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Minimum Exit Pupil for Planetary Viewing?

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#1 jpengstrom

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Posted 28 July 2021 - 06:27 PM

I have a Meade 8" f10 SCT and am considering getting a better eyepiece for planetary viewing.  I currently use the 7mm Meade orthoscopic eyepiece which gives me a .7 exit pupil and, even at such a low exit pupil Jupiter and Saturn are so bright.  Now granted, seeing is probably the most important factor in planetary views, but we're in that time of year where high pressure and high humidity (and the still air that goes along with it) dominate the weather forecasts here in North Texas.  According to Astropheric and my own views of Jupiter/Saturn/Moon I'm often getting excellent seeing.

 

So how low of an exit pupil can I reasonably go?  I'm considering the Baader Morpheus 6.5mm or 4.5mm eyepiece for planetary.  I wear glasses and the eye relief that the 7mm Meade offers is not enough - I'm having to cram my eyeglasses up against the eyepiece and even then the 5 mm eye relief that the 7mm offers is not enough.

 

Any advice or suggestions would be appreciated, thanks!



#2 Bean614

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Posted 28 July 2021 - 06:50 PM

"So how low of an exit pupil can I reasonably go?"...????

 

There is NO correct universal answer to that question!  Everyone's eyes are quite different, and the answer depends entirely on YOUR eyes!  For me, anything over a 5mm exit pupil means blackouts and kidney beaning, and I'm 73.  Jon Issacs,  a frequent contributor to these pages, and accepted expert on most topics, is also 73, but his eyes EASILY handle exit pupils up to 7mm!

So if someone actually gives you an answer to your question, run, quickly, in the other direction!


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#3 MitchAlsup

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Posted 28 July 2021 - 06:53 PM

Realistically, 7mm is about as high as you can go power wise. (It was when I had my C11).....

 

On the other hand, #4 ND filter will get rid of most of that brightness, preserving your night vision.


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#4 jpengstrom

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Posted 28 July 2021 - 06:57 PM

Realistically, 7mm is about as high as you can go power wise. (It was when I had my C11).....

 

On the other hand, #4 ND filter will get rid of most of that brightness, preserving your night vision.

Thanks Mitch.  Yeah, I thought about an ND filter to reduce the brightness but since I really need more eye relief with my glasses (I have enough astigmatism that I need my glasses) I figured I would at least get some advice on exit pupil/power too.  I was leaning toward the Morpheus 6.5 - sounds like it's the best choice.


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#5 Jon Isaacs

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Posted 28 July 2021 - 06:58 PM

I believe jp is asking small an exit pupil is reasonable for viewing the planets.  

 

With my 10 inch, I usually top at about 400x on the best nights, that's a 0.6mm exit pupil.  With a 4 inch, I would push it a little more, about 200x or a 0.5mm exit pupil.

 

Jon


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#6 jpengstrom

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Posted 28 July 2021 - 07:02 PM

I believe jp is asking small an exit pupil is reasonable for viewing the planets.  

 

With my 10 inch, I usually top at about 400x on the best nights, that's a 0.6mm exit pupil.  With a 4 inch, I would push it a little more, about 200x or a 0.5mm exit pupil.

 

Jon

Yep - that's exactly what I'm asking.  And it sounds like the .45 exit pupil (444x) is trying to push it too much and I would be better off with the .65 exit pupil (308x) on the 8" SCT.


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#7 jpengstrom

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Posted 28 July 2021 - 07:15 PM

"So how low of an exit pupil can I reasonably go?"...????

 

There is NO correct universal answer to that question!  Everyone's eyes are quite different, and the answer depends entirely on YOUR eyes!  For me, anything over a 5mm exit pupil means blackouts and kidney beaning, and I'm 73.  Jon Issacs,  a frequent contributor to these pages, and accepted expert on most topics, is also 73, but his eyes EASILY handle exit pupils up to 7mm!

So if someone actually gives you an answer to your question, run, quickly, in the other direction!

I guess I'm confused by your comment.  Isn't exit pupil defined as the focal ratio of the scope divided by the focal length of the eyepiece?  If so then lower exit pupils would be "harder" on the eyes and result in blackouts/kidney beaning/floaters/etc.  And the exit pupil of 2 for my 20mm eyepiece is very easy to view.


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#8 Asbytec

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Posted 28 July 2021 - 07:36 PM

I believe jp is asking small an exit pupil is reasonable for viewing the planets.  

 

With my 10 inch, I usually top at about 400x on the best nights, that's a 0.6mm exit pupil.  With a 4 inch, I would push it a little more, about 200x or a 0.5mm exit pupil.

 

Jon

My eyes agree with Jon and at least one other CN contributor. For Jupiter's bright low contrast detail I normally top out at 0.6mm exit pupil through my MCT and about 0.5mm in my Newt. (Truthfully, I may be as low as 0.5mm in my MCT, but there is some leeway in the operating focal length and resulting magnification). Much higher than that and, for me, bright low contrast detail tends to fade. The limb of the planet is still sharp, just the low contrast detail fades as the planet's image becomes larger and a bit dimmer (per unit of area, normally per square arc second). 

 

On Saturn, the same story. Saturn has bright and a lot of low and very low contrast detail in it's cloud tops. I do not notice much degradation that might be visible at lower magnification, so I top out around 0.5mm exit pupil, too. The rings are much brighter with a bit higher contrast detail (and some low contrast detail). I can push the rings a bit higher, say around 600x in my 8" Newt or about 0.3mm. Mars is bright and has some high contrast detail. In my MCT, I have hit Mars as high as 400x or about 0.4mm with good results. 

 

Typically, in my experience, the brighter and more high contrast detail, you can push it a little higher as much as seeing might "allow" or until the image becomes too dim for our eyes to see well. This seems to be generally true with everything, including the moon, small bright planetary nebulae, and even galaxies. I have heard some folks are amazed at Jupiter's bright low contrast detail at low as 0.3mm exit pupils. But, for me, such ludicrous magnification does not do me any favors. 

 

And it sounds like the .45 exit pupil (444x) is trying to push it too much and I would be better off with the .65 exit pupil (308x) on the 8" SCT.

 

You could do both. It depends on the planet and the bright high and low contrast detail visible. And seeing, of course. But the former might be good for the outer gas giants, Mars, and Saturn's rings. The latter is better for Jupiter and Saturn, generally. Be sure your scope is well collimated and thermally stable. 

 

Yes, if you wear glasses some generous eye relief is needed. However, you may find the small exit pupils help correct astigmatism. We're using so much less of our astigmatic iris, so you may not need glasses if astigmatism is the problem. You can consider a Barlow lens if you can find one that delivers the desired exit pupils with your existing eyepieces. They generally extend eye relief while adding magnification to an eyepiece. 

 

Isn't exit pupil defined as the focal ratio of the scope divided by the focal length of the eyepiece?

 

I am not sure the exit pupil is defined by that, but it can be used to calculate the exit pupil. Using the focal ratio, the math contains the aperture and focal length and magnification with the eyepiece focal length. Really, in my view and since the exit pupil is an image of the entrance pupil, magnification is the ratio of the entrance pupil (aperture) to the exit pupil. Or equally, the exit pupil is the ratio of aperture to magnification. I prefer this as a definition, but either work to calculate the exit pupil.


Edited by Asbytec, 28 July 2021 - 08:07 PM.

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#9 jpengstrom

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Posted 28 July 2021 - 08:10 PM

My eyes agree with Jon and at least one other CN contributor. For Jupiter's bright low contrast detail I normally top out at 0.6mm exit pupil through my MCT and about 0.5mm in my Newt. (Truthfully, I may be as low as 0.5mm in my MCT, but there is some leeway in the operating focal length and resulting magnification). Much higher than that and, for me, bright low contrast detail tends to fade. The limb of the planet is still sharp, just the low contrast detail fades as the planet's image becomes larger and a bit dimmer. 

 

On Saturn, the same story. Saturn has bright and a lot of low and very low contrast detail in it's cloud tops. I do not notice much degradation that might be visible at lower magnification, so I top out around 0.5mm exit pupil, too. The rings are much brighter with a bit higher contrast detail (and some low contrast detail). I can push the rings a bit higher, say around 600x in my 8" Newt or about 0.3mm. Mars is bright and has some high contrast detail. In my MCT, I have hit Mars as high as 400x or about 0.4mm with good results. 

 

Typically, in my experience, the brighter and more high contrast detail, you can push it a little higher as much as seeing might "allow" or until the image becomes too dim for our eyes to see well. This seems to be generally true with everything, including the moon, small bright planetary nebulae, and even galaxies. I have heard some folks are amazed at Jupiter's bright low contrast detail at low as 0.3mm exit pupils. But, for me, such ludicrous magnification does not do me any favors. 

 

You could do both. It depends on the planet and the detail visible. And seeing, of course. But the former might be good for the outer gas giants, Mars, and Saturn's rings. The latter is better for Jupiter and Saturn, generally. Be sure your scope is well collimated and thermally stable. 

 

Yes, if you wear glasses some generous eye relief is needed. However, you may find the small exit pupils help correct astigmatism. We're using so much less of our astigmatic iris, so you may not need glasses if astigmatism is the problem. You can consider a Barlow lens if you can find one that delivers the desired exit pupils with your existing eyepieces. They generally extend eye relief while adding magnification to an eyepiece. 

 

I am not sure the exit pupil is defined by that, but it can be used to calculate the exit pupil. Using the focal ratio, the math contains the aperture and focal length and magnification with the eyepiece focal length. Really, in my view and since the exit pupil is an image of the entrance pupil, magnification is the ratio of the entrance pupil (aperture) to the exit pupil. Or equally, the exit pupil is the ratio of aperture to magnification. 

Asbytec, thanks for the detailed and helpful reply! I appreciate it - posts like yours not only give me better understanding, but also help with the "why's" and "how's" rather than just trial and error and it's one of the reason I appreciate this forum.

And, you are quite correct about my "exit pupil is defined by" comment.  I should have been more precise and said "exit pupil is equal to" because the definition should encompass the relationship between entrance pupil/aperture, magnification (which in itself is the relationship between aperture, focal ratio of the scope, and focal length of the eyepiece) , and exit pupil.

 

Seeing as viewing Jupiter and Saturn (gas of the planet more so than the rings) is my current priority that does steer me toward the 6.5mm Morpheus but I'll keep the 4.5mm in my mind for the future.


Edited by jpengstrom, 28 July 2021 - 08:15 PM.

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#10 Sandy Swede

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Posted 28 July 2021 - 08:26 PM

According to my cheat sheet:

 

Exit pupil = Eyepiece Focal Length / Focal Ratio of Scope, or

 

Exit pupil = Objective Diameter / Magnification, or

 

Eyepiece Focal Length = Focal Ratio of scope x Exit Pupil, so, if your pupil is 5mm and your scope's FR is 10, your recommended max eyepiece FL is 50mm.

 

If my cheat sheet is wrong, please let me know where and the correct value.  Thanks.


Edited by Sandy Swede, 28 July 2021 - 08:27 PM.

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#11 luxo II

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Posted 28 July 2021 - 08:29 PM

I'll sometimes go to 0.4 or 0.33mm exit pupil on Mars or Saturn in excellent seeing... that's 750X... Jupiter not so much.


Edited by luxo II, 28 July 2021 - 08:32 PM.

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#12 jpengstrom

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Posted 28 July 2021 - 08:56 PM

According to my cheat sheet:

 

Exit pupil = Eyepiece Focal Length / Focal Ratio of Scope, or

 

Exit pupil = Objective Diameter / Magnification, or

 

Eyepiece Focal Length = Focal Ratio of scope x Exit Pupil, so, if your pupil is 5mm and your scope's FR is 10, your recommended max eyepiece FL is 50mm.

 

If my cheat sheet is wrong, please let me know where and the correct value.  Thanks.

You are correct - I misspoke when I said "focal ratio of scope divided by focal length of eyepiece".  It is indeed focal length of eyepiece divided by focal ratio of the scope, or, alternatively as you say objective diameter (i.e. aperture of the scope) divided by magnification.

And the latter can be restated as:

    exit pupil = aperture / (focal length of scope / focal length of eyepiece)
That is equivalent to:

    exit pupil = focal length of eyepiece * aperture / focal length of scope
Since "aperture / focal length of scope" = "1 / focal ratio of scope" we can further simplify to:

    exit pupil = focal length of eyepiece / focal ratio of scope
Which is the first equation! smile.gif


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#13 Ernest_SPB

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Posted 29 July 2021 - 02:07 AM

Interesting thread.

 

The answer correctness depends on a scope aperture.

Telescopes with relatively small aperture (like 70-120 mm) allow to use effectively exit pupil 0.4 mm and even smaller in Saturn, Mars and Moon observing.

Owners of telescopes with large aperture (8-10" and more) are mostly limited with exit pupil 0.7 mm in diameter and even large in their planetary observing. And meet certain difficulties with extra brightness of observed objects.

I think the aperture is not a primary source of the difference, but just a gate for stronger atmospheric distortions. Large scopes are affected more by poor seeing conditions and it prevents them from use in diffraction limits.

By the way it clues that in rare perfect seeing large telescopes can work effectively with smaller exit pupils, like 0.5-0.6 mm. And waiting for the opportunity it is useful for large scope owners to reserve a short focal eyepiece in EPs box.


Edited by Ernest_SPB, 29 July 2021 - 02:11 AM.

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#14 Asbytec

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Posted 29 July 2021 - 02:31 AM

 

The answer correctness depends on a scope aperture.

Telescopes with relatively small aperture (like 70-120 mm) allow to use effectively exit pupil 0.4 mm and even smaller in Saturn, Mars and Moon observing.

Earnest, great point. I believe there's a trend for folks with smaller apertures toward smaller exit pupils and push higher magnifications because we're starved for image scale. Folks with larger apertures can attain larger image scales at high magnifications and, since the exit pupil is related to the larger aperture, they still result in a slightly larger exit pupil at relatively high magnification.

 

As you point out, this may also be related to seeing conditions in that smaller apertures are less affected, generally, while maintaining a nice image. Higher magnification in larger apertures can more readily reach any seeing limit imposed on a given night.

 

But, at some point, image surface brightness must play a limiting role depending on the type of object we are viewing. Even magnified relatively bright images of planets have a limit somewhere. And of course, our eye plays a role in detecting increasingly dim detail at higher magnification.

 

The best way to know is to simply magnify an object and see how the image appears. As Jon has said, the eye likes a large image and it likes a bright image. The key to productive observing is to find that balance. Doing so in very good seeing, I find Jupiter is best around 0.6mm to 0.5mm. 


Edited by Asbytec, 29 July 2021 - 02:36 AM.

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#15 luxo II

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Posted 29 July 2021 - 03:46 AM

Interesting thread.

 

The answer correctness depends on a scope aperture.

There is no correct answer. Too many uncontrolled (unquantified) variables starting with aperture, the quality of the scope, the quality of the eyepieces, seeing and the choice of target itself, and then there's the question of the observers eyesight and observing experience.

 

I really wish some of you would stop thinking there is a one-answer-rules-all, or polarised "you're with us, or against us" approaches. Wars have started that way.


Edited by luxo II, 29 July 2021 - 03:49 AM.


#16 Redbetter

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Posted 29 July 2021 - 03:58 AM

There are a number of competing limitations for minimum exit pupil for planetary (max magnification for a given scope.)  Part is simply personal preference.  I stop at the point at which I see the most detail, even if the seeing seems steady.  Going beyond that reveals less to my eye, so the only reason to go higher mag/smaller pupil is if I need some scale to confirm separation of an object/detail from another.  Others will operate at lower mag/inch larger exit pupil for aesthetic reasons (looks crisper), while some like to operate at smaller exit pupil even though they admit it is adding no more detail (truly empty magnification.)

 

Limiting factors:  

  • Personal preferences as noted above.  This includes sensitivity to floaters/texture of the eye, as well as astigmatism, acuity, etc. Image brightness and color are reduced by smaller exit pupil, so there are trade offs.   This is more of an exit pupil preference
  • Seeing.  This nearly always effectively caps the optimal magnification for my eye, even with a 60mm scope.  With the 20" it always does.  Seeing presents more of a magnification limitation, not an exit pupil limitation.  In mediocre seeing the larger scopes will be dialed back far more, but still might be running considerably higher magnification than the smallest scopes.
  • The optics, essentially an exit pupil limitation (or mag/inch).  Even when the seeing is steadiest, I don't find a good 80 f/5 achro will support nearly the same exit pupil/magnification as an ED80.  (The 80 f/5 is out of gas by 30x/inch for planetary detail.)  And an 80 f/11 achro is somewhere in between, but closer to the ED.  My ED's top out around 50x/inch for planetary detail to my eye sometimes a little more although by 60x/inch they all seem to be a little past topped out.  My 8" SCT in rock steady seeing tops out at around 39-40x/inch for my eye.  My 127 Mak is similar, only slightly better at roughly 42x/inch.   Our 10" Dob has only had a couple of nights where the seeing was briefly steady enough to push it, but the seeing always faltered before I could get past ~42x/inch...so I don't know what its limit is, I suspect about 45x/inch but I haven't had the seeing to explore it.  
  • The target.  For some this seems to matter a great deal, but I don't find as much target sensitivity as some report.  I find the sweet spot is roughly the same when conditions are good and planets well placed, whether I am looking at Jupiter, Mars, Saturn or the Moon.  The same things seem to be limiting detail to my eye despite the Moon having about 20x the surface brightness of Saturn (and it is the outlier.)  A possible exception is Neptune, which isn't revealing detail past how well its disk and color are resolved.  With Neptune the loss of color (graying or even relative reddening vs the bluish hue expected) is noticeable in the smallest apertures at the same pupil I use for bright planets.  Neptune is roughly 10x dimmer than Saturn, and deep into the mesopic range at ~15 MPSAS at 0.5mm exit pupil.  In 60 to 80mm aperture its size is of the same approximate scale as the airy disk, so the lack of photons and resolution conspire against it with respect to color.

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#17 Redbetter

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Posted 29 July 2021 - 04:13 AM

Considering the practical question about what is best for the OP's 8" SCT, my standard response is to move in small increments on the high end of the scale.  An 8" SCT that still seems to have room to run with a 7mm is a good scope in my experience.  Mine has done well with 7 in 2" mode (longer scope focal length of about 2220mm measured by drift timing.)  But it is not clear how much room is left, so the jump to a 4.5 is unlikely to pan out.  If this scope is set up with a 2" mirror refractor diagonal, then I would strongly recommend making a more modest jump.

 

While I am not enamored of the very small increment going from 7 to 6.5mm, it could prove to be the sweet spot.  I would prefer something around 6 if this is with a 1.25" diagonal.  My experience has been that moving from 7 to a 13 w/2x TV Barlow for 6.5 was not adding anything; but keep in mind accounting for the actual 2220mm operating focal length, the 7 was effectively operating as a 6.4mm indexed to f/10, and the 13 w/2x Barlow in this configuration was more like 5.9mm.  


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#18 luxo II

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Posted 29 July 2021 - 05:07 AM

Most SCT's IMHO run out of puff at 8-9mm. A 7mm eyepiece doesn't show anymore detail, just a dimmer and blurrier image.


Edited by luxo II, 29 July 2021 - 05:07 AM.

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#19 Rick-T137

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Posted 29 July 2021 - 06:24 AM

Most SCT's IMHO run out of puff at 8-9mm. A 7mm eyepiece doesn't show anymore detail, just a dimmer and blurrier image.

I would tend to agree if you're referring to the affects of seeing on an 8" telescope. Most nights I can use my 8mm but even some nights are so bad that 11mm is the lowest I can go. But I have had some nights where I could use my 6mm and the view showed more detail compared to the 8mm (ie: Jupiter, Mars).

 

I think most SCT's are not properly cooled and collimated and therefore don't perform their best when given an opportunity - thus they run out of puff. 

 

Clear skies!
 

Rick


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#20 luxo II

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Posted 29 July 2021 - 06:48 AM

In 40 years I’ve only ever used 7mm in scts when collimating them but never saw any benefit on moon or planets. And that includes nights of excellent seeing.
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#21 noisejammer

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Posted 29 July 2021 - 06:59 AM

I find 0.55 mm is about the smallest usable for the combination of my skies (typically 0.6 - 0.7 "), my scope (TOA150), my eyepieces (ZAO II) and my floater-filled eyes. In this case, I think my eyes are the limiting factor.

 

Depending on the seeing, I'd be looking at magnifications from 200-250 for an average 8" SCT. This corresponds to exit pupils of 0.8-1.0 mm.


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#22 Rick-T137

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Posted 29 July 2021 - 08:41 AM

In 40 years I’ve only ever used 7mm in scts when collimating them but never saw any benefit on moon or planets. And that includes nights of excellent seeing.

I would appear my experience with SCTs and yours differs. I've owned 4 of them in the last 25 years (3 Meades and 1 Celestron) and I could easily use them at over 300x when seeing permitted. I guess I got lucky with my SCT optical quality. One of the best views I've had in my life was through my late friend's (Geoff Gaherty) Celestron C11 - breathtaking views of Jupiter and Saturn at over 300x.

 

I guess the old acronym YMMV applies here. 

 

Clear skies!

Rick


Edited by Rick-T137, 29 July 2021 - 08:43 AM.

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#23 Voyager 3

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Posted 29 July 2021 - 09:09 AM

In 40 years I’ve only ever used 7mm in scts when collimating them but never saw any benefit on moon or planets. And that includes nights of excellent seeing.

I guess it must be seeing as you seem to be experienced and would have known all the musts like collimation , cooling etc .

My point is your Excellent seeing maybe others' average seeing ...
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#24 Redbetter

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Posted 29 July 2021 - 10:08 AM

Most SCT's IMHO run out of puff at 8-9mm. A 7mm eyepiece doesn't show anymore detail, just a dimmer and blurrier image.

Not my experience at all.  When I initially put the 7T1 into the diagonal of my 8" SCT while turned to Mars, it was love at first sight.  The 9mm remained my mainstay eyepiece for typical nights (which would be very good nights where I am now), but when the seeing reached reached 7/10, 8/10 or got rock steady, the 7mm was the one revealing fine detail at the limit of what the scope could do.  And like I said, with the 2" TV refractor diagonal and visual back, the eyepiece was effectively running at 6.4mm if it had been f/10 focal length..

 

Sounds like you have never had a decent SCT, and that's a shame.  Mine was sharper than the others I looked through back in the 90's, particularly compared to the Meades of that era.  When I first joined a local club, an officer/professional photographer with his own studio looked through my SCT at the double Antares and said, "that's a good optical tube, if you ever want to sell it, I want it."  And he was a Meade guy.   The Celestrons of the past decade have been similar to mine, when I have looked through them and they were collimated.  I've seen some that needed a touch up to collimation based on a quick star check.


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#25 tommm

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Posted 29 July 2021 - 12:03 PM

There is no correct answer. Too many uncontrolled (unquantified) variables starting with aperture, the quality of the scope, the quality of the eyepieces, seeing and the choice of target itself, and then there's the question of the observers eyesight and observing experience.

 

True, but I agree with Ernest about seeing. At my location even 1mm exit pupil is rarely useful for viewing planets with my 22" scope.  It's not the optics because on those rare nights the views are stunning - at least to someone used to worse conditions, in FL maybe just routine.
 


Edited by tommm, 29 July 2021 - 12:05 PM.



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