Do telescopes have a 'sweet spot' that works best with a certain eyepiece?

[jetstream]

Gerry,

Indeed they are both excellent eyepiece and you are very lucky to have them. Like myself having a few half decent eyepieces we are able to make comparisons though sadly many are not. I am absolutely sure anyone would be more than happy with any view from the type of glass we keep in our cases.

Alan

What makes me smile is when eyepieces like the 18mmBCO,10mmBCO ($90) chug right along with the best of the best, heck these 2 eyepieces show less distortion than some of my TV's and other glass, amazing really. Open discussion on eyepieces and scopes here has helped many, including me, make some knowledgeable purchases and yes we are lucky. Does your 18" dob have a favorite faint galaxy eyepiece yet Alan? Very nice scope you have there

[mikeDnight]

I've never worried about which eyepiece delivers that sweet spot in my collection. I simply do the same as others have already mentioned, by tayloring the magnification to the seeing conditions.

Something else I've noticed and something I've done for years without ever mentioning it, is to place the target being observed just below the centre of the field. For whatever reason, I see detail more easily at this point in the field of view.

Also, when studying linear features, such as the belts of Jupiter, I turn my diagonal so that the belts are vertical. Again for whatever reason, fine detail is more easily discernable when I do this. Even during sweeping, objects are easier to detect if I sweep vertically rather than horizontally. There is likely a scientific reason for these things and others may have a different experience, but it shows that we, the observers, are an integral part of the telescope and not separate from it. It also shows that different observers essentially alter the performance of a telescope.

Mike

[Ags]

I always find that an eyepiece that produces the optimum image for me is one that gives magnification near or equal to the telescope aperture, so for my 100mm ED, the optimum mag would be 100x, just a 'rule of thumb' guide that I use.

Would this rule apply if you had a 16" (400mm) dob?

I find the best magnification depends on the target and the sky conditions. 50-150 for DSOs, 150-220 for planets.

[SpaceWalker]

Would this rule apply if you had a 16" (400mm) dob?

I find the best magnification depends on the target and the sky conditions. 50-150 for DSOs, 150-220 for planets.

This rule of course works only up to ~300x and everything above limited by the atmosphere.

The  magnifications you've researched will work only for your specific scopes or for the  similar apertures, so you can't compare correctly your magnifications, say, with  ones on the 16" Dob which gathering significantly more light so can provide brighter and sharper images at higher magnifications. I suspect that the 16" Dob can effectively work on clusters of small galaxies at 250-300x, but your scopes are limited to 150x. But you can easily do the comparison through exit pupil which serves as a universal measure of amount of light coming into the observers eye at a certain magnification.  So the exit pupils for DSOs on your scopes and  on the 16" Dob can be the similar, but the magnifications will be different.

[Ags]

I understand my scopes are small, and this restricts my magnification particularly for planets; however many of the best open clusters are not going to fit in a x200 view. While a scope might theoretically perform best at 1mm exit pupil, you have to factor in the size of whatever you are looking at.

[SpaceWalker]

Well, I'm just saying about convenience of using of the exit pupil as a universal measure facilitating different scope comparisons, eyepiece selection etc. IMO, you shouldn't take this literally as if each scope would have only one *sweet spot*. Am I understand correctly what you mean?
 
You may know well that there is a concept of choosing eyepieces based on exit pupil. For example, check out this guide by Al Nagler, Choosing an Eyepiece - Step by Step. IMO, it's not correct thinking that your ST80 will work only at 80x, and your SCT - at 103x, but... from my experience... when I used f/13 refractor it worked best under most seeing conditions with an eyepiece of 12.5mm... my 8" Dob indeed works best at ~200x when the weather allows, in this case the size and the seeing interact in their specific way, but it's really not the only sweet spot. For example, for most open clusters  everything inbetween 1mm-2mm exit pupil (or 200x-100x) works fine, but there are many small ones so I prefer whenever possible to observe closer to 200x.

This concept suggests to pick certain eyepieces providing several such *sweet spots* which consider various sizes and brightness of DSOs as well. Magnification just follows exit pupil and helps to control optimal focal length spread and avoid unnecessary redundancy.  An arbitrary example. For example, for your f/5 ST80 these *spots* will be the  0.75mm, 1mm, 1.4mm, 2mm, 2.8mm,  4mm, 6mm (or 5mm and 7mm, depending on your preferences) which correspond to lineup ~3.75mm, 5mm, 7mm, 10mm, 14mm, 20mm, 30mm. As you can see the Pentax XWs or TV Naglers include all of these focal lengths since the f/5 scopes are most popular. So, just seven *sweet spots*, you can have at least one for any size DSO. Of course, you can skip some of them.  It would be hard to make a lineup for a variety of scopes based on magnifications without preliminary experimenting. But we can do it more or less precisely just on the basis of eye pupil.
 
So the 1mm exit pupil usually is a sweet spot for planetary under most observing conditions, for your ST80 it should be 5mm eyepiece that'll gives you 80x; 1.4mm - sweet spot for open clusters (of course, not for all, but for most of them because they fit the FOV), for your ST80 it's 7mm eyepiece giving 57x;  2mm - the best human eye visual acuity for perception of diffuse, low contrast, low brightness DSOs including diffuse nebulae and galaxies, the most numerous categoriy of astronomical objects, for your ST80 it's 10mm eyepiece giving 40x   well this usually can be stretched to 3mm exit pupil or 15mm eyepiece, 27x ; etc... Could you say that all this is wrong?
 
Same things for Celestron 4E, 0.75mm exit pupil > 9.75mm eyepiece > 136x
                                                  1mm                    > 13mm eyepiece     > 102x .....so, the *sweetest spot*,  let say the 13mm Hyperion, 102x, 0.7*AFOV... not sure about planets, but what about open clusters, is 0.7* enough?
                                                  1.4mm                  >  18.2mm                > 83x   ..... let say the 17mm Hyperion, 78x,   0.9*AFOV   is it OK for open clusters?
                                                     2mm                  >  26mm                   >  51x
                                                     3mm                  >   39mm                  >  34x      
 
Well, as I said it's just an arbitrary example for illustration, of course SCT has some specifics and probably you just need 3 eyepieces and can skip 1.4mm exit pupil etc.

[alan potts]

There is some interesting reading in this thread and I guess it shows more than anything how we are all different, at the end of the day if it works for you, so what to theory.

I have many times placed Juptier, Mars Venus and Saturn in the same position as Mike earlier described when he observes but never tried, or thought of, putting the belts verticle. I will give this a go as i never believe in dismissing something until I have given it a try.

Alan 

[Mr Spock]

For planets the best combination of detail and contrast comes at x1 per mm. Below that you lose detail and above that you lose contrast and brightness. For my C9.25 x235 with a 10mm eyepiece is spot on, and so I have found in practice.

[alan potts]

Michael,

With respect that is just not right for everyone, lets say we have a LX 200 16 inch, using your maths that would mean a power of a shade over X400, for me it would mean X304, these levels of power are just not useable on a regular basis.

In my books on the 12 inch contrast goes to the wall at around X 220 Jupiter and a bit higher for Saturn, Mars needs more but rarely looks good in out seeing conditions and I suspect mine are normally better than most. I personally find the ideal power on Jupiter is around the X180 mark on all of my scopes apart from the 18 inch, that will take X230 well for the same target.

Alan.

[John]

My approach is to have a box full of eyepieces (or two ) and to play around with them until I get the most pleasing view of the object I'm looking at.

Very un-scientific and I'm full of admiration for those that carefully work out what will work best.

[Piero]

I just put in an eyepiece until I find one that makes images look good on that night. It then pretty much stays in there all night. Forget the theoretical - practical is of more use!!

John

This is an experimental approach and actually works fine, but can be quite expensive!

When I was teenager I went to an astronomy shop to add an eyepiece to my 'collection' (25Kellner and 10 ploss sirius) to use with my Newton 114 F8. At the time there weren't forums, google(!), and I didn't have a clue of what the exit pupil was. So I asked the seller of this shop (who I already cited in another thread) if hecould help me, and this guy suggested me to buy 

a TV Plossl 20mm (which I still have). At the time in Italy this eyepiece cost something comparable to £250 nowadays (no kidding... my newton 114 cost a value proportional to something like £600).. Anyway, it was a large sum. However, since this was my hobby and I saved for it, I could afford that eyepiece, and I bought it after I was told that that would have been my medium power eyepiece.. To be honest, with my inexpert eyes I wasn't able to discern a noticeable improvement over the 25K apart for a slightly increase in magnification and a slightly decrease in image brightness. Eventually, I became fond of that eyepiece because it was my top piece of equipment, but I did not use as I should. 

What I learnt from this is not the 'sweet spot'.. but I leant that 1) you can try different eyepieces, but this is going to be expensive. Some people like to try and I have high respect for curiosity, but I am not keen to spend a fortune and re-sell just things because they don't fit nicely with what I have. This is a personal opinion/feeling of course; 2) you don't need all the available eyepieces but a small set of lenses having reasonable 'distances'.

The latter is the main reason why my plossl 20 did not fit with the Kellner 25 and the kellner 25 was actually preferable. Theoretically the kellner 25 gave a larger field of view and 3.3mm exit pupil, whereas the plossl 20 gave 2.7mm exit pupil and a little bit more in magnification. Between a brightness of 3.3mm and 2.6mm there wasn't a substantial difference to select the 25Kelner as low power only and the plossl 20 as medium power only (which would have been the case if I had a 32mm and 20mm eyepieces combined with an F8 telescope). Surely, one could spend words and time on the quality of the tele vue glass compared to a normal celestron kellner, but the point here is that they were quite overlapped.. 

In conclusion, of course it is possibly to buy all eyepieces focal lengths and spending the entire night trying to find out the best view for that specific target that specific night under those specific sky conditions. If this is what makes a person happy, I don't see anything wrong with that, as long as money does not become a serious concern.

Certainly this approach works.

However, most of the people just want to see some targets and don't necessarily spend a fortune. In this case, understanding the exit pupil can give some *indication* on how to chose a set of eyepieces. To me, the exit pupil is a much better measure than pure magnification. The best would be to choose after thinking in terms of exit pupil and field of view if possible. There will be some nights in which an user cannot reach the best view for that specific night under those specific sky conditions, but if you have a decent selection of eyepieces without (major) overlapping, you will still enjoy jupiter, andromeda, ring nebula, the moon etc.. 

So eventually, I think also this approach works..

Gerry

The only down side of that is the other night my 15mm TV Plossl was superb on Juptier in the 12 inch, better than the 14mm Delos. The night before it was the other way around. In the same scope also the 14mm D was better than the 17mm E the other night, so never the same as far as I can see, everything else was the same on given nights.

Alan

I also found sometimes the views through my TV plossl 20mm (yup, the one cited above), somehow more transparent than my tv panoptic 24mm.. However, I don't think this is a fair comparison.. I should have compared a my 20mm with a tv plossl 25mm and my panoptic 24mm with a pan 19mm and see.. 

I suspect your case is similar as you compared 15 TV plossl with 14mm Delos, and 14mm Delos with 17mm Ethos. These have (at least) different coatings, number of glass and field of view.. all factors that can contribute to make you perceive objects differently, as your eye behaves differently.