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robininni
scholastic sledgehammer

Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing [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

min = H * (16/63)

There are more formulas in this post

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
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [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

min = H * (16/63)

There are more formulas in this post

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 [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
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [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
Postmaster

Reged: 02/22/10

Loc: nj
Re: Secondary Sizing [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 [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 [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
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [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.

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))

 Post Extras:
Jason D
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [Re: robininni]

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
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Secondary Sizing [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
scholastic sledgehammer

Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing [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
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Secondary Sizing [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 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
Carpal Tunnel

Reged: 09/19/07

Loc: Somewhere in the Orion Spur
Re: Secondary Sizing [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.

 Post Extras:
Vic Menard
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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
scholastic sledgehammer

Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing [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)

 Post Extras:
Vic Menard
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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
scholastic sledgehammer

Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing [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

 Post Extras:
robininni
scholastic sledgehammer

Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing [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
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [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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