42 replies to this topic

[N3p]

This is a question about my secondary mirror and the Mel's calculator.

 

Starting this thread following Don's suggestion to do so, good idea.

 

===================

https://www.cloudyni...ussdob-project/

post #20

 

"Don H"

 

For your diagonal, a 2.1" may still be good for 12" instead of 12.5". Others may have better supported opinions to go one way or another. If you start a thread just for that, those who know most might reply. Here is a screenshot form Mel's calculator showing both. The 9.5" distance from diagonal to focal plane might be the deciding factor.

 

===================

This is a screenshot of the calculator with my settings + a GSO focuser Crayford, my first choice of focuser.

 

https://postimg.cc/Js1HC9jg

 

My objective is to have a general purpose telescope good on Planets and DSOs, for visual only and not really for tasks requiring a very large fully illuminated field such as photography and variable star observing.

 

BUT I don't want to notice any dimming at the outer edge of the field while observing.

 

Which secondary would you recommend between the 2.14 and 2.6 minor axis for my project?

Edited by N3p, 17 June 2022 - 10:35 PM.

[Scott E]

My 12.4" f/5.5 uses a 2.14" secondary (17%) obstruction. One thing you need to keep in mind is the inside diameter of the tube. My scope is only about 13.5" at the far end. This means that, no matter how big the secondary, the tube limits how much off-axis light will make it to the eyepiece. For mine, that means that I could make the secondary bigger, and that would increase the diameter of the fully illuminated field (to a point) but the edge will never be 75 %.

 

Try using Newt: https://stellafane.o...b/newt-web.html

 

This shows the ray trace graphically (including your tube) and you'll see what I mean.

[Garyth64]

There are 4 things that influence the size of a secondary: Diameter of the primary, its focal length, the distance between the secondary and the image plane, and the diameter of the image plane.

As said, this scope will not be used for photography, where a larger than normal illuminated field would be used, for a larger than normal secondary.  But, a larger illuminated field may also be wanted for the larger 2" eyepieces.

 

According to Texereau's book, he uses a illuminated field diameter that corresponds to the scope's focal length.  He multiplies the focal length by .009. So for your scope, and looking at post #20, the scope is a 12" f/5.5 with a focal length of 66", and the distance between the secondary and the image plane is 9.5" (?).

 

So, for those parameters, and an illuminated field of .6", we get:

 

(12" - .6" )  x  9.5"    +    .6"       =      2.24"

            66"

 

If it were my scope, I think I would try a 2.24" secondary, but it only blocks 3.6% of the incoming light.  Going to a 2.6" secondary, it only blocks 4.3%, .7% more of the incoming light.  So a larger secondary would work well too.  And if you wanted a illuminated field of 1", the 2.6" secondary would also work.

 

I don't use Mel's calculator, I like doing it the old-school way.

Edited by Garyth64, 18 June 2022 - 08:33 AM.

[jpcannavo]

This is one of those questions that can lead to analysis by paralysis in the attempt to balance the relevant constraints. To try and avoid this I find it helpful to think in terms of optimizing different kinds of observing. So let’s look at 12” F6. For high power/high contrast work, with the 2.6” you are at approx 22% central obstruction. I think this still small enough to allow excellent visual work with excellent seeing, and you would be hard pressed to see much contrast/MTF gain at 17% with the 2.1”  Now consider the other observing end, i.e. low power/high etendue  viewing.  Consider that you are at F6. This means that you need quite long focal length EPs to get to lowest useable powers (I.e. w/o exceeding your fully dilated pupil). And here you will encounter some large aperture stops:

 

41 mm Panoptic with 46 mm field stop takes you 0.9” off axis ( just under 7 mm exit pupil)
31 mm Nagler with 41 mm field stop takes you 0.8” off axis

At somewhat higher power/smaller exit pupil the 21 mm Ethos with its 100 degree field has a 36 mm field stop which still takes you 0.7” off axis

etc…

So now look at vignetting with the 2.1”. For my taste this is a bit too tight. On the other hand 2.6” is maybe on the generous size. So absent a custom in between size, I like to go with Mike Lockwood’s rule of thumb and err on the larger size when stuck in the middle. So in short, for me, in an all purpose 12” F6, the gain in contrast/MTF with 2.1” vs  2.6” does not offset the loss of field illumination. I’d choose the 2.6”.

But that’s just one opinion! Speaking of ML, his piece on diagonal size really pares it down in a nice way.

 

http://www.loptics.c.../diagonals.html

 

Joe

[N3p]

My 12.4" f/5.5 uses a 2.14" secondary (17%) obstruction. One thing you need to keep in mind is the inside diameter of the tube. My scope is only about 13.5" at the far end. This means that, no matter how big the secondary, the tube limits how much off-axis light will make it to the eyepiece. For mine, that means that I could make the secondary bigger, and that would increase the diameter of the fully illuminated field (to a point) but the edge will never be 75 %.

 

Try using Newt: https://stellafane.o...b/newt-web.html

 

This shows the ray trace graphically (including your tube) and you'll see what I mean.

In my case if the mirror is 12", the inside diameter of the upper tube assembly (UTA) will be 14" + the wall, a wrapping of around, maybe 1/8" thick and then a wooden board of about 1/2" or 5/8" where the focuser will be resting.

 

From the center of the diagonal through the exterior wall of the UTA we talk about a distance of 7.62" ~ 7.75", then the focal plane is at 10.42" with the GSO focuser according to Mel's calculator. 

 

I'll test the numbers with the Stellafane numbers today and see.
 

[N3p]

There are 4 things that influence the size of a secondary: Diameter of the primary, its focal length, the distance between the secondary and the image plane, and the diameter of the image plane.

As said, this scope will not be used for photography, where a larger than normal illuminated field would be used, for a larger than normal secondary.  But, a larger illuminated field may also be wanted for the larger 2" eyepieces. Good point,  I will use with the telescope my 34mm 68d explore scientific with a field stop of 38.6mm, if it's possible, I don't want vignetting!!!

 

According to Texereau's book, he uses a illuminated field diameter that corresponds to the scope's focal length.  He multiplies the focal length by .009. So for your scope, and looking at post #20, the scope is a 12" f/5.5 with a focal length of 66", and the distance between the secondary and the image plane is 9.5" (?). Hemm with the specific GSO focuser in Mel,s calculator, the focal plane diagonal distance is 10.42"

 

So, for those parameters, and an illuminated field of .6", we get:

 

(12" - .6" )  x  9.5"    +    .6"       =      2.24" (or 2.399 with the GSO)

            66"

 

If it were my scope, I think I would try a 2.24" secondary, (I was told to buy Antares secondary, they apparently have the best secondary, I need to pick either 2.14, 2.6 or 3.1) but it only blocks 3.6% of the incoming light.  Going to a 2.6" secondary, it only blocks 4.3%, .7% more of the incoming light.  So a larger secondary would work well too.  And if you wanted a illuminated field of 1", the 2.6" secondary would also work. That is a really good question, I really don't understand what amount of fully illuminated field is required. I don't want to notice dimming at the end of the field and vignetting with my 2" eyepiece.

 

I don't use Mel's calculator, I like doing it the old-school way. (Ok)

 

[N3p]

This is one of those questions that can lead to analysis by paralysis in the attempt to balance the relevant constraints. To try and avoid this I find it helpful to think in terms of optimizing different kinds of observing. So let’s look at 12” F6. For high power/high contrast work, with the 2.6” you are at approx 22% central obstruction. I think this still small enough to allow excellent visual work with excellent seeing, and you would be hard pressed to see much contrast/MTF gain at 17% with the 2.1”  Now consider the other observing end, i.e. low power/high etendue  viewing.  Consider that you are at F6. This means that you need quite long focal length EPs to get to lowest useable powers (I.e. w/o exceeding your fully dilated pupil). And here you will encounter some large aperture stops:

 

41 mm Panoptic with 46 mm field stop takes you 0.9” off axis ( just under 7 mm exit pupil)
31 mm Nagler with 41 mm field stop takes you 0.8” off axis

At somewhat higher power/smaller exit pupil the 21 mm Ethos with its 100 degree field has a 36 mm field stop which still takes you 0.7” off axis

etc…

So now look at vignetting with the 2.1”. For my taste this is a bit too tight. On the other hand 2.6” is maybe on the generous size. So absent a custom in between size, I like to go with Mike Lockwood’s rule of thumb and err on the larger size when stuck in the middle. So in short, for me, in an all purpose 12” F6, the gain in contrast/MTF with 2.1” vs  2.6” does not offset the loss of field illumination. I’d choose the 2.6”.

But that’s just one opinion! Speaking of ML, his piece on diagonal size really pares it down in a nice way.

http://www.loptics.c.../diagonals.html

 

(Ok i read the webpage right now, my telescope will be 12" F5.5)

 

Joe

 

[N3p]

 

Ah man I wish I could skip that part, as I am very poor at seeing things from numbers.

[Jon Isaacs]

I agree with Joseph. You want a general purpose scope, the 2.14 inch secondary is just too small.

 

- The fully illuminated field is only about 0.25°. 

 

- If you're using an Astros stems style secondary holder, the secondary will actually only be about 2.0" in diameter 

 

- Secondary positioning will be more critical 

 

-  You will be using the edge of the secondary at high magnifications. The edge is likely to be the least flat part of the secondary, a larger secondary gets those high power views away from the edge.

 

- At F/5.5, eyepieces like the 41 mm Panoptic make good sense. It's down about 0.6 magnitudes at the edge with a 2.14 inch secondary.

 

This a screen shot of Mel's calculator with a 1.8 inch = 46 mm field stop.

 

 

A 22% diagonal is plenty small enough for planetary. Mak owners make sacrifices just to get a 22% CO.

 

Jon

 

 

[N3p]

I agree with Joseph. You want a general purpose scope, the 2.14 inch secondary is just too small. Thanks for your opinion, very appreciated.

 

- The fully illuminated field is only about 0.25°. 

 

- If you're using an Astros stems style secondary holder, the secondary will actually only be about 2.0" in diameter I don't see any other option then the Astrosystem secondary holder right now, so by default, it's going to be that. When you say that the secondary will be 2.0, do you mean because of the lip around the secondary mirror, like this picture bellow?

 

 

- Secondary positioning will be more critical 

 

-  You will be using the edge of the secondary at high magnifications. The edge is likely to be the least flat part of the secondary, a larger secondary gets those high power views away from the edge.

 

- At F/5.5, eyepieces like the 41 mm Panoptic make good sense. It's down about 0.6 magnitudes at the edge with a 2.14 inch secondary. (looks like a bit too much)

 

This a screen shot of Mel's calculator with a 1.8 inch = 46 mm field stop. (ahh ok ok I see it now, a 1.8 max field diameter, with the 2.6" I get 0.8" fully highly illuminated diameter THEN I get a gradual loss of magnitude from 0.4 to 0.9 at the end of that field, 0.9 is were I get a total maximum loss of +-0.33 magnitude which is within the acceptable limits.

 

Conclusion I should not notice any light dimming at the edge during visual and I would get no vignetting with something like a 41mm Panoptic.

Screenshot_20220618-081322.png

 

A 22% diagonal is plenty small enough for planetary. Mak owners make sacrifices just to get a 22% CO. Ok, looks like it's settled for the Antares 2.6mm minor axis mirror with the AstroSystem holder and spider.

 

Jon

Edited by N3p, 18 June 2022 - 11:30 AM.

[Garyth64]

For the illuminated field diameter that Texereau uses for his calculations, he used the diameter of the Moon in the sky, 1/2*.  So with a scope with a 66" f.l. the diameter of the illuminated field is the diameter of the Moon at the image plane which will be 66" x .009 or .59".

[N3p]

Hello, I am back... Back again to discuss secondary mirrors! it's not over. CN's collective brainpower is required once more.

 

I am looking at a GSO 12" F5 tube right now, checked the specs and the secondary mirror they installed on it is kind of huge, 88mm minor axis, nothing less then a 29% obstruction.

 

Using the calculator, of course, the field is fully illuminated with a 3.5mm mirror (88mm) but 9% of aperture is lost because of the choice of secondary, so maybe a lost of contrast.

 

(With the GSO Crayford focuser)

 

But that is not all, It,s possible to buy the mirror only that is a 12" F5, surprise, they recommend a 70mm secondary in the chart (18mm smaller)

 

https://agenaastro.c...-f-5-ad017.html

 

What's going on.. I am confused and a bit tired of this, what do you think of this ?

Edited by N3p, 22 June 2022 - 08:33 PM.

[MitchAlsup]

An mid-sized F/5 should need a secondary between 18% and 20% (general purpose:: 25% photographic:: 16% planetary)

 

(I am getting away with 17% CO in my 20" F/4)

[Scott E]

It's not the light loss from the central obstruction that matters (and I mean you would NEVER see a difference). It's the the added diffraction! That you will see. There is absolutely NO reason to go bigger than 2.6" for your scope, Even for photography. Your fully illuminated field is about 3/4 of a degree and still over 75% at the edge. I know you're worried about vignetting, but again, you won't see falloff that small, NEVER! You'd be hard-pressed to see a drop of 40%. I know you don't believe that but you won't.

 

Also, I glued my 2.14" secondary with silicon and 2.6" is still small enough to glue (no shroud). The shroud just adds central obstruction and cuts aperture. It's only slight but if you don't need it, don't use it.

[Jon Isaacs]

Np3:

 

Are you sure the tube you are looking at is F/5?  The 88 mm secondary is stock at F/4.

 

Also, you need to be using the actual focal plane to secondary distance. 10.4" is probably close but I'd guess 11". 

 

7" tube + 3" focuser + 1" back focus. 

 

Jon

[brebisson]

Hello,

 

Or, you keep the 2nd mirror and you can create a realy short/compact telescope! with a long drawtube/focusser..

[N3p]

Np3:

 

Are you sure the tube you are looking at is F/5?  The 88 mm secondary is stock at F/4.

 

Also, you need to be using the actual focal plane to secondary distance. 10.4" is probably close but I'd guess 11". 

 

7" tube + 3" focuser + 1" back focus. 

 

Jon

Yes I was told that the 12" F5 is coming with the 88mm secondary, the same specifications as on the Agena site:

https://agenaastro.c...description-tab

But I raised that point to the seller and it's not a problem for them to supply the telescope with a 70mm minor axis mirror instead of a the 88. 

 

This is the illumination with the focal plane at 11". Without overthinking this too much, I have 2 choices, 70 or 88, according to the numbers from the calculator it appears to me that the 70mm (2.76") is the best choice of the 2.

 

I just want to be certain first.

 

 

 

It's not the light loss from the central obstruction that matters (and I mean you would NEVER see a difference). It's the the added diffraction! That you will see. There is absolutely NO reason to go bigger than 2.6" for your scope, Even for photography. Your fully illuminated field is about 3/4 of a degree and still over 75% at the edge. I know you're worried about vignetting, but again, you won't see falloff that small, NEVER! You'd be hard-pressed to see a drop of 40%. I know you don't believe that but you won't.

 

Also, I glued my 2.14" secondary with silicon and 2.6" is still small enough to glue (no shroud). The shroud just adds central obstruction and cuts aperture. It's only slight but if you don't need it, don't use it.

Ok I thought that the size of the central obstruction was the indicator of more or less diffraction. For instance, a larger secondary would cause more diffraction then a smallest possible choice.

 

I read an article by a guy called Gary he says that the aim is to keep the central obstruction at about 20% with the smallest possible secondary that will not cause a drop of illumination too drastic at the end of the field, at least nothing that can be noticeable visually, in case the goal is visual. (So it's probably more then 20% obstruction but certainly less then 30%)

 

My current F8 F5 8" has an obstruction of 28.x% but it's designed for astrophotography, they probably wanted to have a very good illumination for photography.

Edited by N3p, 23 June 2022 - 08:25 AM.

[Piero DP]

The secondary mirror of my 12" f/6 dob is 2.25" (effective). The focuser is a low profile helical Antares FOCH and I use it with 1.25" eps only without PC2. Because of these choices, this size is more than sufficient and a smaller size would probably still work fine.

Having said that, in my opinion a secondary mirror of decent size is in general a better choice because it relaxes a lot of constraints whilst adding negligible downsides.

[Jon Isaacs]

It's not the light loss from the central obstruction that matters (and I mean you would NEVER see a difference). It's the the added diffraction! That you will see. There is absolutely NO reason to go bigger than 2.6" for your scope, Even for photography. Your fully illuminated field is about 3/4 of a degree and still over 75% at the edge. I know you're worried about vignetting, but again, you won't see falloff that small, NEVER! You'd be hard-pressed to see a drop of 40%. I know you don't believe that but you won't.

 

Also, I glued my 2.14" secondary with silicon and 2.6" is still small enough to glue (no shroud). The shroud just adds central obstruction and cuts aperture. It's only slight but if you don't need it, don't use it.

 

The fully illuminated field with the 2.6 inch secondary is actually ~0.38 degrees.... What would be difficult to see is the difference between a 2.6 inch secondary and a 70mm = 2.75" secondary.  

 

Silicone can lead to pinched optics.. 

 

Jon

[Piero DP]

Silicone can lead to pinched optics..

Jon

Agreed. That's a real concern.

Even worse, gluing optics to the support gives in general unpredictable results. One finds out by testing the telescope during the year as temperatures change.. it's a quick but trial and error approach which can lead to frustration and waste of precious observing time.

Generally speaking for both primary and secondary mirrors, with an adequate cell / holder, the outcome is reasonably predictable instead (e.g. are the optics incorrectly supported laterally / behind? Expect astigmatism and / or spherical aberration).

[Vic Menard]

The link in post #17 is for a 12-inch f/5 Newtonian Reflector Telescope.

 

If you read closer, item 4-Focuser and Accessories, "...comes with a 1.25" eyepiece adapter with a compression ring to match 1.25" eyepieces and accessories, and it includes a 35mm-long 2" extension tube to let you achieve focus with a 1.25" or 2" eyepiece when using the telescope for visual use."

 

The secondary mirror is configured for photo/visual use (and a longer secondary mirror to focal plane distance to accommodate a camera body). I would not try to optimize the secondary mirror to reduce the central obstruction without addressing the longer intercept distance, which will likely require a longer OTA.

 

The scope is incorrectly advertised--it should read, 12-inch f/5 Photo/Visual Newtonian Reflector Telescope.

Edited by Vic Menard, 23 June 2022 - 08:17 AM.

[Garyth64]

"What's going on.. I am confused and a bit tired of this, what do you think of this ?"

 

In #12, the value you have for the field diameter is 1.6".  That is why it suggests a large secondary of 3.5"  (88mm). 

Since you're not using the scope for photography, you do not need that large of a secondary.    If you plugged in a field diameter of 1", the secondary size would reduce to 2.9".

 

And according to Texereau, a secondary size, with an field diameter (or diameter of the image plane) of .54", (60 x .009), then you could get away with a 2.53" secondary.

[N3p]

The link in post #17 is for a 12-inch f/5 Newtonian Reflector Telescope.

 

If you read closer, item 4-Focuser and Accessories, "...comes with a 1.25" eyepiece adapter with a compression ring to match 1.25" eyepieces and accessories, and it includes a 35mm-long 2" extension tube to let you achieve focus with a 1.25" or 2" eyepiece when using the telescope for visual use."

 

The secondary mirror is configured for photo/visual use (and a longer secondary mirror to focal plane distance to accommodate a camera body). I would not try to optimize the secondary mirror to reduce the central obstruction without addressing the longer intercept distance, which will likely require a longer OTA.

 

The scope is incorrectly advertised--it should read, 12-inch f/5 Photo/Visual Newtonian Reflector Telescope.

Ah this is exactly the kind of information I was looking for, thanks!

 

I see the logic behind GSO's choice to install a 88mm instead of a 70mm, it's the same thing as with my current F5 8" telescope, because of photography. I think it's a pretty good telescope even with a large secondary.

 

I'll follow your advice Vic and keep the original choice, the 88mm mirror.

 

[Starman1]

This is a question about my secondary mirror and the Mel's calculator.

 

Starting this thread following Don's suggestion to do so, good idea.

 

===================

https://www.cloudyni...ussdob-project/

post #20

 

"Don H"

 

For your diagonal, a 2.1" may still be good for 12" instead of 12.5". Others may have better supported opinions to go one way or another. If you start a thread just for that, those who know most might reply. Here is a screenshot form Mel's calculator showing both. The 9.5" distance from diagonal to focal plane might be the deciding factor.

 

===================

This is a screenshot of the calculator with my settings + a GSO focuser Crayford, my first choice of focuser.

 

https://postimg.cc/Js1HC9jg

 

My objective is to have a general purpose telescope good on Planets and DSOs, for visual only and not really for tasks requiring a very large fully illuminated field such as photography and variable star observing.

 

BUT I don't want to notice any dimming at the outer edge of the field while observing.

 

Which secondary would you recommend between the 2.14 and 2.6 minor axis for my project?

That depends on the intercept distance (secondary to focal plane) and the size of the field in your lowest power eyepiece.

One way to use Mel's calculator is to enter the fieldstop of your lowest power eyepiece in the illuminated field size (e.g. 31mm Nagler = 1.65"), and select the diagonal size that doesn't descend below -0.4 magnitudes at the edge of the field shown.

 

As to what the intercept distance is, it is the sum of:

--mirror radius

--clearance to inner tube wall

--thickness of tube

--height of base of focuser off the tube

--height of focuser racked all the way in

--1" additional to focal plane to allow things like barlows and coma correctors to come to focus and have nearsighted friends look through the scope.

 

So, take a 12" scope.  It is hard to see an intercept distance in a properly designed scope will be less than about 114mm.

As a rule of thumb, I always add 4.5" (114mm) to the mirror radius.

Why?

A minimum of 19mm clearance between mirror and tube to keep upper opening from vignetting the incoming light and allow ventilation of the tube without significant air currents in the optical path.

A usual 25mm between the base of the focuser and the I.D. of the upper tube assembly to allow for the focuser board and ring and to keep the drawtube out of the optical path of the scope.

A low profile focuser 45mm tall.  In reality, many are taller.

A focal plane clearance of 25mm over the focuser.

Hence, 114mm.

 

If any of those numbers is larger than that, then you will add length to the intercept distance.  If the scope is used for photography, the secondary is larger (as it is in your example).

A purely visual 12" f/5 scope would be fine with a much smaller 66mm secondary, but that would assume a shorter intercept distance.

 

So, plugging in the numbers, and keeping the edge of field light loss to < 0.4 magnitudes, 

[Starman1]

"What's going on.. I am confused and a bit tired of this, what do you think of this ?"

 

In #12, the value you have for the field diameter is 1.6".  That is why it suggests a large secondary of 3.5"  (88mm). 

Since you're not using the scope for photography, you do not need that large of a secondary.    If you plugged in a field diameter of 1", the secondary size would reduce to 2.9".

 

And according to Texereau, a secondary size, with an field diameter (or diameter of the image plane) of .54", (60 x .009), then you could get away with a 2.53" secondary.

A field diameter of 1.65" is entered if the low power eyepiece is, say, a 31mm Nagler, and that is a distinct possibility on a 12" scope.