scope of choice for the leo triplet?

[jnb]

Looking for the leo triplet last night I tried with three telescopes; a 5" f5 newtonian, 8" f5 newtonian and 4" f10 achromat. All of these were tried with a 25mm plossl. I would have expected the 8" to work well because it has most aperture, I would have expected the faster newtonians to work well because they should give a better surface brightness and I would have expected the shorter focal length to work well because it would have the biggest field of view.

When I tried them I found that the refractor worked best. Is there any reason that would be the case? It would have been the one I would have expected to perform worst!

[DRT]

I don't know the answer to this but it could be because you are getting mire magnification with the refractor.

Are all three scopes of similar quality in terms of optics?

[dweller25]

Hmmm

Well the magnification used with the 200mm F/5 reflector and the 100mm F/10 refractor was the same so the image should have been brighter in the reflector.....

But I do agree that refractors are best       

[Gazabone]

I read some time ago (and I can't remember where - may well have been somewhere on SGL) that refractors give more contrast than reflectors. Given that the triplet is fairly faint (mag 9.4 to 10.3) I guess the better contrast makes them stand out that bit better.

[DRT]

Hmmm

Well the magnification used with the 200mm F/5 reflector and the 100mm F/10 refractor was the same so the image should have been brighter in the reflector.....

But I do agree that refractors are best     

Oops - it's obviously too early in the morning for me to be doing mental arithmetic

[John]

I'm a big refractor fan (I've currently got 4 of them and only one dobsonian !) but I've always found aperture to work best on deep sky objects especially the fainter ones such as galaxies. I have seen the Leo Triplet with my 4" refractor but the views through 8", 10" and 12" dobs have always made them easier to find and  shown more form and shape to them. I generally use a lowish power (not my lowest though) wide angle eyepiece showing about 1.3 degrees of sky for most of my galaxy chasing.

My skies are moderately light polluted.

Did you find the Leo Triplet in the end ?

[acey]

Looking for the leo triplet last night I tried with three telescopes; a 5" f5 newtonian, 8" f5 newtonian and 4" f10 achromat. All of these were tried with a 25mm plossl. I would have expected the 8" to work well because it has most aperture, I would have expected the faster newtonians to work well because they should give a better surface brightness and I would have expected the shorter focal length to work well because it would have the biggest field of view.

When I tried them I found that the refractor worked best. Is there any reason that would be the case? It would have been the one I would have expected to perform worst!

Any scope will show the Leo triplet if the sky is dark enough. The surface brightness of the image depends solely on the exit pupil (aperture divided by magnification). If the exit pupil is at its maximum useful size (equal to eye pupil) then the surface brightness of the image is theoretically equal to that of the target, assuming no light loss in the telescope. As exit pupil decreases (magnification increases) the surface brightness of the image reduces. So for an image with highest surface brightness you want a scope with minimum light loss at lowest useable power.

This is not necessarily the best view, however, since a very small image will show little detail. Larger aperture means a higher minimum useable magnification, hence a larger image at maximum possible surface brightness. Raising magnification above minimum useable will lower surface brightness but reveal more detail, especially very small or pointlike ones (e.g. galaxy cores).

In scotopic (dark-adapted) vision it appears from laboratory and anecdotal evidence that a pupil size of roughly 3mm is optimal. This would suggest that the "best" view in a telescope is usually one with an exit pupil of about that size. Since exit pupil is the eyepiece focal length divided by the f-number of the telescope, we have in the present case an exit pupil of 5mm for both the f5 Newtonians, and a 2.5mm exit pupil for the f10 refractor. So purely on the basis of the workings of the human eye, one expects the refractor to give best view.

A different test would be to have all the scopes giving the same exit pupil, i.e. the Newtonians have an eyepiece with half the focal length of the one in the refractor. Then one would expect the 8" to win.

[cotterless45]

Well put, I was out in the New Forest with a pair of 15x70 bins, with a black sky , galaxies were clearly on view. Bit different from back home, where contrast or lack of it is a galaxy killer,

Nick.

[ronin]

Refractors generally give more contrast, no obstruction in the middle, and the human eye is more responsive to contrast.

So the refractor matches better the way the eye operates.

[SteveB72]

I also tried last night and could only see the two Messier galaxies of the trio and I have seen them all previously with the same scope and EP (my 400P and both 30mm and 25mm EP). There was some haze or high thin cloud in the UK making all but the brightest galaxies visible from where I was observing (Woking). There was a noticable halo around Venus to the naked eye.

I would try again with beter seeing conditions.

[jnb]

I could get M65 and M66 but the 3rd component was elusive. Although it was a night which looked like it should have been good, being still and clear with no condensation there seemed to be a high layer of haze and the skies were never getting properly dark. That said as a comparison of the scopes it was still valid.

[YKSE]

Great explanation Acey, this is the kind of general understanding of optical system benifits all of us.

[michael.h.f.wilkinson]

Refractors generally give more contrast, no obstruction in the middle, and the human eye is more responsive to contrast.

So the refractor matches better the way the eye operates.

Sorry, not my experience. As Acey puts it: exit pupil must be taken into consideration. At the same exit pupil, my C8 with its LARGE central obstruction has the APM 80mm triplet apochromatic refractor for breakfast on galaxies and most other faint fuzzies. My best views ever of the Leo Triplet was with teh C8 and Nagler 31T5 "Panzerfaust". This closely matches the 3mm exit pupil rule of thumb, but the real reason may simply be that it framed the triplet nicely. The optimal exit pupil depends very much on the actual object, and I do tend to swap EPs a lot during my galaxy hunts.

[DRT]

I viewed this last night for the first time. I could just see the two Messiers but not the third one.

I used a 100ED f/9 refractor with a T5 Nagler 31mm against a Celestron Omni 150 f/5 Newt with a 17mm Ethos. The Newt won but not by much.

[jnb]

I viewed this last night for the first time. I could just see the two Messiers but not the third one.

I used a 100ED f/9 refractor with a T5 Nagler 31mm against a Celestron Omni 150 f/5 Newt with a 17mm Ethos. The Newt won but not by much.

Your eyepieces probably made a big difference. I had a choice of 25mm plossl or 8mm BST. Shorter eyepieces would have helped darken the background and bring out the galaxies but 8mm would have just given be too small a field of view to accommodate all three.

[Piero]

Were your Newton collimated and cooled down properly? 

It can be surprising, but I perceive differences at 15x whether I observed immediately after taking the telescope out from inside or I observe after 15min.. And this using a 60mm, low power. The Airy discs are quite different too. 

[jnb]

I couldn't comment on the collimation of the Newtonians as they were not mine. They were definitely cooled down as we had been setup for some 3 - 4 hours by this point.

[Special K]

Nice point by Acey to get the scopes working at the same level to make a true judgement. It is quite amazing how the sweet spot for viewing is a fraction one way or the other.

I recall being surprised to get a good view of the trio in my ED80 at all, but that was with a 12.5mm EP, which turned out to be a fair compromise. The 1.66 exit pupil is below optimum but the darkening of surrounding sky due to higher mag gave the necessary contrast to make it look 'good.' With the right combination most any scope will give some surprising results!

This is a great time of year for the Trio and I must get my skates on

[swamp thing]

For objects like the Leo triplet to look their best you want aperture........Lots of aperture.

[Piero]

I would be curious to see how the triplet appear with a 20".. but might suffer from aperture fever afterwards! 

well, at least I already have a decent finder to attach! 

[michael.h.f.wilkinson]

I have some idea, as I observed M51 and M101 from Olly's dark site with his 20" Dob. My 31T5 gave about the same FOV as in my C8, but the result is gobsmackingly good (never mind optimal exit pupil, a dark site means you can be a lot less fussy). I would love to hunt through Leo and Virgo (or any constellation, come to think of it ) with that scope

[Special K]

Nice point by Acey to get the scopes working at the same level to make a true judgement. It is quite amazing how the sweet spot for viewing is a fraction one way or the other.

I recall being surprised to get a good view of the trio in my ED80 at all, but that was with a 12.5mm EP, which turned out to be a fair compromise. The 1.66 exit pupil is below optimum but the darkening of surrounding sky due to higher mag gave the necessary contrast to make it look 'good.' With the right combination most any scope will give some surprising results!

This is a great time of year for the Trio and I must get my skates on

Well I got my skates on last night and now have to correct my findings! In the 80mm frac the best view of these galaxies was with the 17mm Hyperion and that makes more sense. Bigger pupil exit, etc. It was still a hard time to spot the second of the triplet in my LP skies but was rewarding! So M65 and M66 revisited, while the third was missed altogether. I later rewarded my straining eyes with M51, the two central cores of which, were clearly visible. This ridge of high pressure has been great! Clear skies. K