65 replies to this topic

[Starman1]

Here is an interesting question.

 

My most frequently used magnifications fall between 150x and 200x with my 12.5" scope, though an all-night range will be larger.

This is because:

• the exit pupil is between 1.59 and 2.12mm, which is a good point for visual acuity.
• the framing of deepsky objects is good because my eyepieces are all 78°+
• the drift time I get in that range is long, so little scope movement is called for
• the sky in the eyepiece is adequately dark, unlike with lower powers
• seeing almost never interferes with image sharpness at those magnifications.

So, I recently went from 12.5" to 16", a gain of 63.9% in light gathering, or a little more than 0.53 magnitudes.

 

For equal brightness, the same exit pupils in the 16" result in a magnification range of 192-256x.

That also means the same visual acuity in my eye.

 

So here are some questions for those of you who have a fair amount of experience with a variety of scope sizes:

[of course, over time I'll decide this on my own, but I'm interested in hearing others' experiences]

1. Is the larger scope so much more bothered by seeing that I won't actually be able to use the same exit pupil range on the 16"?
2. Ignoring the seeing versus aperture question, will the higher magnifications just always result in poorer seeing?
3. Since the larger scope has a focal length 73mm shorter than the smaller scope, will I just prefer the same eyepieces in the larger scope because the magnification range would be pretty much the same, just with larger exit pupils?
4. More importantly, since magnifications below 150x seem to have fairly bright sky backgrounds in the 12.5", will magnifications below 192x in the 16" seem to have fairly bright backgrounds in the 16"?

My experience with scopes of >16" has been largely at extremely dark sites (21.5mpsas and darker), so I'm interested in hearing the experiences of others who have gone to apertures of 16" and more who also use

those apertures in skies brighter than 21.5mpsas.

 

I admit to pretty much already knowing about the answers to my questions, but I thought it would be an interesting issue to bring up in the Eyepieces Forum because it impacts what eyepieces you would buy to outfit different sizes of scope.

If a moderator thinks this post fits better in the Reflector's Forum, I'm amenable to having it moved, though the issues raised might also affect the choice of eyepieces if moving up in aperture from a 3" refractor to a 6" one.

 

[Jon Isaacs]

Don:

 

I have to think you pretty much know the answers ..  and you also know that I do a lot of observing with 3 and 4vinch refractors and Dob's of 12.5 inches, 16 inches and 22 inches.

 

My 12.5 inch is F/4.06 and the 16 inch and 22 inch are F/4.4, F4.7 and F/5.05 with the Paracorr.

 

For most DSOs, seeing is not big issue it is for the planets and double stars. It's there for sure... 

 

My scopes are relatively close in focal ratio and I tend to use the same eyepieces for the same objects, it's basically exit pupil based. I'll probably use slightly smaller exit pupils in the 12.5 inch.

 

My observing habits have changed since switching from star hopping to the PiFinder.  For much of the night, the 9 mm or 7 mm XWAs serves as the base magnification and I work up and down from there, mostly up in magnification. In the 16 inch, it's mostly the 7 mm, 300x, in the 12.5 inch, also the 7 mm, 210x.. 

 

You're in a different situation, quite different focal ratios . 

 

Jon

[25585]

Enjoy the larger exit pupils and bigger light show the 16" gives you. Seeing more at the same magnifications as for your 12" would be what I would be interested in. When I went from 8" to 12", that was the main factor (10" is closer to 12"), which also helped with narrow bandpass use too. 
 

My favourite DSOs are globular clusters, and the 12" shows them easier and more detailed, so I envy you your 16", apart from its collimation. Red stars are also brighter, and another favourite of mine.

[Redbetter]

1. The typical seeing of where one observes is the biggest factor in how high of a magnification one will typically use with a large/very large scope.  The sort of  medium exit pupil you have favored with the 12.5 is not likely to be problematic with the 16" from what you have described of your seeing.
2. I might not be reading the question correctly, but the seeing itself shouldn't be all that much different.  The finer resolution and greater surface brightness of the 16 at the same scale will tend to make the seeing look worse--particularly at high power when looking at a star.
3. My guess is you will probably favor the same eyepieces, although I suspect you will find you want to increment up more frequently.
4. I would expect the sky background to appear brighter below 192x in the 16" to appear brighter than in the 12.5.  However, my most used star/galaxy hopping eyepiece with the 20" is a 16T5 at ~156x.  It provides good scale and TFOV for many star clusters and clusters of galaxies.  It also does a better job of revealing the extent of galaxy's outer envelopes.  (For some larger and lower surface brightness galaxies such as Barnard's, Sculptor Dwarf, and Fornax Dwarf, I use a 20, 26, or even 31T5 to see as much as possible.)

Most of my galaxy hops are with the 16T5, although sometimes I need wider, and other times I jump right in with the 9T6 at 278x.  The 156x works well for familiarization with the field around the targets--since there are usually several galaxies visible.  From there I typically go to 278x because that is a favored scale and exit pupil (1.8mm) for my eye.  Sometimes seeing will limit me to 227x, and there are a few low surface brightness galaxies (some UGC's) that are better seen with this brighter pupil.

 

If the seeing is mediocre to good I often go to 357x (7T1) to look for details that would otherwise be missed.  This gets me to the typical TLM for the session, so I can make out the threshold field stars, resolve galaxy cores, knots, supernovae, and some hidden companions.  357x is what I favor for looking for the various small PGC and 2MASSX galaxies, but this is seeing dependent.

 

If the seeing is good or even better, then I will used 500x (5T6) to look for the smallest stuff.  I sometimes even pull out the 3-6 TeleVue zoom to go higher when trying to resolve something specific, including Vesta and other asteroids that reach an appreciable size.  I test 500x in many sessions with mediocre seeing as well, trying to go a little deeper, but it is mostly unsuccessful with the seeing reducing the limiting magnitude when I go that high.

 

I don't get seeing here that is steady enough to make anything higher than that favorable other than for specific brief looks/applications.  I would certainly use more if I had the seeing to support it.  I use exit pupils down to ~0.5mm for some DSO's with refractors, resolving globulars, open clusters, higher surface brightness galaxies, etc., so I would certainly go to the 0.6mm range if the seeing made it useful.

 

 

[Starman1]

A 2mm exit pupil in the 12.5" is an 11.5mm eyepiece = 159x (f/5.75 with Paracorr)

In the 16" it is an 8.6mm eyepiece = 203x (f/4.3 with Paracorr)

What I'm figuring is that if I use a magnification in between the two, it will be both larger AND brighter, the real advantage of a larger aperture.

[TOMDEY]

Hi Don,

 

I rather inexplicably always settled on the 13mm Nagler for Deep Sky "little fuzzies", regardless of the scope >>>

 

13.1" F/4.5       used a lot for about two years

17.5" F/4.5       used profusely for about a decade

29" F/4.5          used profusely for over two decades

36" F/3.75        using now more casually

 

And... my ~technique~ was always the same:

 

> 20mm Nagler T2 for finding the object

> 13mm Nagler T1 for most enjoyably savoring the object

> 9mm Nagler T1 when conditions allowed the higher mag to advantage

 

Why I always gravitated to the 13mm Nagler might be simply that it was the revolutionary performer when Al launched it way back when. I didn't even use a Paracorr for many years... the absolute linear field of the 13mm sufficiently constricted to tolerate that well. On the (admittedly somewhat rare) hours and nights when the 9mm provided good resolution --- most memorable and revealing great detail in even the little NGC's.

 

It rather amuses me that Al's eyepiece use tables suggest 350x max usable magnification, regardless of aperture. I seem to have stumbled upon and embraced that long before I read it in his tutorials.      Tom

 

 

[Starman1]

Hi Don,

 

I rather inexplicably always settled on the 13mm Nagler for Deep Sky "little fuzzies", regardless of the scope >>>

 

13.1" F/4.5       used a lot for about two years

17.5" F/4.5       used profusely for about a decade

29" F/4.5          used profusely for over two decades

36" F/3.75        using now more casually

 

And... my ~technique~ was always the same:

 

> 20mm Nagler T2 for finding the object

> 13mm Nagler T1 for most enjoyably savoring the object

> 9mm Nagler T1 when conditions allowed the higher mag to advantage

 

Why I always gravitated to the 13mm Nagler might be simply that it was the revolutionary performer when Al launched it way back when. I didn't even use a Paracorr for many years... the absolute linear field of the 13mm sufficiently constricted to tolerate that well. On the (admittedly somewhat rare) hours and nights when the 9mm provided good resolution --- most memorable and revealing great detail in even the little NGC's.

 

It rather amuses me that Al's eyepiece use tables suggest 350x max usable magnification, regardless of aperture. I seem to have stumbled upon and embraced that long before I read it in his tutorials.      Tom

Al would have loved observing where I do.  There have been nights I used a 300x eyepiece as a finder and spent almost all night going back and forth from 400x to 500x with an occasional stop above 600x.

He would have posted a slightly different comment about seeing, then.

[Procyon]

You don't mind if I take a crack at the eyepiece part for fun?, the rest you will solve on your own for sure. If the Morphs are still good at f/4ish ignore all the Delos.

 

11mm A ---------------159x 2.5mm

10mm E ---------------175x 2.3mm 

9mm T7 -------------- 194x 2.1mm (Or 8.9 Morpheus) (Or 8.8mm XWA)

8mm E ----------------218x 1.9mm (Or 8.1mm Delos) I have a feeling you will spend a lot of time in this area. I can't find one for 1.8mm Exit Pupil though.(7.5mm Sky Rover UltraFlatField @ 60º?  )

7 XWA ---------------- 250x 1.6mm

6.7mm M -------------260x 1.55mm

6mm E ----------------290x 1.4mm

5.5mm T 7 or 5mm XW? Gap fillers.

4.5mm D -------------390x 1mm (Or Morpheus/Ethos)

3.7 E ------------------470x 0.85mm

No Man's land territory/PN's

3mm Delite? --------585x 0.7mm

2.5mm T6 ---------- 700x 0.68mm

or

2.5mm Tak TOE, small field but ultra sharp?

 

Binoviewers?

 

Enjoy the awesome views man. Sounds like a lot of fun.

Edited by Procyon, 13 May 2025 - 09:09 PM.

[TayM57]

 

So here are some questions for those of you who have a fair amount of experience with a variety of scope sizes:

[of course, over time I'll decide this on my own, but I'm interested in hearing others' experiences]

1. Is the larger scope so much more bothered by seeing that I won't actually be able to use the same exit pupil range on the 16"?
2. Ignoring the seeing versus aperture question, will the higher magnifications just always result in poorer seeing?
3. Since the larger scope has a focal length 73mm shorter than the smaller scope, will I just prefer the same eyepieces in the larger scope because the magnification range would be pretty much the same, just with larger exit pupils?
4. More importantly, since magnifications below 150x seem to have fairly bright sky backgrounds in the 12.5", will magnifications below 192x in the 16" seem to have fairly bright backgrounds in the 16"?

Having gone from a 10" to a 24", I can provide some insight although not extensive as I haven't gotten a trailer for the 24" to take it to dark skies yet.

 

1. Yes and no. I noticed that I could "see" the seeing in the 24", and it jumped out to me the first few times I used the 24". After that, it became less of an issue. It is almost as if my brain adjusted and now I don't "see" the seeing. Also, I devised a cooling system for the 24" so it could be that the mirror is better acclimated now with the fan system than it was previously with just the 3 fans in the back.

 

2. Not always. Sometimes I'm surprised by how high I can go in the 24".

 

3. You may find yourself using shorter focal length eyepieces more often because of the higher mags + larger exit pupils.

 

 

I rarely use low power eyepieces in the 24". That is what the 9" f/3 finder is for. It's a joy to go really high power, because I don't get the floaters in the 24" that I do in the 10" when the exit pupil gets small. I also find the large exit pupils makes the view "easy". Details in Jupiter for example, are easier to discern in the 24". Stars in the core of Globs are easy to resolve and readily apparent in the 24".

 

I don't really subscribe to the idea that larger scopes provide "amazing" views relative to smaller scopes. Superlatives are confusing and hard to understand the intent of. It's easier for me to understand the advantage of the 24" (and other larger apertures) in terms of the relationship between magnification and exit pupil, setting aside aperture entirely except when used to calculate the size of the exit pupil.

 

 

In a few years I'll be able to answer your questions with more depth, after I've gotten a few trips with the 24" under the belt. Right now, I've still got to fix the AZ motion, build a mirror box, and get a tall trailer for it.

 

 

 

 

 

 

 

 

 

[Jay_Reynolds_Freeman]

So here are some questions for those of you who have a fair amount of experience with a variety of scope sizes:

[of course, over time I'll decide this on my own, but I'm interested in hearing others' experiences]

1. Is the larger scope so much more bothered by seeing that I won't actually be able to use the same exit pupil range on the 16"?
2. Ignoring the seeing versus aperture question, will the higher magnifications just always result in poorer seeing?
3. Since the larger scope has a focal length 73mm shorter than the smaller scope, will I just prefer the same eyepieces in the larger scope because the magnification range would be pretty much the same, just with larger exit pupils?
4. More importantly, since magnifications below 150x seem to have fairly bright sky backgrounds in the 12.5", will magnifications below 192x in the 16" seem to have fairly bright backgrounds in the 16"?

My experience with scopes of >16" has been largely at extremely dark sites (21.5mpsas and darker), so I'm interested in hearing the experiences of others who have gone to apertures of 16" and more who also use

those apertures in skies brighter than 21.5mpsas.

 

I admit to pretty much already knowing about the answers to my questions, but I thought it would be an interesting issue to bring up in the Eyepieces Forum because it impacts what eyepieces you would buy to outfit different sizes of scope.

If a moderator thinks this post fits better in the Reflector's Forum, I'm amenable to having it moved, though the issues raised might also affect the choice of eyepieces if moving up in aperture from a 3" refractor to a 6" one.

(1) Depends on the nature of the seeing at the particular sites where you observe -- a cynic might say that if the seeing is already terrible at 12-inch, it can't get any terrible-er at 16-inch. Though come to think of it, that might be something an optimist might say ...

 

(2) Magnification does not cause poor seeing, though more magnification will likely make any bad seeing that is present more easily detectable.

 

(3) For deep-sky work, what seems to make the greatest difference for me is how dark the background sky looks. At the same magnification, a 16-inch will show a brighter sky background than a 12-inch; thus for any given object and observing site, I myself would likely end up using somewhat more magnification with a 16-inch than with a 12-inch.

 

(4) I would think, yes.

 

For what it is worth, my deep-sky observing has generally been with slightly smaller telescopes -- lots with a Celestron 14, and a fair amount with 10-inch aperture of several types.

 

 

Clear sky ...

[CrazyPanda]

8" to 12" to 15" in ~21.0 at the darkest and generally poor seeing most of the time.

 

1. No, the difference from 12.5" to 16" will not be that much more bothered by the seeing. At no point after the switch from the 12" and 15" did I feel seeing was any *more* of a limit. Same is generally true of 8" vs 12". Seeing wasn't the limit for deep sky observing - light delivered to the eye and/or magnification was. Aperture difference with respect to seeing is more noticeable for planetary viewing, but there's a very clear progression in visible detail between 8" to 12" to 15" even if you can't reach the magnification potential of the 15" as often as the 8" or 12".
    
2. Not always no, but generally yes, higher magnification is more demanding of the atmosphere. You'll see the effect most noticeably on globular clusters. Less so on less detailed targets like galaxies. The main thing is the extra aperture does render all stars brighter, and thus the speckling pattern of atmospheric turbulence will be more noticeable, especially since it's not only brighter but better resolved. You will see intact Airy disks less often.
    
3. That wasn't true for me in my ~21 skies. I always found myself wanting to take advantage of the magnification potential of the aperture. Low power views (bright exit pupils) look washed out. My club has a site with 21.3 to 21.4 skies and that doesn't change my opinion. I like smaller exit pupils and more magnification and wouldn't simply accept more brightness without increasing the magnification. I might change my tune if I had access to truly dark skies.
    
4. I mean yes technically. Exit pupil is exit pupil. 16" @ 192x is the same as 12.5" @ 150x. The sky brightness will be identical. If you find the brightness below 150x in the 12" objectionable, then you will find it equally objectionable below 192x in the 16".

Edited by CrazyPanda, 14 May 2025 - 09:56 AM.

[lwbehney]

I would like take a crack at question 4 please. I have read that the surface brightness of an extended object cannot be enhanced by a telescope. If this is true, then the brightness of the sky background, which is an extended object also, diminishes in all telescopes with increasing magnification to the same extent. So I would expect the sky background at 192X in your 16" to have a darker sky background than your 12.5" at 150X even with disimilar exit pupils. 

However, the answer may somewhat depend upon the range of magnification used, because if you imagine the background sky as being composed of millions of faint unresolvable stars, then at magnifications lower than the resolution of the Airy Disc, the larger aperture telescope should be gathering more light in aggregate from each of those stars and the background should be brighter. If I presume the atmosphere smears the spurious disc of each star to a diameter of one arc second in common seeing conditions, then it could be that 150X and higher marks the point where the surface brightness of those faint stars truly diminish en masse.  

[CrazyPanda]

I would like take a crack at question 4 please. I have read that the surface brightness of an extended object cannot be enhanced by a telescope. If this is true, then the brightness of the sky background, which is an extended object also, diminishes in all telescopes with increasing magnification to the same extent. So I would expect the sky background at 192X in your 16" to have a darker sky background than your 12.5" at 150X even with disimilar exit pupils.

 

Exit pupil governs brightness of all extended objects. Identical exit pupil = identical brightness.

 

16" (406mm) @ 192x = 2.1mm exit pupil

 

12.5" (317mm) @ 150x = 2.1mm exit pupil

 

These views produce exactly the same brightness per unit area of all extended objects, with the 16" offering 1.28x the magnification of the 12.5".

Edited by CrazyPanda, 14 May 2025 - 05:31 AM.

[ABQJeff]

Hi Don, I know you know all this but it’s fun to play…

Based on my experience going from 6” to 9.25” to 11”…

1. For DSOs no. On planets/doubles/lunar larger scope will always allow and achieve higher max resolution BUT larger scope will also be bothered more by seeing in that you will be able to see a greater difference between good seeing and not. But when seeing clears, you will love having the larger scope. I liken it to traffic. A car is more bothered by stop and go traffic than a bicycle, but when traffic clears (even briefly) the car excels. So if you are always observing in traffic…what are you doing?! You are going to get hit by a car, get out of the road! (haha.)

2. Again, just for planets/lunar/doubles, higher magnification lets you realize the resolution of the scope, but magnification also resolves (detects) any seeing issues more.

3. Depends, on extended/nebulous objects where you want more exit pupil you could stay at same magnification using same eyepieces. But you will likely more often stay at same exit pupil but use the higher magnification to make objects larger and more easily detected (including punchng thru light pollution). What is neat with a larger aperture is you can go between and get higher magnification AND a larger exit pupil vs the smaller scope (as you have discovered).

4. Well you don’t define fairly bright, but I assume you mean: because exit pupil is same will sky brightness be the same? Even though same exit pupil the extra magnification will help you punch thru the light pollution and make objects more noticeable.

For all these reasons, you are discovering why aperture rules.

Edited by ABQJeff, 14 May 2025 - 12:31 AM.

[Redbetter]

I would like take a crack at question 4 please. I have read that the surface brightness of an extended object cannot be enhanced by a telescope. If this is true, then the brightness of the sky background, which is an extended object also, diminishes in all telescopes with increasing magnification to the same extent. So I would expect the sky background at 192X in your 16" to have a darker sky background than your 12.5" at 150X even with disimilar exit pupils. 

However, the answer may somewhat depend upon the range of magnification used, because if you imagine the background sky as being composed of millions of faint unresolvable stars, then at magnifications lower than the resolution of the Airy Disc, the larger aperture telescope should be gathering more light in aggregate from each of those stars and the background should be brighter. If I presume the atmosphere smears the spurious disc of each star to a diameter of one arc second in common seeing conditions, then it could be that 150X and higher marks the point where the surface brightness of those faint stars truly diminish en masse.  

Apparent surface brightness does not change with increasing magnification to the same extent in different apertures.  Instead it is exit pupil dependent.  A larger exit pupil = less dimming.  A larger scope has a larger exit pupil at the same magnification level compared to a smaller one.  The limitation is that once the exit pupil equals or exceeds that of the eye, lower magnification/larger exit pupil will not increase surface brightness any more to the eye.

 

There is inherent surface brightness to the object, and then there is the variable surface brightness of the sky (depending on man-made light pollution, air glow, the Moon, zodiacal light, transparency, etc.)  The object's surface brightness represents an over brightening of the sky--but don't worry about that right now.  Both of these effective surface brightnesses are reduced by exit pupil dimming.  

 

Exit pupil is simply the eyepiece focal length divided by the telescope focal ratio, or expressed another way, telescope aperture divided by the magnification.  The 1x eye exit pupil is assumed to be 7mm.  This is a reference point, don't worry about your eye's actual pupil diameter for the basic dimming calculation.  The assumption is that one will be interested in the dimming with exit pupils smaller than the eye's dark adapted state.

 

The surface brightness is reduced by the reciprocal of the ratio of the square of the exit pupil.  (This is a diameter to area conversion.)  The ratio can then be converted to the magnitude scale.  Here is a table of results:

Exit pupil (mm)    Resultant S.B. (fraction)   S.B. dimming (magnitude scale)

          7                                   1                                       0

          5                                   0.51                                  0.73

          3.5                                0.25                                  1.51

          2.5                                0.128                                2.24

          2                                   0.0816                              2.72

          1.5                                0.0459                              3.35

          1                                   0.0204                              4.23

          0.7                                0.01                                  5

          0.5*                               0.0051                              5.73

          0.4**                             0.0033                              6.22

 

*The central portion of the spurious disk of the faintest/threshold stars is just beginning to have some apparent size by 0.5 mm exit pupil, so extra magnification beyond this does little for stars even if the seeing is still. 

**In pristine dark sky, the overall night sky will be ~22 mpsas (although the back ground might be a magnitude dimmer or so, once all visible stars are subtracted.)  The eye is reaching an absolute limit somewhere at or beyond 28 mpsas.  So in pristine sky, by 0.4mm exit pupil the sky in the eyepiece is as dark as the eye can recognize, and increasing magnification will dim the target, reducing contrast for both extended and stellar objects.   

[Astro-Master]

Here is an interesting question.

 

My most frequently used magnifications fall between 150x and 200x with my 12.5" scope, though an all-night range will be larger.

This is because:

• the exit pupil is between 1.59 and 2.12mm, which is a good point for visual acuity.
• the framing of deepsky objects is good because my eyepieces are all 78°+
• the drift time I get in that range is long, so little scope movement is called for
• the sky in the eyepiece is adequately dark, unlike with lower powers
• seeing almost never interferes with image sharpness at those magnifications.

So, I recently went from 12.5" to 16", a gain of 63.9% in light gathering, or a little more than 0.53 magnitudes.

 

For equal brightness, the same exit pupils in the 16" result in a magnification range of 192-256x.

That also means the same visual acuity in my eye.

 

So here are some questions for those of you who have a fair amount of experience with a variety of scope sizes:

[of course, over time I'll decide this on my own, but I'm interested in hearing others' experiences]

1. Is the larger scope so much more bothered by seeing that I won't actually be able to use the same exit pupil range on the 16"?
2. Ignoring the seeing versus aperture question, will the higher magnifications just always result in poorer seeing?
3. Since the larger scope has a focal length 73mm shorter than the smaller scope, will I just prefer the same eyepieces in the larger scope because the magnification range would be pretty much the same, just with larger exit pupils?
4. More importantly, since magnifications below 150x seem to have fairly bright sky backgrounds in the 12.5", will magnifications below 192x in the 16" seem to have fairly bright backgrounds in the 16"?

My experience with scopes of >16" has been largely at extremely dark sites (21.5mpsas and darker), so I'm interested in hearing the experiences of others who have gone to apertures of 16" and more who also use

those apertures in skies brighter than 21.5mpsas.

 

I admit to pretty much already knowing about the answers to my questions, but I thought it would be an interesting issue to bring up in the Eyepieces Forum because it impacts what eyepieces you would buy to outfit different sizes of scope.

If a moderator thinks this post fits better in the Reflector's Forum, I'm amenable to having it moved, though the issues raised might also affect the choice of eyepieces if moving up in aperture from a 3" refractor to a 6" one.

Question 1. No, the seeing should look about the same unless the seeing is really bad.

                2. No, I found moving from a 10" SCT to an 18" Obsession has allow me to use higher powers than I thought would be possible, powers 350x or more on night with average seeing on DSO's, 800x on Planetary Nebule on nights with good seeing, and on rare nights with perfect seeing powers of 1,300x on Bright Planetary Nebule, will blow you away.

                3.  I think you'll enjoy exit pupils more in the 1.5mm to 1.2mm more often.

                4.  On the 18" I usually just use the 13 Ethos (160x) to find the object, then add the AP Barlow (258X) to check the seeing then crank the power up on the barlow to any power between 258x to 402x to perfectly frame the DSO, or adjust for the seeing, if the seeing is good I can use the TV Big Barlow with the 13 Ethos for any power between 320x and 560x, It's the closest thing I've found to having an Ethos Zoom.

 

Clear Skies with your new scope.  

 

Bruce

Edited by Astro-Master, 14 May 2025 - 01:13 AM.

[BGazing]

I saw this topic pop out last night, but I was too tired to reply, not surprised that it attracted so many replies.

You know the answers way better than I do  (and I asked you about precisely the same issues when moving to my 14), but here we go. I don't have a 16, but my best skies are comparable to yours, and my mediocre skies are around 21.2 at the moment.

I've changed from 12 to 14, f/5.5. with P2 to f/5.1, 1725mm to 1940mm.

I favor the same exit pupils, and may even back off a bit if the sky is darker. Also, some of the objects will become harder to frame properly unless you back off a bit. I noticed that in 14 although the jump was much smaller. I recall you writing that you rarely, if ever, revisit objects when you move up in aperture, perhaps it will be different now? I also recall that you wrote that you pursue threshold objects for given aperture - if that is the case, I see no reason not to stick to the same exit pupils, and benefit from higher magnifications. Looking back at how I used my C8, it was - again - around the same pupils, sometimes smaller as I sought more magnifications.

Although DSO seeing requirements are different from those for the planets, it is unlikely that 16 will put a cap on your magnifications. My guess is that you will continue favoring the same exit pupils, and that your choice of magnification will be more dicated by the overall sky brightness and the desire to frame the object. You will have more leeway than in your 12. 

 

Time for a 7mm T7? 

Edited by BGazing, 14 May 2025 - 05:39 AM.

[Ernest_SPB]

Hm...

What is the discussion?!

It is so natural - yes, more aperture allows you to observe with higher magnification. It is only gain of larger aperture.

Edited by Ernest_SPB, 14 May 2025 - 02:38 AM.

[25585]

Hm...

What is the discussion?!

It is so natural - yes, more aperture allows you to observe with higher magnification. It is only gain of larger aperture.

Not the only gain though.

[CHASLX200]

The bigger the better in my seeing as long as the optics are top notch. A 18" can loaf around at 650x on Jup and still be bright.

[Ernest_SPB]

Not the only gain though.

Can you list other? Size, weight, sensitivity to seeing, price, mobility...

[sixela]

The bigger the better in my seeing as long as the optics are top notch. A 18" can loaf around at 650x on Jup and still be bright.

Tell me you live in Florida without telling me you live in Florida ;-).

[Starman1]

Can you list other? Size, weight, sensitivity to seeing, price, mobility...

At the same exit pupil as the smaller scope, the images will be larger.

At the same magnification as the smaller scope, the images will be brighter.

As I see it, if the magnification is only increased slightly over the smaller scope, the exit pupils will be larger AND the magnification higher, so the DSO can be both brighter AND larger.

 

In my case, the aperture increase is quite modest, so finding that point might not be easily done as long as I use the same eyepieces.

But I can see the larger aperture can yield images that are larger and brighter; you just need to pay attention to the exit pupil and the magnification.

 

The other things you mention are all the negatives to a larger aperture.

 

Do observers who move to larger apertures stick to the same exit pupils they used in the smaller apertures.

My experience in the field says no, they don't.  They tend to use slightly larger exit pupils than they did in the smaller scopes because of being seeing limited.

They do use slightly higher magnifications, but not enough to equal the exit pupils in the smaller scopes.

A person who often uses 200x in an 8" scope does not necessarily use 400x often in a 16" scope.

Edited by Starman1, 14 May 2025 - 09:54 AM.

[CrazyPanda]

Can you list other? Size, weight, sensitivity to seeing, price, mobility...

Well stars are brighter regardless of the magnification employed. For extended objects yes - bigger aperture's only advantage is more magnification. But for stars, it's brightness. Globs look fuller and brighter.

[Houdini]

My experience in the field says no, they don't.  They tend to use slightly larger exit pupils than they did in the smaller scopes because of being seeing limited.

They do use slightly higher magnifications, but not enough to equal the exit pupils in the smaller scopes.

I agree.

In my (rather extreme) case there isn't much difference in power between the 25" and the 43". They're both mostly seeing limited.

 

With the 43" the lowest power is around 200X, and I get very bright images with an exit pupil of 6 mm.

350X is used quite often as well (over 80% of the nights), but 500X or above is used less frequently (maybe 40% of the nights).

 

It's more or less the same with the 25", except that I can use a lower power of around 120X.

Except for the lowest power the exit pupil considerations disappear completely, it's all about seeing.

Edited by Houdini, 14 May 2025 - 10:11 AM.