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Jason D
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [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
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [Re: Vic Menard]
#5739289 - 03/17/13 06:09 PM

Referring to info in this post
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
scholastic sledgehammer

Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing [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
scholastic sledgehammer

Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing [Re: Jason D]
#5739506 - 03/17/13 07:44 PM

Quote:

Referring to info in this post
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
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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
Pooh-Bah

Reged: 08/31/09

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

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
Post Laureate

Reged: 07/21/04

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

Good point Mitch!

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Jason D
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [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
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [Re: Jason D]

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
Vendor (Woden Optics)

Reged: 07/07/04

Loc: Rancho Cordova, CA
Re: Secondary Sizing [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.

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Jason D
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [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
Vendor (Woden Optics)

Reged: 07/07/04

Loc: Rancho Cordova, CA
Re: Secondary Sizing [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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Vic Menard
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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
Postmaster

Reged: 10/21/06

Loc: California
Re: Secondary Sizing [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
Vendor (Woden Optics)

Reged: 07/07/04

Loc: Rancho Cordova, CA
Re: Secondary Sizing [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
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles
Re: Secondary Sizing [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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Vic Menard
Post Laureate

Reged: 07/21/04

Re: Secondary Sizing [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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