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Equipment Discussions >> Reflectors

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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Secondary Sizing
      #5734136 - 03/15/13 01:21 PM

What's your criteria for the size of a secondary for a particular aperture and focal ratio?

Thanks,

Rob


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KerryR
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Re: Secondary Sizing new [Re: robininni]
      #5734251 - 03/15/13 02:42 PM

Here's a good article on the topic:

Sizing up Newtonian Secondary-- Gary Seronik

It should answer most of your questions.

Note that there is some debate regarding the size of the fully illuminated field.

Edited by KerryR (03/15/13 02:43 PM)


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FineArt
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Reged: 01/26/12

Re: Secondary Sizing new [Re: KerryR]
      #5734257 - 03/15/13 02:43 PM

There is no link.

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KerryR
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Re: Secondary Sizing new [Re: FineArt]
      #5734262 - 03/15/13 02:43 PM

Just fixed it.

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dan_h
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Reged: 12/10/07

Re: Secondary Sizing new [Re: robininni]
      #5734286 - 03/15/13 02:59 PM

I simply refer to Mike Lockwood's article regarding secondary sizes. It includes an easy to use chart.

http://www.loptics.com/ATM/diagonals.html

dan


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FineArt
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Reged: 01/26/12

Re: Secondary Sizing new [Re: dan_h]
      #5734317 - 03/15/13 03:18 PM

From the Gary Seronik article:

"Selecting the best size for the secondary mirror of a Newtonian telescope is an exercise in compromise. A diagonal that is too large will block incoming light and exaggerate image-harming diffraction effects, while one that is too small will fail to deliver all the light from the primary mirror to the eyepiece. Certainly, the second of these is the greater evil of the two."

That is true for premium optics. For a Cheap Chinese primary most of the problems will be around the outer edge of the mirror. I have set up my 10" scope to throw away the outer 1/2 inch of light giving me a 9" scope. The quality of the image on my sensor has improved greatly from the large secondary that came with the scope.

If I later get a resurfaced/high end mirror I will have to extend the distance between the 2 mirrors.


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FineArt
member


Reged: 01/26/12

Re: Secondary Sizing new [Re: FineArt]
      #5734425 - 03/15/13 04:11 PM

Here is a shot taken with that 10" dob.

http://farm9.staticflickr.com/8521/8559623069_ed2e864be5_o.jpg

No cropping. The image is not as good as a premium 300 prime. That's ok, the range is more comfortable for wildlife.


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: FineArt]
      #5734498 - 03/15/13 04:58 PM

I'm confused, here's a quote from Gary Seronik website about the secondary sizing:

"But how do you know that the focal length really is exactly 48 inches? One way to check is to use a clean length of wood or a wooden yardstick, if it is long enough. Slide this down the tube until it just makes contact with the edge of the primary mirror. Next, mark the wood where it crosses the middle of the focuser hole. Measure this distance with a tape measure, and add it to L. This is the telescope’s focal length. To find the minimum secondary size, simply divide L by f. In the case of an 8-inch f/6, L is often about 9 inches. Dividing 9 by 6 gives a minimum secondary size of 1.5 inches.

The same telescope with a low-profile focuser rather than the standard tall rack-and-pinion model might have L as little as 6 inches. Such a configuration would allow you to use a diagonal only 1 inch across. In fact, the most effective means of keeping the secondary small is to use a low-profile focuser. For a given telescope, no other design parameter will have as great an influence on secondary size as focuser height."


So my question is, how does a low profile focuser change where the focal point is? If "L" was calculated as 9" away from the center of the OTA, if you replace the standard focuser with a low profile version, isn't the image still going to come to focus 9" away from the center of the OTA?

I thought the low profile focuser just gave you more room for EPs needing more inward travel?

Thanks,

Rob


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Mirzam
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Re: Secondary Sizing new [Re: robininni]
      #5734504 - 03/15/13 05:04 PM

If you used a low profile focuser with an existing secondary then you are correct, all you gain is some additional in-focus.

However, if you wish to optimize the telescope to use the smallest possible secondary, you will find that the low profile focuser helps with this. It allows the secondary to be placed farther away from the primary, where the reflected cone of converging light is smaller in diameter.

JimC


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: Mirzam]
      #5734537 - 03/15/13 05:26 PM

Quote:

If you used a low profile focuser with an existing secondary then you are correct, all you gain is some additional in-focus.

However, if you wish to optimize the telescope to use the smallest possible secondary, you will find that the low profile focuser helps with this. It allows the secondary to be placed farther away from the primary, where the reflected cone of converging light is smaller in diameter.

JimC




Thanks for clearing it up for me. What you said is what I thought but just making sure I didn't misunderstand something.

Rob


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dan_h
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Reged: 12/10/07

Re: Secondary Sizing new [Re: robininni]
      #5734555 - 03/15/13 05:35 PM

Quote:

So my question is, how does a low profile focuser change where the focal point is? If "L" was calculated as 9" away from the center of the OTA, if you replace the standard focuser with a low profile version, isn't the image still going to come to focus 9" away from the center of the OTA?

I thought the low profile focuser just gave you more room for EPs needing more inward travel?

Thanks,

Rob




You are right in your thinking. The total focal length does not change. In the example given, when L changes from 9" to 6", there is 3" of optical path has to be added to the distance bewtween the primary and the secondary to move the focus position down to the new focuser. So you have to somehow adjust the distance between the primary and secondary mirrors to take up the space. Get longer truss poles or somehow move the primary. More than one person has got caught with this exact scenario when changing to a lower profile focuser.

dan


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ZuoZhao
sage


Reged: 10/16/12

Re: Secondary Sizing new [Re: dan_h]
      #5735066 - 03/15/13 09:55 PM

E....the min sizes means we must use offset or not ?

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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: ZuoZhao]
      #5735437 - 03/16/13 01:40 AM

Where can you buy good quality secondaries larger than 4"? I found several places with 4" and under, but I am looking for a 4.5 to 4.75" secondary, maybe even 5" when you consider the loss caused by the holder cover around the rim.

Thanks,

Rob


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Jon Isaacs
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Re: Secondary Sizing new [Re: robininni]
      #5735526 - 03/16/13 04:49 AM

Quote:

So my question is, how does a low profile focuser change where the focal point is? If "L" was calculated as 9" away from the center of the OTA, if you replace the standard focuser with a low profile version, isn't the image still going to come to focus 9" away from the center of the OTA?

I thought the low profile focuser just gave you more room for EPs needing more inward travel?

Thanks,

Rob




If the scope were designed with the low profile focuser in mind, your Obsession likely uses a JMI DX-1 which is under 2 inch high, then it allows the focal plane to be closer to the secondary which allows for the use of a small secondary.

I use Newt for the Web to check on secondary sizing. It's really a design program but you can use it to evaluate an existing scope. At this point, I am not sure what the motivation would be to increase the size of the secondary, any difference is going to be subtle.

Jon


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Mirzam
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Re: Secondary Sizing new [Re: robininni]
      #5735573 - 03/16/13 06:26 AM

I would try Tom Ozypowski:

Equatorial Platforms

or Mike Lockwood:

Lockwood Custom Optics

JimC


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Fred1
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Re: Secondary Sizing new [Re: Mirzam]
      #5735958 - 03/16/13 10:55 AM

You can call Terry Ostahowski.

And also, Astrosystems.

Edited by Fred1 (03/16/13 11:01 AM)


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: Jon Isaacs]
      #5736028 - 03/16/13 11:21 AM

Quote:

Quote:

So my question is, how does a low profile focuser change where the focal point is? If "L" was calculated as 9" away from the center of the OTA, if you replace the standard focuser with a low profile version, isn't the image still going to come to focus 9" away from the center of the OTA?

I thought the low profile focuser just gave you more room for EPs needing more inward travel?

Thanks,

Rob




If the scope were designed with the low profile focuser in mind, your Obsession likely uses a JMI DX-1 which is under 2 inch high, then it allows the focal plane to be closer to the secondary which allows for the use of a small secondary.

I use Newt for the Web to check on secondary sizing. It's really a design program but you can use it to evaluate an existing scope. At this point, I am not sure what the motivation would be to increase the size of the secondary, any difference is going to be subtle.

Jon




When using Newt for the Web, should I enter the minor axis of the secondary as the **usable** measurement? For instance, my minor axis is 4" but part of that is covered up by the secondary holder around the edge of the secondary.

It would seem the program is looking for actual, usable mirror so I should exclude what is covered up.

Thanks,

Rob


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Jason D
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Re: Secondary Sizing new [Re: robininni]
      #5736041 - 03/16/13 11:26 AM

Quote:

It would seem the program is looking for actual, usable mirror so I should exclude what is covered up.



Correct


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Jason D
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Re: Secondary Sizing new [Re: Jason D]
      #5736052 - 03/16/13 11:33 AM

If you want to calculate the minimal sized secondary mirror for your scope, just use the following accurate formula

min = 4HF/(4F^2-1)
H is the distance between the secondary mirror and the focal plane
F is the F-ratio

For your F4 scope:

min = H * (16/63)

There are more formulas in this post
http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/4519314/...

But the ultimate test to judge of your current secondary mirror size is adequate is to look through a sight-tube with the pupil placed at the focal plane. If you can see the whole primary mirror reflection with little to spare then your secondary mirror is adequate. Of course, to do this test you need to attain excellent collimation first.


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: Jason D]
      #5736137 - 03/16/13 12:09 PM Attachment (9 downloads)

On the secondary to focal plane measurement, do I measure from the physical center of the secondary--mine is marked with a rather large 2mm dot which actually I just realized isn't the exact center of the viewable mirror according to a template.

Or do I measure from where the laser dot coming out of the focuser hits the secondary mirror?

Also, if the physical center of the secondary is good to measure from, could I instead measure from the bolt holding the secondary apparatus in place as long as I have the spider vanes exactly centered? I.E., should that be accurate enough or would it be best to use the actual mirror surface?

If I have to measure the actual mirror surface, how do I do that without taking a chance scratching it since the ruler will be pointed at it and not laid flat on it?

Lastly, I attached a pic of my 2mm dot that was place on the secondary. You can see from the laser dot that the 2mm black dot is not even the 'used' center point so does this dot affect me view?

Thanks,

Rob


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: Jason D]
      #5736162 - 03/16/13 12:18 PM

Quote:

If you want to calculate the minimal sized secondary mirror for your scope, just use the following accurate formula

min = 4HF/(4F^2-1)
H is the distance between the secondary mirror and the focal plane
F is the F-ratio

For your F4 scope:

min = H * (16/63)

There are more formulas in this post
http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/4519314/...

But the ultimate test to judge of your current secondary mirror size is adequate is to look through a sight-tube with the pupil placed at the focal plane. If you can see the whole primary mirror reflection with little to spare then your secondary mirror is adequate. Of course, to do this test you need to attain excellent collimation first.




Your formula can have different outcomes that don't make sense based upon what steps you do first.

This would be a better write out of the formula:


min = 4HF/((4(F^2))-1)

Rob


Edited by robininni (03/16/13 12:25 PM)


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Jason D
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Reged: 10/21/06

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Re: Secondary Sizing new [Re: robininni]
      #5736176 - 03/16/13 12:25 PM

Quote:

Quote:

If you want to calculate the minimal sized secondary mirror for your scope, just use the following accurate formula

min = 4HF/(4F^2-1)
H is the distance between the secondary mirror and the focal plane
F is the F-ratio

For your F4 scope:

min = H * (16/63)

There are more formulas in this post
http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/4519314/...

But the ultimate test to judge of your current secondary mirror size is adequate is to look through a sight-tube with the pupil placed at the focal plane. If you can see the whole primary mirror reflection with little to spare then your secondary mirror is adequate. Of course, to do this test you need to attain excellent collimation first.




Something doesn't compute with your formula. Maybe a 4 is supposed to be a 2?

Rob




You have a 25" scope. I will estimate H to be around 15.5"
which is 1/2 of your aperture plus an estimate 3" to your focal plane which adds up to 12.5"+3"=15.5"

Using 15.5"

min = 15.5 * (16/63) = 3.94"

Currently you have a 4" mirror.

Why did you think the formula did not give reasonable results?

Jason


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: Jason D]
      #5736181 - 03/16/13 12:26 PM

I edited my post, Jason. The formula needed parathensis here and there.

Rob


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Jason D
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Re: Secondary Sizing new [Re: robininni]
      #5736195 - 03/16/13 12:31 PM

Quote:

On the secondary to focal plane measurement, do I measure from the physical center of the secondary--mine is marked with a rather large 2mm dot which actually I just realized isn't the exact center of the viewable mirror according to a template.

Or do I measure from where the laser dot coming out of the focuser hits the secondary mirror?

Also, if the physical center of the secondary is good to measure from, could I instead measure from the bolt holding the secondary apparatus in place as long as I have the spider vanes exactly centered? I.E., should that be accurate enough or would it be best to use the actual mirror surface?

If I have to measure the actual mirror surface, how do I do that without taking a chance scratching it since the ruler will be pointed at it and not laid flat on it?

Lastly, I attached a pic of my 2mm dot that was place on the secondary. You can see from the laser dot that the 2mm black dot is not even the 'used' center point so does this dot affect me view?

Thanks,

Rob




In theory, it is the distance between the focal plane and where the laser beam hits the secondary mirror surface – assuming the scope is well-collimated.

In practice, if you are off my few millimeters then the calculation is still good enough. Do not take chances and scratch your secondary mirror. The risk is not worth that additional 0.1mm in accuracy. Anyway, secondary mirrors come in few pre-defined sizes. Getting the measurement with respect to the central bolt is acceptable.

Do ignore that dot on the secondary mirror.

Jason


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Pinbout
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Re: Secondary Sizing new [Re: robininni]
      #5736220 - 03/16/13 12:47 PM

I couldn't remember if your allowed to multiply before you square or it's square first



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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: robininni]
      #5736223 - 03/16/13 12:48 PM

Okay, so here's what I got:

The actual F ratio is not 4, it is 4.1 based upon the etched focal length of 102 9/16 inches on the rear of the mirror cell. The reflective surface is actually not 25" either, it is more like 24.875" based upon the Catseye template and a ruler, but I'm just going to go with 25" since were are talking about .02 in F ratio difference.

As originally received from Obsession (I have to extend the truss tubes by 1 inch because they had been cut by the previous owner by that much), I get a center of secondary (measured by a centered secondary holder bolt in the spider bane) to the focal plane measurement of: 13.125" (secondary holder bolt to inner edge of OTA)

+ 2.945" (complete focuser length when the draw tube is flush with the inside of the OTA)

- .915" (the difference between how far the focuser sticks out toward the astronomer in the previous calculation versus it being racked all the way in)

+ .425" (the distance toward the astronomer in which the focal plane is beyond the fully racked in focuser)

So... 13.125+2.945-.915+.425 = 15.58"

With the truss tubes seated fully, it should be a little less than one more inch (because of the angle of the truss tubes) or 16.50"

So.... min visible secondary size as original designed = 4HF/((4(F^2))-1)

= 4*15.58*4.1/((4(4.1^2))-1)

= 3.8573" minor axis secondary (minimum).

I currently have 4" - .315" (covered by holder) = 3.685"

So... as used last night and the night before, my 100% illuminated area is 0. Doesn't seem good.

Using the best guess secondary to focal plane distance with the truss tubes fully seated (which I will do because several EPs barely work otherwise and proper Paracorr 2 settings can't be used on many EPs at the 'factory' truss tube length) of 16.5", I get a needed secondary size of:

4*16.5*4.1/((4(4.1^2))-1)

= 4.0851" minor axis secondary (minimum).

Even worse which would make sense seeing how after moving the truss tubes out I thought the views looked a little better.

So I really need a secondary size of 4.0851" + .315" (secondary blocked by holder) = 4.4" minimum but really bigger if I am doing DSO and not just planetary viewing, correct?

Does this all sound right?

How can I determine what my current % of illumination is a the center of the field of view?

How can I determine how much more minor secondary axis is needed to get the 100" illumination out beyond the perfect center of the focal plane?

Thanks,

Rob

Edited by robininni (03/16/13 12:56 PM)


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: Jason D]
      #5736226 - 03/16/13 12:50 PM

Quote:


Do ignore that dot on the secondary mirror.

Jason




Does the dot (being 2mm) hurt my view? If so, does it make it worse that the dot is not centered? (sort of like how the secondary shadow covers the center of the primary and that is why the center sticker doesn't hurt the view)

Thanks,

Rob


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Jason D
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Re: Secondary Sizing new [Re: robininni]
      #5736264 - 03/16/13 01:13 PM

Quote:

Does this all sound right?



It does.

Quote:

How can I determine what my current % of illumination is a the center of the field of view?

How can I determine how much more minor secondary axis is needed to get the 100" illumination out beyond the perfect center of the focal plane?






It is all in the link I provided. Here is a copy of what is in that link:



Referring to the attachment.
Given:
“A” for the secondary minor axis assuming a secondary made off a 45 degree cross section of a cylindar
“S” for the distance between the primary mirror and the secondary mirror (leave sagitta out of it)
“D” for the primary mirror diameter

First, calculate H’ which is the distance between the secondary mirror and the extended apex point along the focuser axis.

H’ = (S(D-2A)-D*sqrt(S^2+A^2-AD))/2(A-D)

Then calculate the secondary mirror offset

Offset = (H'*D^2)/(4*(S+H')^2-D^2)

We can figure out F' which is the F-ratio for the extended cone. It is the ideal F-ratio to reference when setting the sight-tube length
F' = (H'+S)/D

Another formula for the offset

Offset = A/(4*F')

and another

Offset = H'/(4*F'^2-1)

To figure out the smallest secondary mirror size "A(min)", then set H'=H and F'=F (H being the distance between the sceondary mirror and the focal point along the focusre axis)

A(min)/(4*F) = H/(4F^2-1)

A(min) = 4HF/(4F^2-1)

100% illumination area diameter = (H'-H)/F'


Above assumes a secondary mirror made from a 45 degree cross section off a cylinder where

Major axis / minor axis = sqrt(2)

For a secondary mirror made from a 45 degree cross section off a cone,

Major axis / minor axis = sqrt(2) / sqrt(1-1/(4*F'^2))




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Jason D
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Re: Secondary Sizing new [Re: robininni]
      #5736283 - 03/16/13 01:21 PM Attachment (5 downloads)

Quote:

Does the dot (being 2mm) hurt my view? If so, does it make it worse that the dot is not centered? (sort of like how the secondary shadow covers the center of the primary and that is why the center sticker doesn't hurt the view)

Thanks,

Rob



If that dot is located where the laser beam hits the secondary mirror then it has no impact on your view – the secondary mirror shadow will follow the laser beam on its way to the eyepiece. If it is farther away then it does in theory but I highly doubt it will make a difference practically.


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Vic Menard
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Re: Secondary Sizing new [Re: robininni]
      #5736326 - 03/16/13 01:39 PM

Quote:

...Does the dot (being 2mm) hurt my view?



Probably not--but a little acetone on a Q-tip will easily remove it (if it bothers you).

Quote:

If so, does it make it worse that the dot is not centered? (sort of like how the secondary shadow covers the center of the primary and that is why the center sticker doesn't hurt the view)



It's probably close enough to the optical center (the dot appears to be at the mechanical center) that it will fall in the umbra of the secondary shadow. Even if it's slightly outside the shadow, it would have minimal impact on the edge-of-field image performance. In other words, it's not worth worrying about.

And with a 15.5-inch intercept, I wouldn't move too quickly to upsize the secondary mirror... link

(16.5-inches... )

I'm still perplexed why you're having issues with your Paracorr 2. The 21 Ethos should work at the A setting (IIRC, it's one of the closest focusing eyepieces--just like the 31 Nagler). After you have that configuration working, the other eyepieces should be focused using the tunable top. Or does the Paracorr 2/21 Ethos work better with the shortened truss poles (longer, 16.5-inch intercept). I seem to recall having to move my primary forward about 1/4-inch to accommodate focusing the Paracorr 2/31 Nagler...


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Starman1
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Re: Secondary Sizing new [Re: FineArt]
      #5736371 - 03/16/13 02:07 PM

Quote:

From the Gary Seronik article:

"Selecting the best size for the secondary mirror of a Newtonian telescope is an exercise in compromise. A diagonal that is too large will block incoming light and exaggerate image-harming diffraction effects, while one that is too small will fail to deliver all the light from the primary mirror to the eyepiece. Certainly, the second of these is the greater evil of the two."

That is true for premium optics. For a Cheap Chinese primary most of the problems will be around the outer edge of the mirror. I have set up my 10" scope to throw away the outer 1/2 inch of light giving me a 9" scope. The quality of the image on my sensor has improved greatly from the large secondary that came with the scope.

If I later get a resurfaced/high end mirror I will have to extend the distance between the 2 mirrors.



Think of a ray as a flat sheet of light coming into the atmosphere and hitting the front of the scope. The front end of the scope carves out a section of that plane, and focuses the light into a point in the center of the telescope's focal plane.
When the secondary is too small, and the star is exactly in the center of the field, the light from the edge of the mirror misses the secondary.
However, if the star is off axis, the plane of light hits the tube at a slightly different angle, which means more edge on one side will be engaged and more of the mirror on the other side will not.
Hence, though a too-small secondary does stop the scope down, it only trims the exact edge for the on-axis ray. Every other field position on the focal plane WILL use some of the edge you are seeking to avoid.
Hence, stopping down the scope with a too-small secondary mirror really doesn't help.

The way to stop down a mirror is with a ring of black paper cut out to fit over the mirror and prevent light from reflecting from the portion you seek to avoid. That way, even the off-axis rays will not be damaged by the turned edge.


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robininni
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Re: Secondary Sizing new [Re: Vic Menard]
      #5736386 - 03/16/13 02:13 PM

Quote:


I'm still perplexed why you're having issues with your Paracorr 2. The 21 Ethos should work at the A setting (IIRC, it's one of the closest focusing eyepieces--just like the 31 Nagler). After you have that configuration working, the other eyepieces should be focused using the tunable top. Or does the Paracorr 2/21 Ethos work better with the shortened truss poles (longer, 16.5-inch intercept). I seem to recall having to move my primary forward about 1/4-inch to accommodate focusing the Paracorr 2/31 Nagler...




With the truss tubes all the way in (shorter by an 1") the Ethos 21 and Nagler 31 have much more room. And if I recall, the Paracorr 2 works fine with turnable top set at the correct setting for each EP based upon the instructions.

With the truss tubes extended an inch, with the Paracorr 2 in place and tuned correctly to A for the Ethos 21/Nagler 31, I can't even bring the EPs to focus. For other EPs, if I set the paracorr 2 to the correct position it doesn't work so I have to randomly turn the table out more to get them to focus (Ethos 13 and 8mm).

I'll check out the links you and Jason have provided.

Thanks,

Rob


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Starman1
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Re: Secondary Sizing new [Re: Starman1]
      #5736403 - 03/16/13 02:21 PM

1) Since the center of the optical axis will NOT hit the geometric center of the secondary mirror, center-dotting the secondary has no value.

2) You can figure out the secondary to focal plane distance by a sum of:
--mirror radius
--mirror edge to I.D. of tube
--thickness of tube
--height of focuser with drawtube about 3/4" above the focuser (to allow for the fact the focal plane will be above the racked-in focuser to allow for use of barlows and eyepieces requiring in-travel).
And you simply sum all of those measurements. Your actual focal plane can be determined with a piece of translucent scotch tape across the focuser, pointing at the moon. Focus the moon image on the translucent tape and measure the height of the focuser. That's your focuser height for measurement purposes.

3) Secondary size is chosen to not vignette the edge of the field by more than 0.3 to 0.4 magnitudes in the field of your lowest-power, widest-fieldstop eyepiece. It is not necessary to illuminate a field you don't use. If your lowest power eyepiece has a 42mm field stop, it's not necessary to choose a secondary size that illuminates a 46mm field.

4) Many/most secondary holders support the secondary all the way around the edge. The part of the mirror covered does NOT figure in to the secondary size calculation. Only the exposed part of the mirror counts. Hence, if you use a 2.6" secondary and your secondary holder covers 0.1" of the edge all the way around, you are really using a 2.4" secondary, not a 2.6" one. The 2.6" would only figure in to the obstruction, not the illumination.

5) Here is an excellent calculator for graphically showing whether a secondary is the right size. It also shows illumination with other commercial sizes of secondary mirrors:
http://www.bbastrodesigns.com/diagonal.htm


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Fred1
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Re: Secondary Sizing new [Re: Starman1]
      #5736420 - 03/16/13 02:26 PM

While I was going through a similar exercise when ordering my 18" I found the following to be very helpful and easy to use:
http://www.bbastrodesigns.com/diagonal.htm from Jan van Gastel's Astronomy links website.


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Vic Menard
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Re: Secondary Sizing new [Re: robininni]
      #5736432 - 03/16/13 02:32 PM

Quote:

With the truss tubes all the way in (shorter by an 1") the Ethos 21 and Nagler 31 have much more room. And if I recall, the Paracorr 2 works fine with tunable top set at the correct setting for each EP based upon the instructions.



How much room is "much more room"? Perhaps it would be possible to tighten the collimation screws on the primary mirror and get the P2/E21 focus position one or two tenths of an inch above the fully racked in position on your FeatherTouch?

If the 4-inch minor axis is usable--it will almost certainly require carefully optimized mechanicals. Of course, it's also possible that the astigmatism signature you were seeing is being caused by the secondary mirror--but until you've eliminated all the other possibilities--it's probably worthwhile to optimize what you can with what you have.

Incidentally, if 15.5-inches is measured to the center bolt, you can subtract 1/4-inch for offset (using the New Model, the optical axis is offset toward the focuser side of the OTA).

Mel Bartels' site is good for tweaking optimizations and comparing how they work with various secondary mirror sizes, and it also includes user input for defining an exact minor axis secondary mirror.


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robininni
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Re: Secondary Sizing new [Re: Vic Menard]
      #5736475 - 03/16/13 02:59 PM

Quote:


And with a 15.5-inch intercept, I wouldn't move too quickly to upsize the secondary mirror... link

(16.5-inches... )






Vic, that's a cool website! Way fun!

Okay, so I took off the secondary and measured across it (before I had measured the edge of the holder while the secondary was mounted and multiplied by 2--not as precise, sorry) and I actually have 3.875" usable mirror there where previously I thought I had 3.68".

So, Vic, you are right, with the truss tubes out 1", there secondary size is sufficient--but--wouldn't this only be for 100% illumination in a very small central portion of the view? Because I am right barely over the minimum of 3.77" needed?

Anyway, I didn't think about adjusting the primary screws to move the mirror up to move the focal plane out for easier use of my EPs. Just eye-balling it, it looks like I could gain 1/2 inch there.

I am not comfortable with not having the truss tubes all the way seated and I noticed at least one wants to pull out slightly when lowering the altitude of the OTA.

I really like the shorter tubes and since my mirror is barely adequate anyway, and I assume NOT for a broad area of 100% illumination, I think I would still opt for a new larger secondary. Is this silly?

Something else the still bothers me is not being able to fully see the edge of the primary in all directions not matter how I set the secondary when viewing through the empty focuser (or with the sight tube). I guess the numbers above don't lie, but I would think if I can't see it all with my eye, it isn't all being used.

Thanks,

Rob

Edited by robininni (03/16/13 03:22 PM)


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Vic Menard
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Re: Secondary Sizing new [Re: robininni]
      #5736553 - 03/16/13 03:55 PM

Quote:

...with the truss tubes out 1", the secondary size is sufficient--but--wouldn't this only be for 100% illumination in a very small central portion of the view? Because I am right barely over the minimum of 3.77" needed?



It's very close to the minimum size (usually 1/4-inch 100-percent illuminated image diameter), but it should be sufficient for a trial run period (if the numbers work) to get a feel for the scope's personality. Upgrading to a 4.5-inch isn't a bad idea, although it could be expensive, and unless you get real specs on the quality from a reliable source, you're getting into the size range where "off the shelf" can be unpredictable. (Eight new truss poles would be much less expensive if the 4-inch secondary mirror is of good quality and the scope can be optimized to use it effectively.)

Quote:

Anyway, I didn't think about adjusting the primary screws to move the mirror up to move the focal plane out for easier use of my EPs. Just eye-balling it, it looks like I could gain 1/2 inch there.



That may be enough to use your existing truss poles fully seated.

Quote:

I really like the shorter tubes and since my mirror is barely adequate anyway, and I assume NOT for a broad area of 100% illumination, I think I would still opt for a new larger secondary. Is this silly?



It's not silly if you get a good return (increase in performance) on your investment (and the 4-inch with holder has trade/resale value).

Quote:

Something else the still bothers me is not being able to fully see the edge of the primary in all directions not matter how I set the secondary when viewing through the empty focuser (or with the sight tube). I guess the numbers above don't lie, but I would think if I can't see it all with my eye, it isn't all being used.



Numbers don't lie, but you should be able to see your mirror clips. Are you sure your front aperture is only 26.25-inches? That sounds a little small to me...


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robininni
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Re: Secondary Sizing new [Re: Vic Menard]
      #5737735 - 03/17/13 12:02 AM

Quote:

Quote:

Something else the still bothers me is not being able to fully see the edge of the primary in all directions not matter how I set the secondary when viewing through the empty focuser (or with the sight tube). I guess the numbers above don't lie, but I would think if I can't see it all with my eye, it isn't all being used.



Numbers don't lie, but you should be able to see your mirror clips. Are you sure your front aperture is only 26.25-inches? That sounds a little small to me...




I measured again and from the inside to inside of the OTA I get 26.3125".

As for seeing mirror clips, I do see a portion of all three mirror 'clips' (not really clips, but the prongs that would prevent the mirror from falling forward out of the mirror cell). I don't see but about 1/2 of each of these prongs, and I cannot see all edges of the primary at one time (like I can in my Orion 10xtg--I can actually see beyond the edges all the way around on that scope).

I did fully seat the truss tubes and I remeasured the new focal plane from the fully racked-in focuser. It is now 1.269" outside of the focuser where is was .425", so I gained .844".

This puts the new diagonal to focal plane measurement at 16.44" (but perhaps I should subtract .25" for secondary offset toward the primary as the laser dot is about that distance from the physical center of the secondary and the secondary is centered exactly by the spider vanes).

I also unsquared the focuser barely to get a nice concentric circle around the secondary when looking through the sight tube. I couldn't achieve this without doing this or uncentering the secondary via the spider vanes, so I chose the focuser route.

I'll use the scope as is now and see how it does but I think I will want to get the 4.5" secondary or I will always wonder if that could have made a perceivable difference since as it stands, if I understand all of this now, I will not get 100% illumination anywhere in the field of view when taking into account a 25" f4.1 primary, a 3.875" visible secondary and a secondary to focal plane distance of 16.44" (maybe really 16.19" due to secondary offset).

Thanks,

Rob


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robininni
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Re: Secondary Sizing new [Re: robininni]
      #5737843 - 03/17/13 12:52 AM

When you are receiving less than 100% illumination at the focal plane, is there a formula that will show you exactly what percentage of illumination you are receiving?

The formulas Jason provided don't seem to address starting out with less than 100%. For instance, using his formulas I came up with having a 100% illumination area of -.01539". But that doesn't tell me anything I don't already know and that is I don't have 100% illumination.

Thanks,

Rob


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Jason D
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Re: Secondary Sizing new [Re: robininni]
      #5737960 - 03/17/13 02:55 AM

Quote:

When you are receiving less than 100% illumination at the focal plane, is there a formula that will show you exactly what percentage of illumination you are receiving?

The formulas Jason provided don't seem to address starting out with less than 100%. For instance, using his formulas I came up with having a 100% illumination area of -.01539". But that doesn't tell me anything I don't already know and that is I don't have 100% illumination.

Thanks,

Rob




You can use the following simplified formula which has around 3% error margin

(AF/H)^2

Above formula will give illumination % at the center of the eyepiece


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Jason D
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Re: Secondary Sizing new [Re: Jason D]
      #5738007 - 03/17/13 04:31 AM

If you want the accurate formula, it is:

% illumination reduction at center = (((2A(H+S))/(H+sqrt( H^2+A^2)))^2)/(D^2)

Above formula should be used only if the secondary mirror is smaller or equal to minimal size


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Vic Menard
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Re: Secondary Sizing new [Re: robininni]
      #5738673 - 03/17/13 02:02 PM

Quote:

...from the inside to inside of the OTA I get 26.3125".



That's a bit undersized. If you exceed a fieldstop of 1.63-inch, the front aperture will become a vignetting source. It's not much, but it's something else to consider.

Quote:

...I do see a portion of all three mirror 'clips'...about 1/2...and I cannot see all edges of the primary at one time...


.
That sounds like you're missing about 1/4- to 3/8-inch around the mirror perimeter from your viewpoint above the focuser.

Quote:

I did fully seat the truss tubes and I remeasured the new focal plane from the fully racked-in focuser. It is now 1.269" outside of the focuser where is was .425", so I gained .844".



Have you tried pulling the primary mirror as far back as possible?

Quote:

This puts the new diagonal to focal plane measurement at 16.44" (but perhaps I should subtract .25" for secondary offset toward the primary as the laser dot is about that distance from the physical center of the secondary and the secondary is centered exactly by the spider vanes).

I also unsquared the focuser barely to get a nice concentric circle around the secondary when looking through the sight tube. I couldn't achieve this without doing this or uncentering the secondary via the spider vanes, so I chose the focuser route.

I'll use the scope as is now and see how it does...



If you always use the Paracorr 2 (even for high magnification) like I do with my f/4 mirror, you may get a little better field illumination than the un-Paracorred configuration (similar to the gain you might realize from using a Barlow lens). You can see where the Paracorr 2/Ethos 21 focuses and then assess the position of the Paracorr 2 field lens and its fieldstop with Mel Bartel's online calculator.

Quote:

...but I think I will want to get the 4.5" secondary or I will always wonder if that could have made a perceivable difference since as it stands, if I understand all of this now, I will not get 100% illumination anywhere in the field of view when taking into account a 25" f4.1 primary, a 3.875" visible secondary and a secondary to focal plane distance of 16.44" (maybe really 16.19" due to secondary offset).



With the Paracorr 2, you may be able to realize 100-percent in the center along with your already acceptable illumination at the edge. The proof, of course, will be the view in the eyepiece.


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Vic Menard
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Re: Secondary Sizing new [Re: robininni]
      #5738707 - 03/17/13 02:13 PM

Quote:

When you are receiving less than 100% illumination at the focal plane, is there a formula that will show you exactly what percentage of illumination you are receiving?



You could always run the online calculator reducing the primary mirror size until it doesn't kick back the "Diagonal too small" warning.

Using the online calculator, I get 24.5-inches with your current numbers, which, compared to a 25-inch clear aperture, is 96-percent (IIRC, you're a little less than 25--at 24.9-inch, you're closer to 97-percent).

Edited by Vic Menard (03/17/13 02:30 PM)


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Jason D
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Re: Secondary Sizing new [Re: Vic Menard]
      #5739289 - 03/17/13 06:09 PM

Referring to info in this post
http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/5736223/...
I calculated the effective aperture percentage which is the same as the maximum illumination to be 91.6%. Effective diameter according to my calculation is 23.93".

I used the following data
Secondary size = 3.685”
Secondary to focal plane distance = 15.58”
Focal length = 102 9/16”
Mirror diameter = 25”
Jason


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robininni
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Re: Secondary Sizing new [Re: Vic Menard]
      #5739490 - 03/17/13 07:39 PM

Quote:

Quote:

...from the inside to inside of the OTA I get 26.3125".



That's a bit undersized. If you exceed a fieldstop of 1.63-inch, the front aperture will become a vignetting source. It's not much, but it's something else to consider.






Thanks for following up on that.. you left me with a real cliff hanger!

So the only EP I use in this scope that would be affected is a Nagler 31mm and it shows to be barely affected with a field stop of 42mm and your calculation of 41.402mm (1.63").

I am going with a 4.5" secondary so will this change what you calculated and make things worse or will it remain the same?

Also, thanks for the calculations of illumination for me (and you too Jason ).

All your help is very appreciated!

Rob


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robininni
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Re: Secondary Sizing new [Re: Jason D]
      #5739506 - 03/17/13 07:44 PM

Quote:

Referring to info in this post
http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/5736223/...
I calculated the effective aperture percentage which is the same as the maximum illumination to be 91.6%. Effective diameter according to my calculation is 23.93".

I used the following data
Secondary size = 3.685”
Secondary to focal plane distance = 15.58”
Focal length = 102 9/16”
Mirror diameter = 25”
Jason




It will actually be a little different since I am going with the truss tubes fully seated (lose an inch but the longer focal length EPs focus so much easier) and this gives me a secondary to focal plane distance of 16.19" when including a .25" offset. I also determined that my initial secondary measurement of 3.685" was a little off. It is more like 3.875". However, since I will getting a 4.5" secondary it will be more like 4.375" .

Rob


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Vic Menard
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Re: Secondary Sizing new [Re: Jason D]
      #5739731 - 03/17/13 09:57 PM

Quote:

I calculated the effective aperture percentage which is the same as the maximum illumination to be 91.6%. Effective diameter according to my calculation is 23.93".

I used the following data
Secondary size = 3.685”
Secondary to focal plane distance = 15.58”
Focal length = 102 9/16”
Mirror diameter = 25”
Jason



I used a different set of data for my earlier calculations. Plugging your data into the online calculator I get 24.25-inches (94-percent of 25CA). I expected better accuracy from Bartel's online calculator. What do you suspect is the cause of the discrepancy?

(I come up with an effective focal ratio of 4.229 for both triangles using Bartel's solution. It seems to be accurate. What am I missing?)

Edited by Vic Menard (03/17/13 10:13 PM)


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Vic Menard
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Re: Secondary Sizing new [Re: robininni]
      #5739751 - 03/17/13 10:04 PM

Quote:

...I am going with a 4.5" secondary so will this change what you calculated and make things worse or will it remain the same?



The slight vignetting from the front edge of the UTA will remain the same, but it's almost certainly inconsequential. The other concern with such close spacing is tube currents, but hot or cold, they tend to hug top or bottom, and your focuser is mounted on the side, so probably no need to worry there either.

Quote:

Also, thanks for the calculations of illumination for me...



It was a fun exercise on a relaxing Saint Patty's Day.


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MitchAlsup
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Re: Secondary Sizing new [Re: Vic Menard]
      #5739755 - 03/17/13 10:05 PM

I just wanted to add::

AQs you push the focuser height lower and lower, you get to the point where the first lense (of the Paracorr) is right on the edge of the optical field. When the first lens is this low, it can be illuminated by a very large field and this makes it difficult to properly shield the opticas from stray light.

Backing off just a little bit makes the light shield opposite that of the secondary significantly smaller.

Backing off quite a bit, makes the shield almost unnecessary.


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Vic Menard
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Re: Secondary Sizing new [Re: MitchAlsup]
      #5739780 - 03/17/13 10:16 PM

Good point Mitch!

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Jason D
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Re: Secondary Sizing new [Re: Vic Menard]
      #5739928 - 03/18/13 12:51 AM

Quote:

Quote:

I calculated the effective aperture percentage which is the same as the maximum illumination to be 91.6%. Effective diameter according to my calculation is 23.93".

I used the following data
Secondary size = 3.685”
Secondary to focal plane distance = 15.58”
Focal length = 102 9/16”
Mirror diameter = 25”
Jason



I used a different set of data for my earlier calculations. Plugging your data into the online calculator I get 24.25-inches (94-percent of 25CA). I expected better accuracy from Bartel's online calculator. What do you suspect is the cause of the discrepancy?

(I come up with an effective focal ratio of 4.229 for both triangles using Bartel's solution. It seems to be accurate. What am I missing?)




Online tools use simplified formulas with good approximations. For example, when I input the above specs into http://www.bbastrodesigns.com/diagonal.htm the side calculated the the minimum secondary size to be 3.797684 which is really (H/f)*D where "H" is the distance between secondary and focal plane, "f" is the focal length, and "D" is the primary mirror aperture.

Min = (15.58/102.5625)*25 = 3.797684

But (H/f)*D is an approximation. The actual formula is what I provided in an earlier post and using it provided the value 3.854946 – about 1.5% discrepancy.

The online simplified formulas come with good approximation. I am not suggesting everyone stop using them. In general the error margin has been below 3%.

Jason


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Jason D
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Re: Secondary Sizing new [Re: Jason D]
      #5739934 - 03/18/13 12:59 AM Attachment (4 downloads)

Here is the catch. The line in red in the attachment is not equal to the minor axis of the secondary mirror. The simplified formulas assume it is equal; however, it is actually little smaller since it does not run through the geometric center of the secondary mirror.

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Cary
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Re: Secondary Sizing new [Re: Jason D]
      #5739952 - 03/18/13 01:17 AM

I'll throw my stuff in the ring as well. I while back I coded up a utility for calculating Newtonian diagonal sizes.

Here's the dowload link: http://www.opticwavelabs.com/files/OwlDiagonalCalcSetup.exe

And here is a youtube tutorial: http://www.youtube.com/watch?v=xfhCLdsl_zY&feature=share&list=UUrL_pr...


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Jason D
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Re: Secondary Sizing new [Re: Cary]
      #5739978 - 03/18/13 01:49 AM

You are using the same simplified formulas like others. There is nothing wrong with that since these simplified formulas give good results.
For example, in your video you have calculated the minimum secondary size to be 2.116" when the actual value is 2.141" -- a discrepancy of 1.16%.
Jason


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Cary
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Re: Secondary Sizing new [Re: Jason D]
      #5741037 - 03/18/13 04:05 PM

It's been a long time since I wrote it so I'm sure there are better formulae.

A couple of things become quickly apparent though. First, elliptical flats are manufactured in a limited number of sizes so one is always compromising. Secondly the minimum size is basically not usable because you always need some sort of working FOV.

Then on top of all that you have the mechanical alignment issues.

It would seem to me that the 1% "error" gets eaten up in all of the other factors.


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Re: Secondary Sizing new [Re: Cary]
      #5741064 - 03/18/13 04:17 PM

Quote:

...It would seem to me that the 1% "error" gets eaten up in all of the other factors.





The problems arise when, for whatever reason, someone decides to push the envelope.


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Jason D
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Re: Secondary Sizing new [Re: Cary]
      #5741067 - 03/18/13 04:18 PM

Quote:

It would seem to me that the 1% "error" gets eaten up in all of the other factors.




No disagreement.

The point of the last few post exchanges between me and Vic is to figure out the source of the discrepancy. I was not advocating the use of the more accurate formulas.

But I believe it is important for others to understand that tools on the web are accurate within 0.1" which is probably more accuracy that we need. No need to list too many digits to the right of the decimal point.

Jason


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Cary
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Re: Secondary Sizing new [Re: Jason D]
      #5741154 - 03/18/13 05:01 PM

Quote:

Quote:

It would seem to me that the 1% "error" gets eaten up in all of the other factors.




No need to list too many digits to the right of the decimal point.

Jason




True True! What I hope is that folks are paying attention to the FOV falloff issues. No sense buying these MONSTER eyepieces with massive field stops and then under sizing the diagonal.


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Starman1
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Re: Secondary Sizing new [Re: Jason D]
      #5741202 - 03/18/13 05:22 PM

I have a hundred books on astronomy that address the issue of secondary size. Whether 30% or 40% or 25% (NEWT) is a more reasonable light loss at the edge of the field is one issue (there isn't a consensus, alas), but there also is no complete agreement on just exactly WHY one should try to maintain a minimum sized secondary.

Some books on optics state that a 20% obstruction is essentially the same as no obstruction to the eye and that up to 25% has negligible effect, but that larger has a serious deleterious effect on the image quality. All go into great lengths to describe what happens when the MTF is modified by the secondary obstruction.

Meanwhile, most cats have 30-40% obstructions and many of them have pretty sharp optics (as far as I can see). My 5" Mak rivals an Apo of the same size, yet has a 30% secondary.

So it seems to me far too many people worry needlessly about the size of their secondary mirrors. Newtonians automatically have smaller secondaries, percentage-wise, than all cats. If adding a couple percent means better edge illumination for the low power eyepieces, then that's what we should do.

We pay for bigger scopes to gather light. Why stab ourselves in the back by making the I.D. of the UTA too small or using a too-small secondary when the effects of that won't really matter to the axial image but will matter to the majority of the field of view? Do people really buy big dobs to ONLY look at the planets on axis? I seriously doubt it. Plus, anyone who has observed for a few years knows seeing matters 1000X more than secondary obstruction percentage.

So if the optimum size for a secondary is in between two commercial sizes, I would advocate ALWAYS opting for the larger size. You won't see the effect in image quality, but you might see the effects in image brightness in the outer 50% of the field with one that is too small.

And I implore the makers of telescopes to stop making the I.D. of their scopes too small so that the front ends of their scopes cause vignetting. The secondary mirror already adds enough--you don't have to add even more by making the opening too small. And please use a calculator to figure the correct secondary size. Though I don't see it often on the low-priced commercial dobs, I often see a too-small secondary chosen for some of the high-end scopes.

Even the interferogram on my 1/30 wave secondary (it's actually a lot better than that, supposedly, but I distrust the measurement) shows most of the error is right at the rim of the mirror, and that is masked off by the holder. If the mirror is a little on the small side, that edge may become more important and I might be tempted to use it. People should recognize that fact, too, and go a little larger just to be sure.

Many good reasons not to try to push the limits on small sized secondaries. No really good reason to worry about going a little larger.
The visual observer may not need the large size of a photographic-specific secondary (designed to fully illuminate the field), but we also don't need to try to keep the secondary to the absolute minimum size, either.

IMO, of course.


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Re: Secondary Sizing new [Re: Starman1]
      #5741368 - 03/18/13 06:33 PM

Don,
All good points.
About the only reason I can think of to go a bit smaller is $$$. Five inches and up, especially with excellent numbers, gets expensive in a hurry. And unfortunately, cost doesn't guarantee quality.

That said, I did say "a bit smaller". I can't think of any reason to go too small.


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Re: Secondary Sizing new [Re: Jason D]
      #5741388 - 03/18/13 06:41 PM

Quote:

...The point of the last few post exchanges between me and Vic is to figure out the source of the discrepancy.



What's remarkable is that because of a difference between the secondary mirror minor axis and the chord that passes through the optical center (offset) amounting to five hundredths of an inch (a bit over two hundredths on either side)--the percentage light loss increases 2.5-percent!


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Re: Secondary Sizing new [Re: Starman1]
      #5741531 - 03/18/13 07:40 PM

Quote:



We pay for bigger scopes to gather light. Why stab ourselves in the back by making the I.D. of the UTA too small or using a too-small secondary when the effects of that won't really matter to the axial image but will matter to the majority of the field of view? Do people really buy big dobs to ONLY look at the planets on axis? I seriously doubt it. Plus, anyone who has observed for a few years knows seeing matters 1000X more than secondary obstruction percentage.

So if the optimum size for a secondary is in between two commercial sizes, I would advocate ALWAYS opting for the larger size. You won't see the effect in image quality, but you might see the effects in image brightness in the outer 50% of the field with one that is too small.

And I implore the makers of telescopes to stop making the I.D. of their scopes too small so that the front ends of their scopes cause vignetting. The secondary mirror already adds enough--you don't have to add even more by making the opening too small. And please use a calculator to figure the correct secondary size. Though I don't see it often on the low-priced commercial dobs, I often see a too-small secondary chosen for some of the high-end scopes.

Many good reasons not to try to push the limits on small sized secondaries. No really good reason to worry about going a little larger.

IMO, of course.




Well said, Don!

I don't pretend to have much knowledge about astronomy or related optics or telescope building at my current level of experience.... BUT... I'm learning. I've learned a lot with my experience buying the used 25" scope.

With enough feedback from guys like you and Vic and Jason, I feel confident in my judgement that my secondary is too small at the current truss tube length of my new (used) scope and that it was only marginally adequate at best with the original truss tube length (1 more inch) if I was okay having issues using Paracorr 2 and only having the central .163" FOV fully illuminated was acceptable to me.

Then I learned something else which I didn't know was even a consideration and it is that of the OTA causing vignetting.

While I was focused on the secondary, it was Vic that first brought it to my attention that an OTA internal diameter of 26.31" was too small for a 25" mirror. The end of the world? No. Kind of dumb when another .5" would eliminate all vignetting? Yes. And actually, at my current truss tube height I need another .88" of internal diameter. (Question about this below)

While I am grateful for the knowledge, it leaves me questioning why a premium telescope would be setup and sold being deficient to barely adequate in these areas?

They are marketed as being designed to perform best with new 2" EPs such as Naglers and Panoptics but yet I would think you would want a larger 100% illumination area than .163" using these types of EPs. Am I wrong?

Furthermore, if you don't have enough focuser in-travel for a particular 2" eyepiece you want to use, it is recommended to cut the truss tubes in .25" increments until you get there. This sounds reasonable until you realize (like I did) that the tolerances of the provided secondary size and ID of the OTA don't have *room* to do this and not cut down performance as well.

Now to say something constructive: Its movements are smooth, its structure is sturdy, and the electronics installation is top notch.



Now my question:

On the OTA vignetting, using Newt for the Web, I noticed that at least some of the area that is vignetted is actually wider than a 2" focuser so how much peripheral view am I losing with a 2" setup? With a 3 inch focuser I understand I would fully experience the vignetting assuming I was using a 3 inch EP, is this correct?

Next question:

How do EPs' exit pupil related to the area of the focal plane? For instance, a 7mm exit pupil is only .275" so does this mean that as long as the center .275" is fully illuminated the whole FOV in that EP will be fully illuminated? I would think not or else why would anyone want a focal plane with .5" fully illuminated? Please explain.

Rob


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Re: Secondary Sizing new [Re: robininni]
      #5742129 - 03/19/13 12:53 AM

You can do the trig if you want, but a rule of thumb is that the minimum size for the I.D. of the OTA at the front lip is the mirror diameter plus the field stop of the largest field stop eyepiece.
If that's a 31 Nagler, as an example, that would mean 25 + 1.654" (42mm ) = 26.654"
And that's only the minimum size, which depends a lot upon how deep inside the UTA the secondary mirror is.

The trig is simple:
the tangent of 1/2 the maximum field size in degrees = tube clearance distance/primary-to-opening distance
Example: 12.5" f/5. Max field =1.5 degrees with 31 Nagler.
Distance from primary to opening: 60"
So tangent of .75 degrees = clearance/primary-to-opening distance.
tangent of .75 x primary to opening distance = 0.785" clearance (1 side)
UTA = 12.5" + (2 x 0.785, or 1.57")
As you can see, the proper clearance comes very close to 1/2 the field stop of the largest eyepiece used. So an easier-to-remember rule is that UTA I.D. = mirror + largest field stop.
Calculated I.D. for the example: 14.1"
UTA I.D. from rule of thumb: 14.154"
[the scope was constructed with 14.25" I.D. My last 12.5" came with a 13.25" UTA I.D. I reconstructed the UTA to eliminate the error. It had a secondary properly sized for the larger opening. As I said, some commercial scopes aren't logically made.


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Re: Secondary Sizing new [Re: robininni]
      #5742181 - 03/19/13 01:58 AM

Quote:

Next question:

How do EPs' exit pupil related to the area of the focal plane? For instance, a 7mm exit pupil is only .275" so does this mean that as long as the center .275" is fully illuminated the whole FOV in that EP will be fully illuminated? I would think not or else why would anyone want a focal plane with .5" fully illuminated? Please explain.

Rob






They are very different.
Consider some star located in your EP’s FOV. To see that star at maximum brightness, all of its photons that reflect off the primary mirror will have to also reflect off the secondary mirror. That is the first hurdle it needs to overcome. The second hurdle to overcome is to ensure all of its photons make it through your eye’s pupil.
The first hurdle has to do with the 100% illumination calculation and the second hurdle has to do with the exit pupil calculation. They are different and somewhat independent.
It is possible for a star to survive the first hurdle and end up within the 100% illumination field then fails the second hurdle and end up having part of its light clipped by your eye’s pupil.
100% illumiation tells you which stars survived the first hurdle. Exit pupil tells you if each star will survive the second hurdle.

Jason


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Re: Secondary Sizing new [Re: Vic Menard]
      #5742208 - 03/19/13 02:35 AM

Quote:

Quote:

...The point of the last few post exchanges between me and Vic is to figure out the source of the discrepancy.



What's remarkable is that because of a difference between the secondary mirror minor axis and the chord that passes through the optical center (offset) amounting to five hundredths of an inch (a bit over two hundredths on either side)--the percentage light loss increases 2.5-percent!




I re-did the math to double check using the following data:
Primary aperture: 25”
Secondary size: 3.685”
Focal length: 102.5625”
Secondary to focal plane: 15.58”


According to Bartel's site:
Effective aperture is 24.2582" or at 94.15% illumination
Minimum secondary size is 3.7977"

According to my equations:
Effective aperture is 23.9281" or at 91.61% illumination
Minimum secondary size is 3.8549"

So, a difference of 0.0572” between the chords that pass through optical versus geometric centers translates to a difference of 2.54% in illumination.


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Re: Secondary Sizing new [Re: Starman1]
      #5742218 - 03/19/13 02:46 AM

Quote:

You can do the trig if you want, but a rule of thumb is that the minimum size for the I.D. of the OTA at the front lip is the mirror diameter plus the field stop of the largest field stop eyepiece.
If that's a 31 Nagler, as an example, that would mean 25 + 1.654" (42mm ) = 26.654"
And that's only the minimum size, which depends a lot upon how deep inside the UTA the secondary mirror is.

The trig is simple:
the tangent of 1/2 the maximum field size in degrees = tube clearance distance/primary-to-opening distance
Example: 12.5" f/5. Max field =1.5 degrees with 31 Nagler.
Distance from primary to opening: 60"
So tangent of .75 degrees = clearance/primary-to-opening distance.
tangent of .75 x primary to opening distance = 0.785" clearance (1 side)
UTA = 12.5" + (2 x 0.785, or 1.57")
As you can see, the proper clearance comes very close to 1/2 the field stop of the largest eyepiece used. So an easier-to-remember rule is that UTA I.D. = mirror + largest field stop.
Calculated I.D. for the example: 14.1"
UTA I.D. from rule of thumb: 14.154"
[the scope was constructed with 14.25" I.D. My last 12.5" came with a 13.25" UTA I.D. I reconstructed the UTA to eliminate the error. It had a secondary properly sized for the larger opening. As I said, some commercial scopes aren't logically made.




You have made one major assumption. You assumed the optical axis is coincident with the OTA/UTA mechanical axis. However, if the scope happened to apply the "new mode" then you need to factor in the angle between the optical and mechanical axes.

Jason


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Re: Secondary Sizing new [Re: Starman1]
      #5742302 - 03/19/13 05:31 AM

Quote:

You can do the trig if you want, but a rule of thumb is that the minimum size for the I.D. of the OTA at the front lip is the mirror diameter plus the field stop of the largest field stop eyepiece.
If that's a 31 Nagler, as an example, that would mean 25 + 1.654" (42mm ) = 26.654"
And that's only the minimum size, which depends a lot upon how deep inside the UTA the secondary mirror is.

The trig is simple:
the tangent of 1/2 the maximum field size in degrees = tube clearance distance/primary-to-opening distance
Example: 12.5" f/5. Max field =1.5 degrees with 31 Nagler.
Distance from primary to opening: 60"
So tangent of .75 degrees = clearance/primary-to-opening distance.
tangent of .75 x primary to opening distance = 0.785" clearance (1 side)
UTA = 12.5" + (2 x 0.785, or 1.57")
As you can see, the proper clearance comes very close to 1/2 the field stop of the largest eyepiece used. So an easier-to-remember rule is that UTA I.D. = mirror + largest field stop.
Calculated I.D. for the example: 14.1"
UTA I.D. from rule of thumb: 14.154"
[the scope was constructed with 14.25" I.D. My last 12.5" came with a 13.25" UTA I.D. I reconstructed the UTA to eliminate the error. It had a secondary properly sized for the larger opening. As I said, some commercial scopes aren't logically made.




Rob
Setting aside other considerations and focusing only on Vignetting, some scope builders (Obsession) do keep things a bit tight wrt secondary size and UTA inside diameter, it is important to remember, however, that the effects of each of these are not equivalent: Field illumination drops off much more rapidly when inadequate secondar size is the soutce of vignetting than when the source is the ID of the UTA.

To see why, imagine that both the UTA and secondary are both just large enough to match the size of the light cylinder/converging light cone on axis - with no additional diameter. Now consider in a scope the size of yours, the light path for an object one inch off axis roughly at the location of the Secondary and ID of the UTA. Assume for simplicity that the distances from the secondary to the primary, and the UTA to the primary are approximately the same. Now in such a case, the relative portion of the light cone missing the secondary by one inch results in a much larger loss of light (i.e. when subtracted from the total surface area of the secondary) and much larger loss of off-axis illumination, than when compared to the relative portion of the light cylinder missing (i.e. being cut off by) the UTA by one inch. As such, vignetting due to a tight UTA, results in a far less steep fall off in field illumination. Now I am not condoning this particular aspect of telescope design (and of course there are some rationale here, such as decreasing the central obstruction etc), but it is important to understand the difference.
Joe

Edited by jpcannavo (03/19/13 06:20 AM)


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Re: Secondary Sizing new [Re: Jason D]
      #5742313 - 03/19/13 05:58 AM

Quote:

Quote:

Next question:

How do EPs' exit pupil related to the area of the focal plane? For instance, a 7mm exit pupil is only .275" so does this mean that as long as the center .275" is fully illuminated the whole FOV in that EP will be fully illuminated? I would think not or else why would anyone want a focal plane with .5" fully illuminated? Please explain.

Rob






They are very different.
Consider some star located in your EP’s FOV. To see that star at maximum brightness, all of its photons that reflect off the primary mirror will have to also reflect off the secondary mirror. That is the first hurdle it needs to overcome. The second hurdle to overcome is to ensure all of its photons make it through your eye’s pupil.
The first hurdle has to do with the 100% illumination calculation and the second hurdle has to do with the exit pupil calculation. They are different and somewhat independent.
It is possible for a star to survive the first hurdle and end up within the 100% illumination field then fails the second hurdle and end up having part of its light clipped by your eye’s pupil.
100% illumiation tells you which stars survived the first hurdle. Exit pupil tells you if each star will survive the second hurdle.

Jason




Rob

All of above by Jason correct, but to emphasize the distinction between these two issues:

Field illumination - and its fall off due to vignetting - is realized (i.e. comes out in the wash) at the real image formed at the focal plane. One could photograph this real image and measure all the effects of vignetting being considered here: i.e. UTA, secondary size etc. The exit pupil, on the other hand, can be thought of as an optical aperture created by the eyepiece through which the eye "looks at" that real image. As such, (and to a first order approximation, i.e. a few fine points still lurking here) exit pupil issues/considerations - such as it being larger than the eye's anatomic pupil - are "after the fact" of field illumination fall-off (vignetting) at the focal plane, and will instead only bear on loss of light/brightness of the second real image (in this optical path) formed at the retina. Consequently, while the dynamics of size match and alignment, between the anatomic pupil and exit pupil do bear on brightness of the real image across the retina (again some fine points lurking here) they are independent of field illumination at the focal plane.

Hope this helps!

Joe

Edited by jpcannavo (03/19/13 06:32 AM)


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robininni
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Re: Secondary Sizing new [Re: jpcannavo]
      #5742383 - 03/19/13 07:55 AM

Thanks for the answers, but I think I need to rephrase my question better to get the answer I'm really looking for:

How much of the focal plane does an EP use? Is it dependent on the EP's field stop?

If so, then for some EP's of short focal length (and small field stop)such as an Ethos 8 used in a telescope with a fully illuminated focal plane diameter of .5" would get full illumination assuming they are centered in the focuser and the 100% illumination area is centered in the focuser, correct?

Others with large field stops (Nagler 31) won't be fully illuminated even with good design principles because the field stop is too large. Is this right?

Also, back on OTA vignetting, if the 75% vignetting point (using Newt of the Web) is outside of 2" using a 2" focuser (2" focal plane?), then this vignetting would not be noticeable, right? Only if I was using a 3" focuser and 3" EP would I notice this?

Thanks,

Rob


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Re: Secondary Sizing new [Re: robininni]
      #5742466 - 03/19/13 09:04 AM

Quote:

...How much of the focal plane does an EP use? Is it dependent on the EP's field stop?



Assuming there are no other lenses in between, yes.

Quote:

If so, then for some EP's of short focal length (and small field stop)such as an Ethos 8 used in a telescope with a fully illuminated focal plane diameter of .5" would get full illumination assuming they are centered in the focuser and the 100% illumination area is centered in the focuser, correct?



Since I use a Paracorr with my 8mm Ethos, I'm guessing it would be at least as good as without the Paracorr.

Quote:

Others with large field stops (Nagler 31) won't be fully illuminated even with good design principles because the field stop is too large. Is this right?



Before TeleVue redesigned the visual Paracorr (Type 1), the Paracorr itself was causing vignetting with the 31 Nagler. The new visual design seemed to correct the issue, although I'm not certain the issue was eliminated. The New Type 2 visual performance seems to be at least equal (if not better) than the corrected Type 1 with respect to field illumination.

Quote:

Also, back on OTA vignetting, if the 75% vignetting point (using Newt of the Web) is outside of 2" using a 2" focuser (2" focal plane?), then this vignetting would not be noticeable, right? Only if I was using a 3" focuser and 3" EP would I notice this?



I never could understand what they were trying to accomplish with this tidbit of information. As Joe has already pointed out, vignetting from the front aperture is inconsequential compared to light loss from an undersized secondary. Even when the front aperture is equal to the primary mirror diameter, the light loss is rarely more than a few percent. I'm usually more concerned with keeping the light path unobstructed, and tube wall currents managed.

With regards to matching the telescope's exit pupil to your anatomical pupil, I tend to favor keeping the exit pupil slightly smaller (perhaps by a millimeter or more, depending on your visual acuity). Using less of the edge of your anatomical pupil reduces scatter and keeps the image in the sweet spot, at least that's been my experience (with my vision).

Optimizing a Newtonian is all about trade-offs. Years ago, an f/4 Newtonian would primarily be used as a RFT unless you also had an interchangeable cassegrain secondary to push the focal ratio up to f/15 (one of my first project scopes many years ago). Modern f/4 Newtonians are much more versatile, which makes them much harder to optimize for the way they end up being used. The good news is that larger (visual) apertures with short focal ratios scale up easier, providing excellent performance across a wide range of magnifications.


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Re: Secondary Sizing new [Re: Jason D]
      #5742636 - 03/19/13 10:52 AM

Quote:

Quote:

You can do the trig if you want, but a rule of thumb is that the minimum size for the I.D. of the OTA at the front lip is the mirror diameter plus the field stop of the largest field stop eyepiece.
If that's a 31 Nagler, as an example, that would mean 25 + 1.654" (42mm ) = 26.654"
And that's only the minimum size, which depends a lot upon how deep inside the UTA the secondary mirror is.

The trig is simple:
the tangent of 1/2 the maximum field size in degrees = tube clearance distance/primary-to-opening distance
Example: 12.5" f/5. Max field =1.5 degrees with 31 Nagler.
Distance from primary to opening: 60"
So tangent of .75 degrees = clearance/primary-to-opening distance.
tangent of .75 x primary to opening distance = 0.785" clearance (1 side)
UTA = 12.5" + (2 x 0.785, or 1.57")
As you can see, the proper clearance comes very close to 1/2 the field stop of the largest eyepiece used. So an easier-to-remember rule is that UTA I.D. = mirror + largest field stop.
Calculated I.D. for the example: 14.1"
UTA I.D. from rule of thumb: 14.154"
[the scope was constructed with 14.25" I.D. My last 12.5" came with a 13.25" UTA I.D. I reconstructed the UTA to eliminate the error. It had a secondary properly sized for the larger opening. As I said, some commercial scopes aren't logically made.




You have made one major assumption. You assumed the optical axis is coincident with the OTA/UTA mechanical axis. However, if the scope happened to apply the "new mode" then you need to factor in the angle between the optical and mechanical axes.

Jason



You are correct. With the "tilt" of the optical axis in the "New Model" collimation, there is rationale for having the I.D. of the UTA be even larger than I calculate to account for that tilt.
You know, back in the '50s the standard was to have the I.D. of the tube be at least 2" larger than the mirror. Those guys may have known what they were doing.


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Re: Secondary Sizing new [Re: robininni]
      #5742651 - 03/19/13 10:59 AM

Quote:

Thanks for the answers, but I think I need to rephrase my question better to get the answer I'm really looking for:

How much of the focal plane does an EP use? Is it dependent on the EP's field stop?

If so, then for some EP's of short focal length (and small field stop)such as an Ethos 8 used in a telescope with a fully illuminated focal plane diameter of .5" would get full illumination assuming they are centered in the focuser and the 100% illumination area is centered in the focuser, correct?

Others with large field stops (Nagler 31) won't be fully illuminated even with good design principles because the field stop is too large. Is this right?

Also, back on OTA vignetting, if the 75% vignetting point (using Newt of the Web) is outside of 2" using a 2" focuser (2" focal plane?), then this vignetting would not be noticeable, right? Only if I was using a 3" focuser and 3" EP would I notice this?

Thanks,

Rob



Vic answered your questions, but I will add that it is useful to think of the telescope as having an image produced on its focal plane and the eyepiece merely a magnifying glass used to look at a portion of that focal plane.
Low power eyepieces see a larger portion of that focal plane and magnify less. High power eyepieces see a smaller portion of that focal plane and magnify more.
If the secondary size is chosen to provide 100% illumination over, say, the center 1/2" of the focal plane, then an eyepiece whose field stop was 1/2" or less would see a 100% illuminated field from edge-to-edge.
Of course, the exit pupil of that eyepiece might not fill your eye's pupil, so the image would be dimmer than a low power eyepiece with some edge-of-field illumination drop off.


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