46 replies to this topic

[Glory Eye]

I first became interested in the study of how aperture size is related to magnification and exit pupil as a result of having to contend with spates of bad seeing because of which, I have many times been limited to 200x magnification or lower.  In such circumstances I would ask myself, “If I had more aperture (i.e. larger scope) would it improve my observing and the images obtained when my magnification is limited?” Another way to ask the question is to consider being limited to 200x and you have a 16” and a 20” telescope to observe with, how would they compare? The following is a presentation of what I learned. Unfortunately, I can’t rely on direct observation as I don’t personally know a single soul within a 1000 mile radius that has a telescope let alone a 20” that I can look through. Therefore my analysis is solely  based on the governing mathematics and image graphs.

 

Magnification, aperture, and exit pupil are mathematically related. A change in one variable will affect the other two. Normally, aperture is a fixed value as it is the size of your primary mirror. Aperture won’t change without the purchase of a new telescope with a hopefully larger or sadly smaller mirror. Aperture being fixed there is a linear inverse relationship between the other two variables, meaning as one goes up, the other goes down. Stated another way, as you increase magnification, your exit pupil decreases. As a result, while the image gets larger as you increase magnification, it also becomes increasingly darker. For example, suppose you’re trying to observe M51, the Whirlpool galaxy. You may put in a low power eyepiece and notice that while it is bright, the image is not large enough to see the details well. In order to see the details more clearly, you increase magnification by inserting a higher power eyepiece. Now however, the image is too dark to see the galaxy well.
What happens if you increase aperture?

 

Continuing the previous example, you can’t see M51 to your satisfaction, so you make a decision, “I need a bigger telescope”. You purchase the telescope, set it up, and once again slew to M51. What do you find? You have had an exponential increase in the brightness of the galaxy. You may not be thinking about the math or understand it at all, but you will sure be smiling. Demonstratable by mathematical analysis, as aperture increase for a given exit pupil, the image will get larger exponentially by the second power. Similarly, for a given magnification, increasing aperture increases the image brightness exponentially by the second power. The following spreadsheet summarizes the mathematical relationship between Magnification, Exit Pupil, and Aperture.

 

 

Something I find very interesting that acts to verify the results, is that the Light Grasp Ratio, which is simply the ratio of each lower mirror area to the area of the 25" mirror, matches the ratios obtained for Brightness % and Image Size % each of which is calculated with different variables than the Light Grasp Ratio. The equations used are listed under "Notes" in the spreadshhet.

 

Here is a graphical illustration of how aperture affects image brightness: What you're seeing is what happens when you hold the magnification constant and decrease aperture or mirror size.

 

 

I credit TOMDEY as he posted a graphic in times past similar to the ones I am posting here that helped me to understand this topic. His graphic also inspired the creation of these gifs. Thank you Tom.

Edited by Glory Eye, 27 November 2020 - 12:16 AM.

[Glory Eye]

Here is another graphic that demonstrates what happens when you hold Exit Pupil constant while decreasing Aperture:

 

 

What you're seeing is that to maintain exit pupil and hence image brightness, you must use lower and lower magnification as Aperture decreases. Conversely, If you want greater image size to see detail more clearly in the objects you observe, you have to have more aperture or in other words, a bigger telescope.

 

Conclusion:

To put it succinctly and bluntly, aperture rules. Being limited to 200x magnification is compensated by large aperture as it greatly increases image brightness and hence the details of any object. Large aperture particularly benefits galaxies with their low surface brightness and would also be a benefit in light polluted observing.

Edited by Glory Eye, 27 November 2020 - 12:30 AM.

[ButterFly]

Very nice.  I was going to suggest what you did in post #2, but you did already!

 

 

Conclusion:

To put it succinctly and bluntly, aperture rules. Being limited to 200x magnification is compensated by large aperture as it greatly increases image brightness and hence the details of any object. Large aperture particularly benefits galaxies with their low surface brightness and would also be a benefit in light polluted observing.

 

It depends on the object.  Some things just don't fit in the field of view of large aperture scopes - as the aperture increases, so does the focal length, usually.

 

The inherent contrast of the features against its surroundings and their sizes are another important factor.  Proper exit pupil and magnification gets you to the good place on your own contrast sensitivity curve to see the details you are after.  Both brightness variations and spatial frequencies help us discern features of objects from their surroundings.  Framing it well at that point is where aperture, focal length, and thus f/ratio, all come into play.  For filtered deep sky, a big exit pupil is ideal because the images are brighter.  For unfiltered deep sky viewing, an exit pupil around 2-3mm is closer to ideal because of our eye's aberrations.  For planetary, with its inherent low contrast, 1mm and below is more suitable because of the tiny low contrast features.  For any scope, a 1mm exit pupil is 1x per mm of aperture.  Not a lot of places on Earth can support such magnifications when the aperture is large.

 

The paradox underlying this is: no telescopic view will ever show an extended object with greater surface brightness than naked eye; nonetheless, we know that telescopes help us see things better.  A zoom eyepiece helps a lot and there is no math involved at the scope!
 

[ngc7319_20]

 

Conclusion:

To put it succinctly and bluntly, aperture rules. Being limited to 200x magnification is compensated by large aperture as it greatly increases image brightness and hence the details of any object. Large aperture particularly benefits galaxies with their low surface brightness and would also be a benefit in light polluted observing.

I believe the last bit is incorrect.  Light pollution contributes a surface brightness which is uniform across an image.  While at a given power (say 200x) a larger aperture will increase the brightness of a galaxy, it will also increase the brightness of light pollution by the same factor.  Hence the ratio of galaxy surface brightness to light pollution surface brightness will be the same regardless of aperture.  This has indeed been my experience in moving from 6" to 8' to 12" to 16" to 24" aperture -- a larger scope doesn't help you overcome light pollution. You may see more details with the larger scope in light polluted skies, but it in no way approaches what would be seen in pristine skies.

 

I think the first display (images at 200x) is also somewhat incorrect.  The sensitivity to different brightness is closer to logarithmic rather than linear.  So the sensation produced by say a 24" scope is greater than for a 16" scope, but no where near twice as great (as you might expect from the surface areas), and so forth.  Consider reference:

 

"...the perceived object brightness changes with the logarithm of object's actual brightness."  etc...

https://www.telescop...ponse.htm#level

 

And others related to Weber's Law and Fechner's law.

Edited by ngc7319_20, 27 November 2020 - 05:58 AM.

[happylimpet]

Here is another graphic that demonstrates what happens when you hold Exit Pupil constant while decreasing Aperture:

 

M-51-Image-Size.gif

 

What you're seeing is that to maintain exit pupil and hence image brightness, you must use lower and lower magnification as Aperture decreases. Conversely, If you want greater image size to see detail more clearly in the objects you observe, you have to have more aperture or in other words, a bigger telescope.

 

Conclusion:

To put it succinctly and bluntly, aperture rules. Being limited to 200x magnification is compensated by large aperture as it greatly increases image brightness and hence the details of any object. Large aperture particularly benefits galaxies with their low surface brightness and would also be a benefit in light polluted observing.

This could become a classic! Nicely done.

[Asbytec]

 

Here is a graphical illustration of how aperture affects image brightness: What you're seeing is what happens when you hold the magnification constant and decrease aperture or mirror size.

 

Very nice thread.

 

Since I observe with a smaller aperture, it dawns on me the other half of the equation is can one observe the 5 or 8" image at 200x? Trade precious surface brightness for a little larger image scale. I observe a lot of bright galaxies between 2mm and just less than 1mm exit pupil, but I do not recall M51 at anything smaller than 2mm (around 100x). Or larger than 2mm, for that matter. 

 

Off topic and intended only for further reading for those interested and have not seen Glenn LeDrew's work on DSO visibility here and an earlier version here. 

Edited by Asbytec, 27 November 2020 - 06:51 AM.

[Jon Isaacs]

I believe the last bit is incorrect.  Light pollution contributes a surface brightness which is uniform across an image.  While at a given power (say 200x) a larger aperture will increase the brightness of a galaxy, it will also increase the brightness of light pollution by the same factor.  Hence the ratio of galaxy surface brightness to light pollution surface brightness will be the same regardless of aperture.  This has indeed been my experience in moving from 6" to 8' to 12" to 16" to 24" aperture -- a larger scope doesn't help you overcome light pollution. You may see more details with the larger scope in light polluted skies, but it in no way approaches what would be seen in pristine skies.

 

I think the first display (images at 200x) is also somewhat incorrect.  The sensitivity to different brightness is closer to logarithmic rather than linear.  So the sensation produced by say a 24" scope is greater than for a 16" scope, but no where near twice as great (as you might expect from the surface areas), and so forth.  Consider reference:

 

"...the perceived object brightness changes with the logarithm of object's actual brightness."  etc...

https://www.telescop...ponse.htm#level

 

And others related to Weber's Law and Fechner's law.

The last statement:

 

"Conclusion:

To put it succinctly and bluntly, aperture rules. Being limited to 200x magnification is compensated by large aperture as it greatly increases image brightness and hence the details of any object. Large aperture particularly benefits galaxies with their low surface brightness and would also be a benefit in light polluted observing."

 

I would say yes and no, it's just more complicated than that.  For faint objects, seeing is not such a factor because the eye cannot resolve the details.  The smallest galaxy in Stephan's quintet is about the size of Jupiter. The eye can't see the detail that might exist. So limiting a 20 inch scope to 200x when viewing galaxies just doesn't make sense.   

 

My experience is that a larger scope will show more under light polluted skies.  With extended objects, galaxies in particular, a larger scope is not going to necessarily show objects visible under dark skies but it will show objects that can't be seen in a smaller scope.

 

It's not really about brightness and contrast, rather it's about magnification, same as it is under dark skies, the optimal exit pupil for a given aperture for a given object doesn't change much so the larger scope provides a larger image at the same brightness, the eye likes this.   

 

NGC2903, it's magnitude 9.0 with a surface brightness of 22.5 mpsas.  It's fairly large but from my urban backyard, I will only see the brightest part.  In my 13.1 inch, it's doable, in my 4 inch, it's not possible.  I can barely see the Veil with an O-lll filter in my 4 inch, only the eastern section. In the 13.1 inch, I see quite a bit of detail and I see the western Veil, at least parts of it too. 

 

I think constant exit pupils are appropriate for aperture comparisons.  Smaller scopes generally need to pushed a little more but 200x in a 4 inch is a 0.5mm exit pupil, too small for dim objects, it's about 200 times dimmer than naked eye. 

 

Jon

[Echolight]

Hate to see it in pictures like that! But it's really obvious just going from 3 inch to 6 inch to 8 inch in my small scopes. With the littlest scope there's a fine line between image detail/brightness and image size/not bright enough. And while I can get by jumping from 30 to 60x in the 6 inch and 50 to 100x in the 8 inch, the 3 inch seems to want something in between 15x and 30x for many objects. Or maybe more accurately, the 3 inch seems to call for something between a not always bright enough 2.67mm exit pupil, and an often too bright 5.33mm exit pupil.

 

And the worst is, it really makes me want a 16 inch!

 

Also I think an ST120 would wup my ED80 for widefield viewing, in spite of field curvature. I'd be able to make more use of the 20-100 and it's 30x in the same focal length 600mm scope thanks to it's greater light gathering ability as well as much larger exit pupil.

Edited by Echolight, 27 November 2020 - 02:25 PM.

[junomike]

Hate to see it in pictures like that! But it's really obvious just going from 3 inch to 6 inch to 8 inch in my small scopes. With the littlest scope there's a fine line between image detail/brightness and image size/not bright enough. And while I can get by jumping from 30 to 60x in the 6 inch and 50 to 100x in the 8 inch, the 3 inch seems to want something in between 15x and 30x for many objects. Or maybe more accurately, the 3 inch seems to call for something between a not always bright enough 2.67mm exit pupil, and an often too bright 5.33mm exit pupil.

 

And the worst is, it really makes me want a 16 inch!

 

Also I think an ST120 would wup my ED80 for widefield viewing, in spite of field curvature. I'd be able to make more use of the 20-100 and it's 30x in the same focal length 600mm scope thanks to it's greater light gathering ability as well as much larger exit pupil.

Got a 16" for this reason.....didn't work, now I want a 20"+

 

For RFT the larger 120 easily best the smaller 80ED unless you're viewing brighter objects (Vega, Sirius, Etc).

[Echolight]

Got a 16" for this reason.....didn't work, now I want a 20"+

 

For RFT the larger 120 easily best the smaller 80ED unless you're viewing brighter objects (Vega, Sirius, Etc).

That's what I was thinkin. More stars in the eyepiece in the same field of view thanks to it's extra magnitude of reach. Plus the larger exit pupil for maybe greater nebulosity, and definitely general brightness at higher powers.

[havasman]

Though these sorts of comparisons are always open to critique, the point here is well taken and expressed clearly enough to be appreciated by folks that have not experienced increased aperture observing. While there are theoretical and even practical penalties to increased apertures in poor observing conditions, the offset of the inherent advantages of aperture is insufficient to negate them.

 

And the arguments in post #4 are well made and recognized.

 

But the value of the OP's work remains high. There can be little doubt that the high value of increased aperture enables larger exit pupils at a given magnification and higher magnification at a given exit pupil ands that these serve extremely well to enable observations of many objects that are unlikely or impossible via small apertures. The graphic presentations are likely to illustrate the matter for a large group of observers.

[gatorengineer]

Your analysis is set to a single magnification, and its alot more complicated than that, you  need to dial in field of view, and consider that its a rare night that you will get to 200x, at least in my seeing.  You also need to consider what you ""like"" to view, if you are a planetary observer, you will likely get a different answer than a person looking at large Nebula.    For some objects, 0.3 degrees is fine but for many others you need  or will want alot more field..  

 

Also the Best Telescope you own is one that you use the most, dont let aperature fever get the better of you.  Currently I own from 45mm to 36".   Since I discovered NV, my favorite and currently most used scope is an 80mm refractor, followed by a 7" Mak Newt.  The 24" gets used maybe once a month on average.  The 36" is at a darksky site and gets used even less.  I can see far more with that little 80MM with NV for Ha sources than I can with the 36" without it.  (I can see barnards loop in an 80mm refractor from suburbia). at 7"F4 with NV nebula look like the pictures.  at 24" plus NV, they are better than the pictures.

 

I am again picking up a 16" this weekend, dont sell that aperature short, it can do a heck of alot, throw night vision on it and you will be blown away (for a whole lot less $ than a 25").  16" for me is feet on the ground and very easy out of the garage. the 24" takes alot more want to, and a ladder.

 

Yes Aperature is nice Aperature + nightvision is unbelievable.

[Glory Eye]

I believe the last bit is incorrect.  Light pollution contributes a surface brightness which is uniform across an image.  While at a given power (say 200x) a larger aperture will increase the brightness of a galaxy, it will also increase the brightness of light pollution by the same factor.  Hence the ratio of galaxy surface brightness to light pollution surface brightness will be the same regardless of aperture.  This has indeed been my experience in moving from 6" to 8' to 12" to 16" to 24" aperture -- a larger scope doesn't help you overcome light pollution. You may see more details with the larger scope in light polluted skies, but it in no way approaches what would be seen in pristine skies.

 

I think the first display (images at 200x) is also somewhat incorrect.  The sensitivity to different brightness is closer to logarithmic rather than linear.  So the sensation produced by say a 24" scope is greater than for a 16" scope, but no where near twice as great (as you might expect from the surface areas), and so forth.  Consider reference:

 

"...the perceived object brightness changes with the logarithm of object's actual brightness."  etc...

https://www.telescop...ponse.htm#level

 

And others related to Weber's Law and Fechner's law.

To address your first point take a look at the article

https://skyandtelesc...elescope-myths/

 

If you look under the heading: "Large telescopes are more impaired by light pollution than small ones", you will see this statement:

 

"The brightness ratio between a galaxy like M31 (say) and the background sky is the same regardless of telescope size. All telescopes are equally impaired by light pollution compared to what they would show under a dark sky. A large telescope's advantages — superior light grasp and resolution — are the same under bright skies and dark ones."

---

 

So I agree that Light Pollution affects all scopes equally. What I meant by large scopes benefit in Light polluted skies is that a sufficiently large scope will still allow you to see spiral arms in M51 even in Bortle 4, whereas you may be able to see some spiral structure in an 8" under dark skies but much more subtle in Bortle 4. I suspect that if I were to get a 20", I would still be affected by light pollution but there is no doubt in my mind that Whirlpool's arms would be significantly brighter than in the 16". Therefore larger aperture would benefit in light polluted skies.

 

As to your second statement, I have looked at the article you cited and Lord-willing, I'll study it until I understand it. I will say that as a test, I adjusted the spreadsheet and the image graph for the Celestron 5" to display what M51 looks like with a 9mm eyepiece. I chose this specifically because I have observed M51 from my house with a 9mm eyepiece which corresponds to a magnification of about 72,2x. The image posted below seems fairly close as far as the bright cores to what I observed, but the spirals were much more subtle than in the picture. I also want to emphasize that brightness and size comparisons are RELATIVE NOT ABSOLUTE. This means that for example, the image brightness for the 16" is not as it may show in reality but that the brightness is how it relatively would compare to the 25" brightness if the image was that particular luminosity.

.

Attached Thumbnails

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[Glory Eye]

Your analysis is set to a single magnification, and its alot more complicated than that, you  need to dial in field of view, and consider that its a rare night that you will get to 200x, at least in my seeing.  You also need to consider what you ""like"" to view, if you are a planetary observer, you will likely get a different answer than a person looking at large Nebula.    For some objects, 0.3 degrees is fine but for many others you need  or will want alot more field..  

 

Also the Best Telescope you own is one that you use the most, dont let aperature fever get the better of you.  Currently I own from 45mm to 36".   Since I discovered NV, my favorite and currently most used scope is an 80mm refractor, followed by a 7" Mak Newt.  The 24" gets used maybe once a month on average.  The 36" is at a darksky site and gets used even less.  I can see far more with that little 80MM with NV for Ha sources than I can with the 36" without it.  (I can see barnards loop in an 80mm refractor from suburbia). at 7"F4 with NV nebula look like the pictures.  at 24" plus NV, they are better than the pictures.

 

I am again picking up a 16" this weekend, dont sell that aperature short, it can do a heck of alot, throw night vision on it and you will be blown away (for a whole lot less $ than a 25").  16" for me is feet on the ground and very easy out of the garage. the 24" takes alot more want to, and a ladder.

 

Yes Aperature is nice Aperature + nightvision is unbelievable.

Admittedly, I at least have a mild case of aperture fever but I am very happy with 16" and have seen incredible things. Some of my best views have been from my back yard in suburban light pollution while others objects are far and away better at my dark site.. I am only dreaming about  20" because I love galaxies and I am limited to the brightest for viewing spiral arms. I'm also dreaming about NV coupled with the 16" and it may become a necessity in the years to come as light pollution will only get worse. What has your experience been like with galaxy viewing with NV? Do you think that 16" + NV would be better or worse than a 20?

[Glory Eye]

Got a 16" for this reason.....didn't work, now I want a 20"+

 

For RFT the larger 120 easily best the smaller 80ED unless you're viewing brighter objects (Vega, Sirius, Etc).

Not to go off topic too much, but I'm throwing around the idea of getting a hold of a descent 20" mirror and using parts of the xx16G  to make a Frankenstein 20. I would have to come up with a OTA and mirror cell for the new primary, but I could make some adapter plates to connect the xx16G Side Panels to the Baseplate at a wider distance to accommodate the larger mirror. That way I'd still have GoTo. I already have a new Anteres secondary. Everything would be done non-destructively so I could assemble the 16" or 20" depending on my preference at the time.  If I had the money, I'd buy a 20" today but alas, reality.

[a__l]

Maksutov. Dependence of the real eye resolution on the pupil diameter (black line).

Red line for 756 mm aperture, magnification 168x

Shtih dotted line - theory.

Draw conclusions for the pictures (above) yourself.

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Edited by a__l, 27 November 2020 - 08:30 PM.

[Jon Isaacs]

So I agree that Light Pollution affects all scopes equally. What I meant by large scopes benefit in Light polluted skies is that a sufficiently large scope will still allow you to see spiral arms in M51 even in Bortle 4, whereas you may be able to see some spiral structure in an 8" under dark skies but much more subtle in Bortle 4. I suspect that if I were to get a 20", I would still be affected by light pollution but there is no doubt in my mind that Whirlpool's arms would be significantly brighter than in the 16". Therefore larger aperture would benefit in light polluted skies.

 

 

For most of us, light polluted skies are 17-19 mpsas. 20.5-21.3 mpsas correspond to Bortle 4 and for many, particularly towards the 21.3 end, that's pretty dark..

 

Jon

[ButterFly]

Maksutov. Dependence of the real eye resolution on the pupil diameter (black line).

Red line for 756 mm aperture, magnification 168x

Shtih dotted line - theory.

Draw conclusions for the pictures (above) yourself.

And it gets worse ... .  Rutten, Telescope Optics, ch. 18, Table 18.2 gives the following representative data:

 

Resolving power of the eye

Contrast (%)   Resolving power (")

89                     74

70                     76

49                     80

...

10                     95

6                       105

2.5                    127

 

Chapter 18 (a great read) ends with a discussion of optimum magnification as a function of contrast and scintillation, based on the above data (which is obviously different for everyone).

 

Again, no telescopic view of an extended object is ever brighter than naked eye.  The magnification a telescope provides is very important for good visibility, and in the presence of less than 100% contrast, the combination of magnification and brightness..

[Asbytec]

Got a 16" for this reason.....didn't work, now I want a 20"+

 

Mike, I went the other way stepping down in aperture. I thought I had lost deep sky forever, but I was wrong. In fact, the smaller aperture caused me to improve my own skill and work more closely with exit pupils (magnification and surface brightness) to see more. I no longer relied on the smallest, brightest image of galaxies just to see them with a large aperture and 7mm exit pupil.

 

I don't believe I regained the full effect from stepping down, but my own views improved dramatically. Then an interesting thing happened. I became satisfied with my improved views through the smaller scope and observing became more rewarding. Not because of what I couldn't see with a larger aperture, but because of what I could see in the smaller one. My whole paradigm changed. And with it aperture fever was tamed. 

 

[Starman1]

At constant exit pupil, the larger scope has a higher magnification, so details in the object are more visible.

At constant magnification, the larger scope has a larger exit pupil, so the image is brighter.

 

But, in practice, the larger scope uses a higher magnification AND has a larger exit pupil because the magnification is not increased in direct ratio to aperture.

So that is a double plus for the larger scope--brighter and larger.

 

As for seeing conditions at higher magnifications, it is a personal matter, I think.  I have viewed on nights when seeing blurred the images at high power 90% of the time.

Most of my observing time was waiting for those moments of clarity.  I think many if not most observers would argue that the magnification was too high for the conditions.

So, what percentage of the time is the image steady as a rock?  If you confine it to 100% of the time, you'll never use anything but low power.

Pickering 10 only occurs once in a blue moon anywhere.

That's why I say seeing is a personal matter--what % of the time you're willing to put up with less than excellent seeing to get those few moments of good or great seeing.

I think if you're willing to wait for those moments, it's not unusual to use 200x in a 4" and 300-400x in a 12.5".

That doesn't mean that all your observing will be done at the higher power, just that it's possible to do so.

I grew up in the Midwest and now live in CA, and I've found the same thing both places.  Great seeing is rare, but good seeing is common.  You just have to wait for it.

[gatorengineer]

A delayed response to the OPs question.  NV is sensitive to Primarily Ha....  it will help some (read incrementally) on face on galaxies.  Edge ons it makes alot better.  Seeing M81/82 with NV in my friends 16 was really impressive (I picked up another 16 today).  I would take the 16 with NV over the 20 without any day.  

[MunichAtNight]

Hello!

 

I run now a F5 dobson / newton with 1.250mm focal length and an aperture of 250 mm. Only for viewing. No photos! I am thinking to get a faster newton with F4 to find and view more easily nebulae and view them some how brighter. Because of limited space in my van I wanted to stay at the same focal length of about 1.250 mm. By investigating www I found this very interesting discussion, which let me understand and hope to be right in sense "more brighter" can be a good idea. I did a spreadsheet to compare the exit pupils based on different magnitudes for different types of newton telescopes.

 

 

Thinking based only on exit pupil, I will get for example with magnification of 50 about 1mm more exit pupil with F4 as with F5  with the same focal length of ~ 1200 mm. Of course I know when using more focal length, that viewing an object as nebulae will be wider. And a combination of bigger focal length and bigger aperture will be better as getting "bigger" and "brighter". I was wondering if I am completely false with my idea to increase only the focal ratio and the real "viewing" effect might be to small as it would make sense to change equipment. and additional, I read that as bigger the focal ratio will be, there are other optical side effects which make problems when doing photos, but what about when the newton is only be used for viewing? Coma corrector needed?

 

Any ideas? I would appreciate very much any help, hint and ideas for this!

 

Kind regards from MunichtAtNight

 

Edited by MunichAtNight, 05 December 2020 - 03:53 AM.

[Jon Isaacs]

Any ideas? I would appreciate very much any help, hint and ideas for this!

 

 

Hello and to Cloudy Night.  

 

Over the years I have been fortunate enough to acquire several Dobs and several are applicable to your situation, the 10 inch F/5 tube Dob, a 12.5 inch F/4.06 truss that was a tube Dob, the 13.1 inch F/5.5 truss Dob and the 16 inch F/4.4 that was a tube Dob and is now a strut/truss scope.

 

For a 300mm aperture or greater, a truss style scope is the best choice.  It takes up so much less room in the car.  This is the 318 mm F/4.06:

 

 

Coma:  At F/5, coma can be seen and a coma corrector is nice to have but one can get by without one.  At F/4, I consider it a necessity. I sometimes view with the coma corrector in my F/5 scopes and even the F/4.4's but never the F/4.06.

 

Each increase in aperture is worthwhile.  The step up from the 250mm F/5 to the 318mm F/4 makes a noticeable difference, the step up from the 318mm to the 406mm, it very apparent in terms of what i see.  

 

Jon

[Starman1]

I think you meant "I sometimes view without the coma corrector in my f/5"

[MitchAlsup]

Thinking based only on exit pupil,

Yes, think about exit pupil

 

But Mel Bartels takes the position that what you want is 1 EP and multiple scopes--one scope for each desired FoV.

So if you are looking at all of M31, you want something like a 8" F/3 and a 21E, but if you are looking at Saturn nebula you want something like 40" F/3 for the light and for the (smaller) FoV and you can still use that same 21E !