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Advice needed on secondary size

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17 replies to this topic

#1 Itz marcus

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Posted 12 September 2019 - 01:06 PM

Hi,

I'm gonna be replacing my secondary with a 1/20 antares secondary. The original secondary was a 70mm.  Antares makes a 2.6 inch which is 66mm and a 3.1 which is 78mm. The 3.1 inch is much more expensive. Can I get away with the 2.6 inch or will I be effectively not be using my full primary mirror. The scope is s synta (orion brand) 12 inch f 4.9 truss tube.

Clear Skies

Itz


Edited by Itz marcus, 12 September 2019 - 01:07 PM.


#2 scopewizard

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Posted 12 September 2019 - 01:33 PM

If it is a visual only scope I would go with 2.6 inch. My 16" F5 has a 2.77 inch secondary.

The smaller your secondary is the better the contrast is.

I was told by a master optician that keeping the surface of a secondary to less that 3% of the surface of a primary, contrast lost is almost to nil.

12" with 2.6" = 4.6%

12" with 3.1" = 6.7%

16" with 2.77 = 3.0%



#3 havasman

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Posted 12 September 2019 - 01:54 PM

That'll depend on the exit pupil of the eyepieces in use and how you weigh the value of fully illuminating them.

Here's the calculator with graphic representation  -  http://www.bbastrode...m/diagonal.htm 

 

For my use I'd rather have a 1/14th wave secondary that fully illuminates my widest field eyepiece than a 1/30th wave that leaves off part of what the primary might deliver to the eyepiece. But your location might not much reward very large exit pupil observing due to the ambient light of the city. So your priorities might differ.



#4 Garyth64

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Posted 12 September 2019 - 02:20 PM

What is the diameter of the UTA?

How far outside the UTA is your focus?

 

If I guess at the diameter of the UTA at 14", and the focus is 4" outside the UTA, then (according to a formula), you should have a secondary of at least 2.67".

 

Your original secondary blocked 5.2% of the incoming light.

 

A 3.1" secondary is only blocking about 6.6% of the incoming light.  A 2.6" secondary blocks 4.6% of the incoming light, but you won't have a fully illuminated field.

 

What's wrong with the secondary that you have?  Do you think that a 1/20 secondary will give you better views?  And maybe the fault is in the  primary. (?)


Edited by Garyth64, 12 September 2019 - 02:23 PM.

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#5 Itz marcus

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Posted 12 September 2019 - 03:50 PM

My secondary tested at 1/2 wave over part and 1 wave over part. That is why I want to replace it. The mirror was tested by Nova and Mr. Dodds found it to be about 1/6 wavefront.

 

According to M. Lockwoods article one adds 4 to the diameter of the scope and divides by the focal ratio. If so then all I technically need is a 2.14 but since its close i would use a 2.6. However, other formulas used measuring the focal length and the distance to the focal plane come out to another number.

What say you guys?

Clear Skies

Itz+



#6 Starman1

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Posted 12 September 2019 - 05:23 PM

It depends on the distance from your secondary mirror to the focal plane.

I assume 

6" mirror radius

3/4" from edge of mirror to tube

1/8" for tube thickness

3" for focuser height including base plate

1" additional to focal plane.

That means 10.875" from secondary to focal plane.

Using a calculator that takes into account the desired illuminated field (I'll use 1.65" for a Nagler 31mm or ES 30mm)

and a light loss at the edge of the field of 0.4 magnitudes, and I get:

2.6" as the very minimum sized secondary.  If it is glued to a stalk like the original.

If it's used in a secondary holder with a lip all the way around, you'll have a bit more light lose at the edge, but 2.6" will still be OK.

And if the distance from secondary to focal plane is a tad shorter, even better for the 2.6" size.

Mike Lockwood's calculation shows a 2.6" size, with a 0.5" fully illuminated circle.

The calculator I used assumed a 0.4" fully illuminated center to the field.

Either way, 2.6".


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#7 Garyth64

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Posted 12 September 2019 - 06:21 PM

My secondary tested at 1/2 wave over part and 1 wave over part. That is why I want to replace it. The mirror was tested by Nova and Mr. Dodds found it to be about 1/6 wavefront.

 

According to M. Lockwoods article one adds 4 to the diameter of the scope and divides by the focal ratio. If so then all I technically need is a 2.14 but since its close i would use a 2.6. However, other formulas used measuring the focal length and the distance to the focal plane come out to another number.

What say you guys?

Clear Skies

Itz+

Hmmm, So for your 12" f/4.9 scope . . .

 

12 + 4 = 16,      16/4.9 = 3.2"

 

maybe I divided wrong.

 

I'll stay with my formula. smile.gif


Edited by Garyth64, 12 September 2019 - 06:22 PM.

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#8 Itz marcus

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Posted 12 September 2019 - 08:03 PM

Hi,

 

Hmmm, So for your 12" f/4.9 scope . . .

 

12 + 4 = 16,      16/4.9 = 3.2"

 

maybe I divided wrong.

 

I'll stay with my formula. smile.gif

Its 4 plus the radius not the diameter so it's (6 = 4)/4.9= 2.04

Clear Skies

Itz



#9 Jon Isaacs

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Posted 13 September 2019 - 03:39 AM

Itz:

 

You really should be calculating the size of the illuminated field from the real parameters. You need to know just how far it is from the center of the secondary to the focal plane. 

 

Don made a guess. My experience with similar Dobs is something likevthis:

 

Radius of tube: 6 inches  plus 1 inch clearance = 7 inches

Racked in focuser = 3 inches

Back focus = 1 inch

 

Distance to focal plane = 11 inches. This is very close to Don's estimate. I imagine he used Mel Bartels Diagonal Off Axis Illumination Calculator.

 

https://www.bbastrod...om/diagonal.htm

 

A 2.6 inch looks good to me, about 0.5 degree fully illuminated circle, down about 0.4 magnitudes at the edge of the field. 

 

One thing about a small illuminated field, the positioning of the secondary becomes critical. A 2.6 inch is a good balance.

 

Another quick calc: you can calculate the minimum possible diagonal size, the diameter that only uses the full aperture at a point in the very center of the field, by dividing the distance to the focal plane by the focal ratio.

 

In your case, that would be about 11"/5 = 2.2".

 

Jon


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#10 Itz marcus

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Posted 13 September 2019 - 07:11 AM

Hi,

Thanx for all the advice. I just pulled the trigger on an 2.6 inch Antares 1/20 secondary. I now am waiting  to get my primary back (tested by Nova Optical) and my secondary from Antares to see if this new secondary changes what I've been seeing. Of course the clouds will come with whichever mirror comes last!!! I will say that the views have been ok just hardly ever able to get above 180x on Jupiter. and detail didn't POP out like it did in my C8 although MUCH brighter.

Clear Skies

Itz



#11 Garyth64

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Posted 13 September 2019 - 07:13 AM

Lockwood's article says nothing about the secondary distance, it is not exact.  I would not use it.

 

In my post, I also used 11" as the secondary to focus distance.

 

Marcus, what is that distance on your telescope?  That distance is critical.

 

The formula that I use is this one:      s = (D - a) x d     + a

                                                                     f

s = ?, the minor axis of the secondary

D = 12", diameter of primary

d = 11", distance of secondary to the image plane

f = 58.8", focal length of primary

a = .52".  this is from using the diameter of the Moon at the image plane,   f x tan31', or 58.8" x .009 = 52"

 

For these parameters, the secondary size is 2.67", if the it's distance to the focus is 11".

 

For fun, what is the diameter of the cone of light at 11" inside the focus?  A simple ratio:

 

12"/58.8" = x/11"   ,   x = 2.24",   your secondary should be larger than that.

 

If your secondary distance is 12", then your secondary size should be 2.86".

 

So what is the distance from the center of your tube to where the focus is?


Edited by Garyth64, 13 September 2019 - 07:23 AM.

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#12 Itz marcus

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Posted 13 September 2019 - 07:54 AM

Hi,

Its apprx 10 inches

Clear skies

Itz


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#13 Garyth64

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Posted 13 September 2019 - 08:13 AM

So, ok then, that gives 2.47" from the formula, so your new 2.6" secondary should work.



#14 Jon Isaacs

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Posted 13 September 2019 - 02:54 PM

Hi,

Its apprx 10 inches

Clear skies

Itz

Does that include the back focus or is it the distance to the top of the racked in Focuser?  Normally commercial Dobs have the focal plane about 1 inch beyond the racked in Focuser, in other words, with the focuser racked out about an inch.

 

Jon



#15 Itz marcus

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Posted 13 September 2019 - 04:46 PM

Hi 

Just checked. With focuser wracked out an inch it is slightly less than 10.5 inches I would say 10 3/8 inches.

Clear Skies

Itz



#16 Asbytec

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Posted 15 September 2019 - 12:31 AM

Hi 

Just checked. With focuser wracked out an inch it is slightly less than 10.5 inches I would say 10 3/8 inches.

Clear Skies

Itz

You don't have to be that precise because the available mirror sizes are not that precise. Running the numbers on Mel Bartel's calculator gives 2.6" and that's consistent with Jon's and Don's estimates above and Gary's calculation of 2.67" (which you will not find unless you go up one size) and it is larger than his 2.24" calculation for the diameter of the light cone at 11".

 

So, you pulled the trigger on target. 

 

Using the radius +4 method, it comes up short by about 0.5". Mel's calculator says no diagonals "meet the criteria". 



#17 Jon Isaacs

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Posted 15 September 2019 - 04:44 AM

Using the radius +4 method, it comes up short by about 0.5". Mel's calculator says no diagonals "meet the criteria".

 

 

Something ain't quite right.  The radius is 6 inches, so radius plus 4 = 10 inches which is smaller than the 11 inches Don and I used.  

 

I think the logic of the radius plus 4 is something like this:

 

Radius

1 inch to focuser board

1.5 inch racked in focuser height.  Anyone buying one of Mike's mirrors is almost certainly using a Feathertouch focuser or other low profile focuser. My JMI's are 1.5 inch racked in. 

3/4 inch for back focus

3/4 inch for off-axis illumination. 

 

Jon



#18 Asbytec

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Posted 15 September 2019 - 05:16 AM

"Anyone buying one of Mike's mirrors is almost certainly using a Feathertouch focuser or other low profile focuser."

Yea, that sounds more like it.


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