TV plossls

[Russko13]

Hi I've decided on a few tv plossls for my skywatcher st120 f/5 the diaganol I'm soon purchasing is a 90degree dielectric star 2 inch with a 1.25 adapter, can anyone recommend one or two 2 inch eyepieces which will work well

[Russko13]

*which will work well for deep sky objects on my 120

[John]

Hi,

Apart from the 55mm which would not really be any good for your scope, all the Tele Vue Plossls are 1.25" format. Unfortunately the Tele Vue eyepiece ranges that have 2" ones in them are quite a lot more expensive - is that OK or would you prefer to find lower budget 2" eyepieces ?

[Laurie61]

HI, the TV plossl  range have one 2" eyepiece and its the 55 mm. With an f5 scope the resulting exit pupil would be 11 mm and too big to use, without wasting light, when the typical eye will dilate to around 7 mm ish. The 32 mm tv plossl gives a 6.5 ish exit pupil which would be fine with your scope.  

[Russko13]

Well I've got a 1.25 adapter that will come with the diaganol I'm purchasing so they shouldn't be a problem to use I was just thinking of adding a few 2 inch eyepieces for wider field views as I can't imagine the plossls performing well enough I big objects like m31 or the Orion Nebula, have looked at a couple of the wider televue lines such as the naglers and the radians but being so expensive if I go down that root I'm only

Gonna be able to get myself 1 or 2 eyepieces at most. (I Spose I can barlow it to get 2 focal lengths mind )

[Laurie61]

Your scope is designed for deep sky / wide field viewing with its short focal length. A 32 mm / 52 dg eyepiece generates 19 x and a field of view of 2.8 degrees in your scope. This will fit 99% of targets into the field of view  If you wanted a bigger F.O.V, without breaking the bank, these seem to be well thought of  http://stargazerslounge.com/topic/186249-maxvision-24mm-68deg/ 

Field of view calculator http://www.nightskies.net/scopetest/scopecalc/index.html 

[bambuko]

...TV plossl  range have one 2" eyepiece and its the 55 mm. With an f5 scope the resulting exit pupil would be 11 mm and too big to use, without wasting light, when the typical eye will dilate to around 7 mm ish...

this is commonly repeated advice on interweb and yet ... Televue themselves advise:

Myth #1: A 7-mm exit pupil gives the lowest useful magnification.

Not so! With a refractor there is no limit on the size of the useful exit pupil. Use whatever is necessary to get the field you need to frame the subject. A reflector's low-power limit is reached when the black spot in the exit pupil (caused by the secondary obstruction) becomes obtrusive.

While a 7-mm exit pupil, by matching that of the eye, does give the brightest views of deep-sky objects, it does not necessarily give the best ones. Higher magnifications, despite their smaller exit pupils, will reveal more details, maintain contrast, show fainter stars, and help bypass defects in the eye itself.

Myth #2: Exit pupils larger than 7 mm waste light and resolution.

With refractors larger pupils do waste aperture. But the magnification is so low that the wasted aperture is of little concern: both image brightness and resolution are as great as possible at that magnification. With reflectors, however, larger pupils do waste light, but primarily because the black spot in the pupil caused by the secondary obstruction becomes larger. Both light loss and field shadowing occur with reflectors, but as with refractors there is no resolution loss because of the low power....

so I'd say why not 55mm?

Listen to advice and make your own mind...

[John]

this is commonly repeated advice on interweb and yet ... Televue themselves advise:

so I'd say why not 55mm?

Listen to advice and make your own mind...

Or even better, try it for yourself. When you do compare you may see why an 11mm exit pupil is not for everyone or every occasion, even with a refractor, despite what Al Nagler says !   

(I have tried it !)

[Russko13]

55mm on my 600mm focal length wil only give me 11x mag mind and being a bino observer at the moment I already know this will not be substantial enough I'm looking at Around a 24-32mm eyepiece really to give at least 20x mag

[Moonshane]

I don't agree with those statements either. maybe they apply more from pristine sites but not where you have to contend with light pollution.

[cantab]

The interesting comparison, IMHO, would be between a scope-EP combination with an oversize exit pupil, and the same setup but with an aperture stop to reduce the exit pupil to normal size. On paper they should be equivalent, but I wonder if the aperture stop wouldn't help reduce aberrations.

Either way, what you're basically doing is using a big scope as though it were a smaller one. Which is fair enough I guess, saves owning and dragging out a smaller scope as well.

[Moonshane]

I just use an aperture mask on e.g. my 16" dob. makes it a 6.7" f11, negating the exit pupil problem but we're heading off into the desert of off topicness.

[John]

I don't agree with those statements either. maybe they apply more from pristine sites but not where you have to contend with light pollution.

Exactly 

A 55mm plossl in my F/6.5 Vixen 102 showed a lovely orange sky. The Nagler 31 shows 95% as much sky but it's much, much darker.

[BillP]

Or even better, try it for yourself. When you do compare you may see why an 11mm exit pupil is not for everyone or every occasion, even with a refractor, despite what Al Nagler says !   

(I have tried it !)

Good advice.  Best to always try for yourself.  But back to the questions of "Why not an 11mm exit pupil?"  With a refractor, because it has no central obstruction to come into view at very low powers, any exit pupil at all will work just fine.  But by working just fine all it means is that nothing aesthetically inappropriate will happen, like the a central obstruction shadow becoming visible, etc.  If you have eyepieces with the same AFOV, then what happens as the exit pupils get larger and larger is that you begin to trade image brightness for less magnification + larger TFOV once the exit pupil gets larger than how large your eye's pupil is dilated.  So nothing wrong with it, just a decision if this is what you want to do.

When making this decision however, best to get all the facts.  In the OPs case with a 120mm f/5 scope, a 55mm Plossl gets them 11x magnification, an 11mm exit pupil, and 4.4 degrees TFOV.  In the process though, if their eye can dilate to 7mm then the light loss from the 11mm exit pupil being truncated by the 7mm pupil dilation of their eye, makes their 120mm scope appear to have the light gathering of a 66mm scope instead.  So they have traded A LOT of light gathering to get that low magnification and wide TFOV.  The thing we can do now is to decide if we can retain our TFOV and reduce our exit pupil some so we are lot losing all that light gathering from out main objective.  This is where eyepieces with larger AFOVs come in as we can increase magnification and retain TFOV, which means the exit pupil will get smaller and we get back some light loss.   When we look at other available eyepieces, there is not any that will get us all the light back while retaining a 4.4 deg TFOV, but we can get close.   For example, if instead of a 55 Plossl a 41mm Panoptic was chosen, then you retain the 4.4 deg TFOV and the exit pupil becomes 8.2mm.  With this combo the 120mm scope appears like a 88mm aperture scope, so A LOT better than what the 55 Plossl does.  A 40mm XW gets it down to 8.0mm while keeping the 4.4 Deg TFOV and lets the 120mm scope operate in light gathering like it was a 91mm aperture scope.  So a little better still.  Finally, if the OP wants to retain all the light gathering of their 120mm f/5 then they will have to sacrifice some TFOV to do that compared to what the 55mm Plossl or 41 Pan or 40 XW can give.  If they use something like a 35mm Panoptic or 34mm ES 68, then they have a 7mm exit pupil retaining all the scopes 120mm aperture's light gathering, but the TFOV will be reduced to about 3.7 degrees.  So they lose some TFOV but instead get to keep all their scope's light gathering.  Can we do better than this?  Yes we can if we look at 82 degree eyepieces.  If we use something like the 31 Nagler or 30mm ES 82, then the exit pupil becomes even smaller at about 6.2mm nicely smaller than what a typical human eye can dilate to, and the TFOV goes up a little over what the Pan and ES68 were giving us to a full 4 degrees TFOV. 

So after the analysis, do we have to give up light gathering and use large exit pupil eyepiece-telescope combinations?  The answer is not always.  We began this journey thinking we had to settle on an 11mm exit pupil to get our low magnification and large 4.4 degree TFOV view which would make our 120mm scope look like it was gathering light like a 66mm scope instead.  But after some analysis of other eyepieces we discovered that we can get back a lot of the light gathering using other eyepieces:

55mm Plossl --> 4.4 deg TFOV and 120mm Scope dims to equivalent of 66mm Scope

40mm XW -----> 4.4 deg TFOV and 120mm Scope dims to equivalent of 91mm Scope

31mm Nagler--> 4.0 deg TFOV and 120mm Scope keeps all its light gathering capability

35mm Pan -----> 3.7 deg TFOV and 120mm Scope keeps all its light gathering capability

So yes, nothing really wrong with using larger exit pupils in refractors.  But that is not the whole story and there is a lot wrong if we just follow advice instead of doing our homework.  As we have seen, with a little analysis we can do a lot better with our 120mm f/5 scope than using a 55 Plossl by looking to eyepieces with larger AFOVs.

[ronin]

OK. So who is going to Astrofest and is going to tell Al he's got it wrong? :biggrin: :biggrin:

[Tiny Small]

These are supposed to be the dogs danglies for the price: http://www.firstlightoptics.com/skywatcher-eyepieces/sky-watcher-swa-70-eyepieces.html

[John]

These are supposed to be the dogs danglies for the price: http://www.firstlightoptics.com/skywatcher-eyepieces/sky-watcher-swa-70-eyepieces.html

I've not seen many reviews / report of them yet. First Light Optics are going to send me 3 to try this week and I'll report back once I've had a chance to see what they are like.

[BillP]

OK. So who is going to Astrofest and is going to tell Al he's got it wrong? :biggrin: :biggrin:

Let's look at some parts of supposed Myth #2:

Part 1 -- "With refractors larger pupils do waste aperture."  <-- CORRECT

Part 2 -- "But the magnification is so low that the wasted aperture is of little concern:"  <-- SUBJECTIVE & AN INDIVDUAL BIAS

Part 3 -- "both image brightness and resolution are as great as possible at that magnification."  <-- PARTIALLY CORRECT, NEED TO ADD AT END: " for a given telescope."

Part 3 is the kicker and leaves out the details where the Devil lives

[AlexB67]

Very nice write up Bill. I often thought that statement was well ... ahum, splutter cough when this topic came up I felt like saying so, refractor or not, but feared that would cause some conflict speaking against the great Mr Nagler   . Good to know to get that info confirmed. 

There again would the great man himself have written those pages and really believe that ?, I doubt if he was cornered and asked. The best way to find out is to meet him at a star party, have a peep in his case alongside his scopes and see what he uses, doubt he would use that combo with an 11mm exit pupil :0)

[BillP]

I would not say any of this is really contrary to TV's web related to exit pupils.  It just goes into more details and provides a more complete picture to things.  Sometimes, when one tried to provide the simplest answer, things get lost and the whole truth of the matter never comes to light.  7mm is also not the cut-off point either, just a convenient number.  So this is another detail left out. As you age your eye can't dilate even to 7mm.  So just depending on your individual nature you might only be able to go to 6mm.  And for younger folks some ave been measured in excess of 8mm.  So next time at the eye doctor, get your max pupil dilation measured so you have all the facts pertiment to your use of equipment (if this max exit pupil thing is important for you).  My rule of thumb is that if I'm doing wide TFOV observing of DSO, then I like to keep it at 5mm or less so it shows ok.  If I'm just looking a big swatches of star fields, then I go to 8.5mm and more...even in my Newts as I'm looking for TFOV and star points perceptually do not dim really since they are point soruces.  Funny, but to answer the max exit pupil question needs a long essay really!