mm. If you live under a very dark sky, you could also consider a larger exit pupil (e.g. 6mm-6.5mm). Under a light polluted sky, an exit pupil as large as 6 can show you the sky as bright grey and in a Newton telescope the shadow of the secondary mirror might appear too. That's why it is generally suggested not to go much beyond 5mm under average skies.

In your telescope a 10mm corresponds to a middle power. This is an eyepiece which will be used a lot for DSOs and can provide you with a better view because 1) the background sky is darker, 2) the object is magnified, so the pattern recognition in your eye-brain works better. Please note that the contrast is the same. When the exit pupil is decreased both the target and the background brightness are decreased. However, firstly the object is increased (and our eye is more capable to recognise a larger object than a small one), and secondly for our eye it is easier to recognise some grey (the target) contrasted with some black (the background), rather than a light grey (the target) with some darker grey (the background). Of course there are limits here too. If too much magnification is used (lower exit pupil), too much target detail is lost and the target might also not be visible. All this explanation works for extended sources (e.g. galaxies, globular clusters, nebulae etc), not for point sources (e.g. stars and planets). With time, you might find that a higher power than a 10mm can be very useful for this targets too.

Don't confuse the exit pupil with the eye relief. The exit pupil is a measure of brightness that an eyepiece can show. It is inversely proportional to the magnification. In the opinion of many members and mine, it is very useful because it allows to compare views between different telescopes. For instance, an exit pupil of 2mm on a 60mm will show an image as bright as the equivalent exit pupil on a 250mm. The magnification will be different and higher on the 250mm though! 

The eye relief is the distance you eye needs to be placed to observe at the eyepiece. This is due to optical schemes. If you don't wear spectacles, you can opt for shorter eye relief eyepieces. Otherwise, it is generally advised not less than 15mm eye relief. Nowadays, there are nice optical schemes with 20mm eye relief which is more than adequate for spectacle wearers.

About exit pupil and eyepieces, I wrote this guide some time ago: 

 

[Jimtheslim]

Thanks for the replies above, they are very useful! I was definitely getting confused between exit pupil and eye relief, I have no idea why. Knowing I should be working out exit pupil gives me something else to calculate which should be interesting as well as cutting down the number of eyepieces I will be looking at in the future. I have a 12.4 plossl which I really like for planets so perhaps this could also be good for DSO's.

Having done some reading, it looks like about 7 to 8mm is about the size of a dilated pupil, so I guess it makes sense to have an exit pupil under this to make sure you get all the light. If it is bigger than your pupil and you are not getting all the light, where does it actually go? Might sound a silly question, but does it mean that simply not all the light makes it into the 'exit' of the eyepiece?

[Piero]

If the eyepiece exit pupil is larger than the eye pupil, the extra light is discarded, not collected / processed by the eye. 

Yes, your 12.4 can be used for DSO of course. With a good barlow 2x, it becomes a 6.2mm (194x on your dobson) which is a well used magnification here in the UK for planetary observation.   

[John]

With an over large exit pupil a proportion of the bundle of light that exits the eyepiece simply can't squeeze through the pupil of your eye. There is no harm in it and sometimes it's fun to scan the skies with a really low magnification but you are not getting the benefit of the full aperture of your scope as I see it.

 

[Pondus]

Hi Jim

Dont forget; open star clusters are also dso`s and for those, Your 32mm might just be Perfect. Not (only) cause of the fov, but also the scale  of what you see.

There are so many beautiful open clusters to observe, also from moderately LP locations, and to me the 30-60x range is Perfect for those Objects.

 

Rune

[spike95609]

I've got the same scope and use a 32mm, almost exclusively as a finder but also for very wide DSOs like the Veil and Beehive Cluster. I do find it very useful as a finder although at 38x magnification the background sky is rather bright, personally I wouldn't want to go any lower. I've been searching for a lot of NGC galaxies in the last few months, only the very bright ones show up well in it, although I can usually detect a hint of something in the view for all but the faintest which are on the edge of my scope's ability under my skies anyway, so it's still very useful as a finder. A 12mm will be great for DSOs. My 32mm and 10mm are easily my most used EPs.

[YKSE]

Indeed, there're many kinds of DSO. Open clusters and Globula clusters are quite different from nebulas and galaxies. The former are point-sources, hgiher magnifcation to around 2mm exit pupil(diffraction limit), will darken the background, but not these point-source DSO, therefore increase the contrast, right framing in around 2mm exit pupils should give you about the best results.

For extended DSO like nebula and galaxies, no magnification can change contrast, contrast in solely determined by the sky darkness and tranparency, no aperture can change contrast either. Higher magnification will dim these DSO just as much as the sky. The benefit of higher magnification is the bigger image of these faint fuzzies, therefore easier for our eyes to see, depite that they're dimmer.

Try to see the Veil nebula in the comming months, with an OIII filter, compare the view in 32mm to 10mm or 15mm, you'll find that 32mm show your much more.

[Jimtheslim]

Thanks, I had no idea the various DSO's could be so different! I think my next move will be to get a half decent 10mm eyepiece. I have upgraded to plossls in 12.4 and 6.44mm as well as getting a celestron x-cel 8mm which I may or may not keep depending on ow I get on with it. I've heard a lot of good things about the Baader Classic ortho's or perhaps a quality 10mm plossl, but I will see how I get on with what I already have. I can't wait to get going with looking for some of these targets. I also think a Telrad might also be in the post to me soon.

[YKSE]

I'm glad can be of some help. I think understanding the differences between different DSO is of advantage in choosing right tool combinations (scopes + eyepieces), so that we can make best out of it.