Telescope Reviews: Secondary Sizing

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robininni
professor emeritus

Reged: 04/18/11

Loc: Stephenville, TX

Secondary Sizing new

new

#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
Carpal Tunnel

Reged: 12/05/07

Loc: SW Michigan

Re: Secondary Sizing [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
member

Reged: 01/26/12

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

There is no link.

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KerryR
Carpal Tunnel

Reged: 12/05/07

Loc: SW Michigan

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

Just fixed it.

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dan_h
Carpal Tunnel

Reged: 12/10/07

Re: Secondary Sizing [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
member

Reged: 01/26/12

Re: Secondary Sizing [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 [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
professor emeritus

Reged: 04/18/11

Loc: Stephenville, TX

Re: Secondary Sizing [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
Post Laureate

Reged: 04/01/08

Loc: Lovettsville, VA

Re: Secondary Sizing [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
professor emeritus

Reged: 04/18/11

Loc: Stephenville, TX

Re: Secondary Sizing [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
Carpal Tunnel

Reged: 12/10/07

Re: Secondary Sizing [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
super member

Reged: 10/16/12

Re: Secondary Sizing [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
professor emeritus

Reged: 04/18/11

Loc: Stephenville, TX

Re: Secondary Sizing [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
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA

Re: Secondary Sizing [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
Post Laureate

Reged: 04/01/08

Loc: Lovettsville, VA

Re: Secondary Sizing [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
Carpal Tunnel

Reged: 09/19/07

Loc: Somewhere in the Orion Spur

Re: Secondary Sizing new

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
professor emeritus

Reged: 04/18/11

Loc: Stephenville, TX

Re: Secondary Sizing new

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
Postmaster

Reged: 10/21/06

Loc: California

Re: Secondary Sizing new

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
Postmaster

Reged: 10/21/06

Loc: California

Re: Secondary Sizing new

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
professor emeritus

Reged: 04/18/11

Loc: Stephenville, TX

Re: Secondary Sizing new

new

[Re: Jason D]
#5736137 - 03/16/13 12:09 PM Attachment (6 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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