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Secondary Sizing

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#1 robininni

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Posted 15 March 2013 - 12:21 PM

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

Thanks,

Rob

#2 KerryR

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Posted 15 March 2013 - 01:42 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.

#3 FineArt

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Posted 15 March 2013 - 01:43 PM

There is no link.

#4 KerryR

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Posted 15 March 2013 - 01:43 PM

Just fixed it.

#5 dan_h

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Posted 15 March 2013 - 01:59 PM

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

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

dan

#6 FineArt

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Posted 15 March 2013 - 02:18 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.

#7 FineArt

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Posted 15 March 2013 - 03:11 PM

Here is a shot taken with that 10" dob.

http://farm9.staticf...d2e864be5_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.

#8 robininni

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Posted 15 March 2013 - 03:58 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

#9 Mirzam

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Posted 15 March 2013 - 04:04 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

#10 robininni

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Posted 15 March 2013 - 04:26 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


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

Rob

#11 dan_h

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Posted 15 March 2013 - 04:35 PM

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

#12 ZuoZhao

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Posted 15 March 2013 - 08:55 PM

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

#13 robininni

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Posted 16 March 2013 - 12:40 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

#14 Jon Isaacs

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Posted 16 March 2013 - 03:49 AM

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

#15 Mirzam

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Posted 16 March 2013 - 05:26 AM

I would try Tom Ozypowski:

Equatorial Platforms

or Mike Lockwood:

Lockwood Custom Optics

JimC

#16 Fred1

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Posted 16 March 2013 - 09:55 AM

You can call Terry Ostahowski.

And also, Astrosystems.

#17 robininni

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Posted 16 March 2013 - 10:21 AM

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

#18 Jason D

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Posted 16 March 2013 - 10:26 AM

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

Correct

#19 Jason D

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Posted 16 March 2013 - 10:33 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.cloudynig...ber/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.

#20 robininni

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Posted 16 March 2013 - 11:09 AM

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

Attached Files



#21 robininni

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Posted 16 March 2013 - 11:18 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.cloudynig...ber/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


#22 Jason D

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Posted 16 March 2013 - 11:25 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.cloudynig...ber/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

#23 robininni

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Posted 16 March 2013 - 11:26 AM

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

Rob

#24 Jason D

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Posted 16 March 2013 - 11:31 AM

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

#25 Pinbout

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Posted 16 March 2013 - 11:47 AM

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

:o :grin:






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