Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Question about f/

  • Please log in to reply
39 replies to this topic

#26 pjmulka

pjmulka

    Apollo

  • *****
  • topic starter
  • Posts: 1,071
  • Joined: 17 Sep 2015

Posted 10 August 2022 - 12:29 AM

So what your saying is that faster scopes can achieve just as large an image with the use of a higher magnification eyepiece? 

Hi:

 

This is the way I think about it:

 

Magnification is a choice, I have a selection of eyepieces, I know the aperture and focal length of the scope,  I choose the eyepiece to provide the view I want.  Don't worry about focal ratio/focal length, there are eyepieces, Barlows to achieve any magnification.

 

For a Dob, the eyepiece height will be very close to the focal length. I'm 6ft. For me, a 68 inch eyepiece height is about the maximum without a stool or ladder. 

 

For planetary, mirror quality is more important than focal ratio. Slower mirrors are easier to make.

 

Personally, I think F/6 is a nice place to be. F/7 is too tall.

 

Jon



#27 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 103,515
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 10 August 2022 - 05:56 AM

So what your saying is that faster scopes can achieve just as large an image with the use of a higher magnification eyepiece? 

Yes.

 

The magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece.

 

If your 10 inch is F/8, then it has a focal length of 80 inches or 2000 mm    If you want 200x, then you can use a 10mm, eyepiece,   If it is F/6, then it has a focal length of 60 inches = 1500 mm.to get that same 200x, you can use a 7.5mm eyepiece.  

 

Jon



#28 pjmulka

pjmulka

    Apollo

  • *****
  • topic starter
  • Posts: 1,071
  • Joined: 17 Sep 2015

Posted 10 August 2022 - 07:48 AM

Okay, I’m just trying to understand this. Then what would be the point of a long focal length mirror? Why would anyone ever make a f/8 or a f/10 for that matter? What’s the advantage of a long mirror?

Yes.

The magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece.

If your 10 inch is F/8, then it has a focal length of 80 inches or 2000 mm If you want 200x, then you can use a 10mm, eyepiece, If it is F/6, then it has a focal length of 60 inches = 1500 mm.to get that same 200x, you can use a 7.5mm eyepiece.

Jon



#29 carolinaskies

carolinaskies

    Skylab

  • *****
  • Posts: 4,125
  • Joined: 12 Dec 2014
  • Loc: Greenville SC

Posted 10 August 2022 - 08:01 PM

Okay, I’m just trying to understand this. Then what would be the point of a long focal length mirror? Why would anyone ever make a f/8 or a f/10 for that matter? What’s the advantage of a long mirror?

 


>>"Yes.

 

The magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece.

 

If your 10 inch is F/8, then it has a focal length of 80 inches or 2000 mm    If you want 200x, then you can use a 10mm, eyepiece,   If it is F/6, then it has a focal length of 60 inches = 1500 mm.to get that same 200x, you can use a 7.5mm eyepiece. "<<<

Jon's assessment seems correct on it's face doesn't it?   But there's a big caveat most people who suggest this method either miss or aren't aware of in how they learned this way.  It works for non-obstructed telescopes, specifically refractors.  If you buy a 150mm F/5 refractor vs an F/8 refractor then indeed a smaller focal length eyepiece will provide the magnification.  Of course this omits discussion about field curvature at fast focal lengths.  But what about obstructed optics like Newtonians?  This is where the eyepiece change doesn't also consider the effect of a large secondary mirror. 

Planetary Newtonians are optimized by making the central obstruction as small as possible to keep the light concentrated in the center of the diffraction pattern and the center of the field of view where the planet is viewed without losing fully illuminating the eyepiece.  This increases the resolution of small details on the planets.  Conversely a widefield(and low focal ratio) Newtonian uses a larger secondary to maximize the focal plane across a larger area of the eyepiece, spreading the light to benefit using more of the exit pupil making dimmer targets brighter. If they used a smaller secondary some of the light would never get reflected out to the eyepiece and it would be lost out the front of the telescope.   

You can find many discussions in ATM forums about sizing the secondary with much debate.  For planetary Newtonians though there is no need to debate because the longer focal length allows the small central obstruction.  There are many discussions about the effects of obstructions and percentages of obstructions to make the Newtonian perform as well as a refractor supposedly does.  But you're not going to find an 8" or 10" refractor and definitely not one that would be inexpensive. 

The other issue with fast Newtonians is coma; when the focal length is shortened the curvature of space of the light reflected by the parabolic mirror increases.  A planetary Newtonian of F/8 doesn't require a coma corrector/Paracorr to correct the general field of view, but even at F/6 a coma corrector improves a good 15-20% of the outer portion of the focal plane. 

Now, if you're not adverse to having to regularly employ the extra weight of the coma corrector you can definitely shorten the focal length.  But be aware the sharpness of small detail is going to drop off as the secondary increases in size and you're using another lens in your system to make corrections.

On quality of mirror surface. If you are capable of grinding a very good 1/20th wave mirror, you can mitigate some of the differences suffered above because more light will concentrate in the inner diffraction.  If you achieve 1/4 or 1/8 wave, on the other hand, a faster mirror will not be nearly as crisp for planetary.   

I've seen planets through very large fast Dobsonians, but those optimized for planetary are always better ,even if smaller in aperture.  It's the nature of the optics game played with Newtonians.  

You have to decide for yourself are you willing to compromise a bit somewhere, and where that will be, either focal length, secondary size, or mirror quality.  


Edited by carolinaskies, 10 August 2022 - 08:11 PM.

  • pjmulka likes this

#30 pjmulka

pjmulka

    Apollo

  • *****
  • topic starter
  • Posts: 1,071
  • Joined: 17 Sep 2015

Posted 11 August 2022 - 02:13 AM

So let me ask you this is a 10” f/7.5 slow enough to be considered a planetary scope?




>>"Yes.

The magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece.

If your 10 inch is F/8, then it has a focal length of 80 inches or 2000 mm If you want 200x, then you can use a 10mm, eyepiece, If it is F/6, then it has a focal length of 60 inches = 1500 mm.to get that same 200x, you can use a 7.5mm eyepiece. "<<<

Jon's assessment seems correct on it's face doesn't it? But there's a big caveat most people who suggest this method either miss or aren't aware of in how they learned this way. It works for non-obstructed telescopes, specifically refractors. If you buy a 150mm F/5 refractor vs an F/8 refractor then indeed a smaller focal length eyepiece will provide the magnification. Of course this omits discussion about field curvature at fast focal lengths. But what about obstructed optics like Newtonians? This is where the eyepiece change doesn't also consider the effect of a large secondary mirror.

Planetary Newtonians are optimized by making the central obstruction as small as possible to keep the light concentrated in the center of the diffraction pattern and the center of the field of view where the planet is viewed without losing fully illuminating the eyepiece. This increases the resolution of small details on the planets. Conversely a widefield(and low focal ratio) Newtonian uses a larger secondary to maximize the focal plane across a larger area of the eyepiece, spreading the light to benefit using more of the exit pupil making dimmer targets brighter. If they used a smaller secondary some of the light would never get reflected out to the eyepiece and it would be lost out the front of the telescope.

You can find many discussions in ATM forums about sizing the secondary with much debate. For planetary Newtonians though there is no need to debate because the longer focal length allows the small central obstruction. There are many discussions about the effects of obstructions and percentages of obstructions to make the Newtonian perform as well as a refractor supposedly does. But you're not going to find an 8" or 10" refractor and definitely not one that would be inexpensive.

The other issue with fast Newtonians is coma; when the focal length is shortened the curvature of space of the light reflected by the parabolic mirror increases. A planetary Newtonian of F/8 doesn't require a coma corrector/Paracorr to correct the general field of view, but even at F/6 a coma corrector improves a good 15-20% of the outer portion of the focal plane.

Now, if you're not adverse to having to regularly employ the extra weight of the coma corrector you can definitely shorten the focal length. But be aware the sharpness of small detail is going to drop off as the secondary increases in size and you're using another lens in your system to make corrections.

On quality of mirror surface. If you are capable of grinding a very good 1/20th wave mirror, you can mitigate some of the differences suffered above because more light will concentrate in the inner diffraction. If you achieve 1/4 or 1/8 wave, on the other hand, a faster mirror will not be nearly as crisp for planetary.

I've seen planets through very large fast Dobsonians, but those optimized for planetary are always better ,even if smaller in aperture. It's the nature of the optics game played with Newtonians.

You have to decide for yourself are you willing to compromise a bit somewhere, and where that will be, either focal length, secondary size, or mirror quality.



#31 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 103,515
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 11 August 2022 - 04:58 AM

I've seen planets through very large fast Dobsonians, but those optimized for planetary are always better ,even if smaller in aperture.  It's the nature of the optics game played with Newtonians.

 

 

Have you ever owned a large aperture Newtonian with good quality optics? It doesn't seem like it.  I can say that currently I have five Dobsonians, a 10 inch F/5, a 12.5 inch F/4.06, a 13.1 F/5.5, a 16 inch F/4.4 and a 22 inch F/4.4.  

 

Under excellent seeing, the planetary views in the 22 inch F/4.4 are to die for, you don't get views like that in smaller scopes.  I know a little bit about Newtonian design, both from a practical point of view and a theoretical point of view.  

 

- Planetary viewing is primarily about the seeing, the stability of the atmosphere.  If you are looking an air column that is not steady, then that degrades the quality of the view. Generally, it is the seeing that limits the contrast and resolution.  Optical quality of the mirror is important, Thermal equilibrium is important. From Detroit, the seeing is going to be the limiting factor the vast majority of the time. 

 

- The effect of the central obstruction is relatively small.  People use SCTs and Maks with much larger central obstructions (35%) for viewing the planets, the CO has the same effect with a SCT as it does with a Newtonian.  One can design an 10 inch F/6 Newtonian with a low profile focuser and a 21% CO and the loss of contrast will be minimal.  

 

http://www.bbastrode...r.html#diagonal

 

- In designing a practical telescope, the length of the OTA is one of the most important because it determines ergonomics, the comfort level.  A 10 inch F/7.5 might provide slightly more planetary contrast than a 10 inch F/6,  but the scope will be awkward to use and manage, particularly for shorter people.  

 

- I consider my 13.1 inch F/5.5 to be an excellent scope for viewing the planets. It has an excellent mirror, a 20% CO and the seeing here can be excellent.  It's focal length is 72 inches, the eyepiece at the zenith is 72 inches.  That is taller than I would like but it's OK.  

 

- I would not build a 10 inch F/7.5 as a planetary scope, it's just too tall.  From 65 inches to 75 inches, from a practical point of view, those are critical inches.  

 

- Coma:  Coma is certainly something to be considered, most people find that at F/6 coma correction is not necessary.  I would probably use it because I have a coma corrector.  This evening, I was viewing the planets in my 10 inch F/5 without a coma corrector, the views were very good. 

 

- You are building a scope to view the planets with your family, the primary concern should be the ease of use by all the members or your family.  A "planetary telescope" is one designed for an experienced planetary observer trying to wring out every last bit of detail possible.  An all around scope will still provide amazing planetary views and will be used more often... 

 

Jon 


  • KWB likes this

#32 KWB

KWB

    James Webb Space Telescope

  • *****
  • Posts: 17,834
  • Joined: 30 Sep 2006
  • Loc: Westminster,Co Elev.5400 feet

Posted 11 August 2022 - 09:33 AM

Have you ever owned a large aperture Newtonian with good quality optics? It doesn't seem like it.  I can say that currently I have five Dobsonians, a 10 inch F/5, a 12.5 inch F/4.06, a 13.1 F/5.5, a 16 inch F/4.4 and a 22 inch F/4.4.  

 

Under excellent seeing, the planetary views in the 22 inch F/4.4 are to die for, you don't get views like that in smaller scopes.  I know a little bit about Newtonian design, both from a practical point of view and a theoretical point of view.  

 

- Planetary viewing is primarily about the seeing, the stability of the atmosphere.  If you are looking an air column that is not steady, then that degrades the quality of the view. Generally, it is the seeing that limits the contrast and resolution.  Optical quality of the mirror is important, Thermal equilibrium is important. From Detroit, the seeing is going to be the limiting factor the vast majority of the time. 

 

- The effect of the central obstruction is relatively small.  People use SCTs and Maks with much larger central obstructions (35%) for viewing the planets, the CO has the same effect with a SCT as it does with a Newtonian.  One can design an 10 inch F/6 Newtonian with a low profile focuser and a 21% CO and the loss of contrast will be minimal.  

 

http://www.bbastrode...r.html#diagonal

 

- In designing a practical telescope, the length of the OTA is one of the most important because it determines ergonomics, the comfort level.  A 10 inch F/7.5 might provide slightly more planetary contrast than a 10 inch F/6,  but the scope will be awkward to use and manage, particularly for shorter people.  

 

- I consider my 13.1 inch F/5.5 to be an excellent scope for viewing the planets. It has an excellent mirror, a 20% CO and the seeing here can be excellent.  It's focal length is 72 inches, the eyepiece at the zenith is 72 inches.  That is taller than I would like but it's OK.  

 

- I would not build a 10 inch F/7.5 as a planetary scope, it's just too tall.  From 65 inches to 75 inches, from a practical point of view, those are critical inches.  

 

- Coma:  Coma is certainly something to be considered, most people find that at F/6 coma correction is not necessary.  I would probably use it because I have a coma corrector.  This evening, I was viewing the planets in my 10 inch F/5 without a coma corrector, the views were very good. 

 

- You are building a scope to view the planets with your family, the primary concern should be the ease of use by all the members or your family.  A "planetary telescope" is one designed for an experienced planetary observer trying to wring out every last bit of detail possible.  An all around scope will still provide amazing planetary views and will be used more often... 

 

Jon 

waytogo.gif

 

Bingo



#33 carolinaskies

carolinaskies

    Skylab

  • *****
  • Posts: 4,125
  • Joined: 12 Dec 2014
  • Loc: Greenville SC

Posted 11 August 2022 - 10:37 AM

You described the best case scenario... 'a good mirror'. I covered that in my statement you glossed over. Also, of course a CC isn't used for planetary since only the center is used.

You may have big/fast dobs, but that doesn't mean you've side by side compared to true planetary optimized newts. I've had 150mm to 400mm dobs that are good, but prefer my SCTs now (150mm-400mm too) they are more ergonomic than my Newts were. And a well executed SCT indeed delivers nice performance with little contrast issues.

I did advise the OP to consider what he wants to prioritize.

A F/7.5 would be a good planetary 10" as well. Like I said, once ratios drop to F/6 it's the whole focal plane that starts exhibiting coma.

Family use of a scope always begs the question of how much will family participate. It might be more advisable to pick up a faster cheap xT8 which younger members will find much easier to use than a truss dob

#34 KWB

KWB

    James Webb Space Telescope

  • *****
  • Posts: 17,834
  • Joined: 30 Sep 2006
  • Loc: Westminster,Co Elev.5400 feet

Posted 11 August 2022 - 01:58 PM

A F/7.5 would be a good planetary 10" as well. Like I said, once ratios drop to F/6 it's the whole focal plane that starts exhibiting coma.

 

A 10 inch F/5 is a very good planetary scope for me. Coma is an optical defect that I have never found to be a deal killer or game changer when using an F/5 reflector. I can function without a coma corrector. My attention is focused in the middle of the field of view of the eyepiece I'm using. No coma there. Using a Nagler type eyepiece helps me when dealing with a defect that bothers me more at F/5, and that's eyepiece astigmatism.

 

What is a deal breaker for me is using a long focal length telescope that would be an 10 inch F/7.5. I'm 71 and 3/4 inches tall, just a freckle under 6 feet. With the telescope pointed at the zenith and both my feet planted on the ground, I'm not tall enough to view through the eyepiece. That requires I stand on a short ladder. This flunks my first requirement for using a telescope - ease of use.

 

My 10 inch F/5 dob fit in every 4 door car I've ever owned across the back seat and when I wanted to get out of my light polluted skies, it required little extra effort to transport it. An 10 inch F/7.5 solid tubed dobsonian would require the use of my Ford F-150 and it's a 4X4, in other words a gas guzzling Jesse in 2022 dollars. No thanks. That flunks my second requirement for using a telescope - ease of transport.

 

It's one thing to recommend a particular longer focal ratio design because on paper it could wring out the " last drop of performance" for the perfectionist. I'm an experience observer using dobsonian telescopes at home and away, I'll take the 10 inch F/5 from an ergonomic standpoint every day of the week and twice on Sunday. It is good enough for me. And plenty long enough to deal with as is.

 

The often used YMMV is applicable here.

 

VkFmSnw.jpg


  • Jon Isaacs likes this

#35 carolinaskies

carolinaskies

    Skylab

  • *****
  • Posts: 4,125
  • Joined: 12 Dec 2014
  • Loc: Greenville SC

Posted 11 August 2022 - 03:10 PM

Lolz... I put my 16" SCT on an electric wheelchair when I was 51 for ease of use. I learned in the 90s my comfort is more important than crooning about a 30" Dob that takes 40 minutes to pu together and requires a 10ft ladder.

If you like dob gymnastics which exist regardless of size don't forget the Doans pills at the end of the night!

The OP IS 5' 11", I'm 6'1" both taller than average. A 5'3" buddy built an F/4.5 16" while I just used a sonotube. A 3-step ladder was all needed for any adult on either if our scopes.

A 10" planetary isn't an issue except in your mind limiting it. And if using it for planets and moon will be well below zenith, so griping about that is disingenuous in my opinion.

Zenith operation is the worst observing location anyway and I and most of my buddies rarely spent our time vertically. At big events it was rare to see any big dob above 80 degrees for long except for special observing like Messier marathons, etc.

If the OP opts for faster optics that's his decision. And regardless of our opinions will decide what's best for his use. I've said my 2 cents about ratios, secondaries, etc... so I'm confident it will be taken as another opinion he'll use in figuring his next moves accordingly

#36 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 103,515
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 11 August 2022 - 03:32 PM


 

You may have big/fast dobs, but that doesn't mean you've side by side compared to true planetary optimized newts. I've had 150mm to 400mm dobs that are good, but prefer my SCTs now (150mm-400mm too) they are more ergonomic than my Newts were. And a well executed SCT indeed delivers nice performance with little contrast issues.

 

 

In your previous posts you point to the central obstruction of a Newtonian as causing issues with contrast.  The central obstructions of your SCTs are much larger, instead of being the 15%-25% of a Newtonian they are over 30%.   The smallest is the 31% CO in the 16 inch, A C-6 has a 37% CO.  

 

You cannot play the game both ways, the effect of the CO is to transfer energy from the central disk to the diffraction rights, this spreads out the energy and reduces the contrast. If you are OK with the 31% CO of your SCT, then a 20% CO of an 10 inch F/6 Newtonian provide significantly better contrast. 

 

It is the optical quality and the aperture that is critical, not the focal ratio, focal ratio is old school thinking.. 

 

jon


  • KWB likes this

#37 KWB

KWB

    James Webb Space Telescope

  • *****
  • Posts: 17,834
  • Joined: 30 Sep 2006
  • Loc: Westminster,Co Elev.5400 feet

Posted 11 August 2022 - 03:37 PM

A 10" planetary isn't an issue except in your mind limiting it. And if using it for planets and moon will be well below zenith, so griping about that is disingenuous in my opinion.

Zenith operation is the worst observing location anyway and I and most of my buddies rarely spent our time vertically. At big events it was rare to see any big dob above 80 degrees for long except for special observing like Messier marathons, etc.

 

How I use a telescope to observe as to what and where in the sky is up to me. I don't care about what you or anyone you observed did or didn't do when using theirs. I use my telescopes how I want. I stated my opinion, you stated yours. Your definition of 10 inch planetary differs from mine. My 10 inch F/5 is my planetary scope. I don't want to deal with a longer telescope. That's my privilege as to scope choice  and I'm entitled to it. One telescope has to be used to view at the zenith and viewing the planets so I'm not being disingenuous as to my preference for an F/5 to do it all.


  • Jon Isaacs likes this

#38 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 103,515
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 11 August 2022 - 03:44 PM

Lolz... I put my 16" SCT on an electric wheelchair when I was 51 for ease of use. I learned in the 90s my comfort is more important than crooning about a 30" Dob that takes 40 minutes to pu together and requires a 10ft ladder.

If you like dob gymnastics which exist regardless of size don't forget the Doans pills at the end of the night!

The OP IS 5' 11", I'm 6'1" both taller than average. A 5'3" buddy built an F/4.5 16" while I just used a sonotube. A 3-step ladder was all needed for any adult on either if our scopes.

A 10" planetary isn't an issue except in your mind limiting it. And if using it for planets and moon will be well below zenith, so griping about that is disingenuous in my opinion.

Zenith operation is the worst observing location anyway and I and most of my buddies rarely spent our time vertically. At big events it was rare to see any big dob above 80 degrees for long except for special observing like Messier marathons, etc.

If the OP opts for faster optics that's his decision. And regardless of our opinions will decide what's best for his use. I've said my 2 cents about ratios, secondaries, etc... so I'm confident it will be taken as another opinion he'll use in figuring his next moves accordingly

 

I am 74, it takes me 5 minutes to roll any of my Dobs out the door and be ready to go and that includes the 22 inch.  

 

My 22 inch F/4.4 requires a 3 step ladder.. My 16 inch F/4.4 requires no ladder. 

 

Near the zenith is the best place to observe the planets. That is where the air column is the least disturbed.  But the important thing to realize is that the eyepiece height is a sine function of elevation angle, the eyepiece height changes very little at higher elevatons. A 10 inch F/7 would have an eyepiece about 70 inches off the ground at the zenith.  At 70 degrees it would be about 67 inches, at 60 degrees it would about 64 inches.  

 

When it comes to designing a practical scope, ergonomics is more important that the size of the central obstruction. 

 

Jon


  • KWB likes this

#39 pjmulka

pjmulka

    Apollo

  • *****
  • topic starter
  • Posts: 1,071
  • Joined: 17 Sep 2015

Posted 15 August 2022 - 02:23 PM

This will be the second scope I’ve built. My first is a 6” dob build straight from the Stellafane website. I ground, Polished and Figured the mirror as well. My point is in reference to an earlier post. I have to drag my wife and kids to the eyepiece kicking and screaming so this new scope I’m working on is all for me! :)

#40 pjmulka

pjmulka

    Apollo

  • *****
  • topic starter
  • Posts: 1,071
  • Joined: 17 Sep 2015

Posted 15 August 2022 - 02:27 PM

I have new questions: I completed 120 sc and am almost done with 220 sc. I’ve been working exclusively MoT since I got close to my target sag. My question is at what grit do I not need to worry about changing the sag either way?


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics