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Equipment Discussions >> Reflectors

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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Secondary Sizing new [Re: Jason D]
      #5741388 - 03/18/13 06:41 PM

Quote:

...The point of the last few post exchanges between me and Vic is to figure out the source of the discrepancy.



What's remarkable is that because of a difference between the secondary mirror minor axis and the chord that passes through the optical center (offset) amounting to five hundredths of an inch (a bit over two hundredths on either side)--the percentage light loss increases 2.5-percent!


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: Starman1]
      #5741531 - 03/18/13 07:40 PM

Quote:



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.

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.

IMO, of course.




Well said, Don!

I don't pretend to have much knowledge about astronomy or related optics or telescope building at my current level of experience.... BUT... I'm learning. I've learned a lot with my experience buying the used 25" scope.

With enough feedback from guys like you and Vic and Jason, I feel confident in my judgement that my secondary is too small at the current truss tube length of my new (used) scope and that it was only marginally adequate at best with the original truss tube length (1 more inch) if I was okay having issues using Paracorr 2 and only having the central .163" FOV fully illuminated was acceptable to me.

Then I learned something else which I didn't know was even a consideration and it is that of the OTA causing vignetting.

While I was focused on the secondary, it was Vic that first brought it to my attention that an OTA internal diameter of 26.31" was too small for a 25" mirror. The end of the world? No. Kind of dumb when another .5" would eliminate all vignetting? Yes. And actually, at my current truss tube height I need another .88" of internal diameter. (Question about this below)

While I am grateful for the knowledge, it leaves me questioning why a premium telescope would be setup and sold being deficient to barely adequate in these areas?

They are marketed as being designed to perform best with new 2" EPs such as Naglers and Panoptics but yet I would think you would want a larger 100% illumination area than .163" using these types of EPs. Am I wrong?

Furthermore, if you don't have enough focuser in-travel for a particular 2" eyepiece you want to use, it is recommended to cut the truss tubes in .25" increments until you get there. This sounds reasonable until you realize (like I did) that the tolerances of the provided secondary size and ID of the OTA don't have *room* to do this and not cut down performance as well.

Now to say something constructive: Its movements are smooth, its structure is sturdy, and the electronics installation is top notch.



Now my question:

On the OTA vignetting, using Newt for the Web, I noticed that at least some of the area that is vignetted is actually wider than a 2" focuser so how much peripheral view am I losing with a 2" setup? With a 3 inch focuser I understand I would fully experience the vignetting assuming I was using a 3 inch EP, is this correct?

Next question:

How do EPs' exit pupil related to the area of the focal plane? For instance, a 7mm exit pupil is only .275" so does this mean that as long as the center .275" is fully illuminated the whole FOV in that EP will be fully illuminated? I would think not or else why would anyone want a focal plane with .5" fully illuminated? Please explain.

Rob


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Secondary Sizing new [Re: robininni]
      #5742129 - 03/19/13 12:53 AM

You can do the trig if you want, but a rule of thumb is that the minimum size for the I.D. of the OTA at the front lip is the mirror diameter plus the field stop of the largest field stop eyepiece.
If that's a 31 Nagler, as an example, that would mean 25 + 1.654" (42mm ) = 26.654"
And that's only the minimum size, which depends a lot upon how deep inside the UTA the secondary mirror is.

The trig is simple:
the tangent of 1/2 the maximum field size in degrees = tube clearance distance/primary-to-opening distance
Example: 12.5" f/5. Max field =1.5 degrees with 31 Nagler.
Distance from primary to opening: 60"
So tangent of .75 degrees = clearance/primary-to-opening distance.
tangent of .75 x primary to opening distance = 0.785" clearance (1 side)
UTA = 12.5" + (2 x 0.785, or 1.57")
As you can see, the proper clearance comes very close to 1/2 the field stop of the largest eyepiece used. So an easier-to-remember rule is that UTA I.D. = mirror + largest field stop.
Calculated I.D. for the example: 14.1"
UTA I.D. from rule of thumb: 14.154"
[the scope was constructed with 14.25" I.D. My last 12.5" came with a 13.25" UTA I.D. I reconstructed the UTA to eliminate the error. It had a secondary properly sized for the larger opening. As I said, some commercial scopes aren't logically made.


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Jason D
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Reged: 10/21/06

Loc: California
Re: Secondary Sizing new [Re: robininni]
      #5742181 - 03/19/13 01:58 AM

Quote:

Next question:

How do EPs' exit pupil related to the area of the focal plane? For instance, a 7mm exit pupil is only .275" so does this mean that as long as the center .275" is fully illuminated the whole FOV in that EP will be fully illuminated? I would think not or else why would anyone want a focal plane with .5" fully illuminated? Please explain.

Rob






They are very different.
Consider some star located in your EPs FOV. To see that star at maximum brightness, all of its photons that reflect off the primary mirror will have to also reflect off the secondary mirror. That is the first hurdle it needs to overcome. The second hurdle to overcome is to ensure all of its photons make it through your eyes pupil.
The first hurdle has to do with the 100% illumination calculation and the second hurdle has to do with the exit pupil calculation. They are different and somewhat independent.
It is possible for a star to survive the first hurdle and end up within the 100% illumination field then fails the second hurdle and end up having part of its light clipped by your eyes pupil.
100% illumiation tells you which stars survived the first hurdle. Exit pupil tells you if each star will survive the second hurdle.

Jason


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Jason D
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Reged: 10/21/06

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Re: Secondary Sizing new [Re: Vic Menard]
      #5742208 - 03/19/13 02:35 AM

Quote:

Quote:

...The point of the last few post exchanges between me and Vic is to figure out the source of the discrepancy.



What's remarkable is that because of a difference between the secondary mirror minor axis and the chord that passes through the optical center (offset) amounting to five hundredths of an inch (a bit over two hundredths on either side)--the percentage light loss increases 2.5-percent!




I re-did the math to double check using the following data:
Primary aperture: 25
Secondary size: 3.685
Focal length: 102.5625
Secondary to focal plane: 15.58


According to Bartel's site:
Effective aperture is 24.2582" or at 94.15% illumination
Minimum secondary size is 3.7977"

According to my equations:
Effective aperture is 23.9281" or at 91.61% illumination
Minimum secondary size is 3.8549"

So, a difference of 0.0572 between the chords that pass through optical versus geometric centers translates to a difference of 2.54% in illumination.


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Jason D
Postmaster
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Reged: 10/21/06

Loc: California
Re: Secondary Sizing new [Re: Starman1]
      #5742218 - 03/19/13 02:46 AM

Quote:

You can do the trig if you want, but a rule of thumb is that the minimum size for the I.D. of the OTA at the front lip is the mirror diameter plus the field stop of the largest field stop eyepiece.
If that's a 31 Nagler, as an example, that would mean 25 + 1.654" (42mm ) = 26.654"
And that's only the minimum size, which depends a lot upon how deep inside the UTA the secondary mirror is.

The trig is simple:
the tangent of 1/2 the maximum field size in degrees = tube clearance distance/primary-to-opening distance
Example: 12.5" f/5. Max field =1.5 degrees with 31 Nagler.
Distance from primary to opening: 60"
So tangent of .75 degrees = clearance/primary-to-opening distance.
tangent of .75 x primary to opening distance = 0.785" clearance (1 side)
UTA = 12.5" + (2 x 0.785, or 1.57")
As you can see, the proper clearance comes very close to 1/2 the field stop of the largest eyepiece used. So an easier-to-remember rule is that UTA I.D. = mirror + largest field stop.
Calculated I.D. for the example: 14.1"
UTA I.D. from rule of thumb: 14.154"
[the scope was constructed with 14.25" I.D. My last 12.5" came with a 13.25" UTA I.D. I reconstructed the UTA to eliminate the error. It had a secondary properly sized for the larger opening. As I said, some commercial scopes aren't logically made.




You have made one major assumption. You assumed the optical axis is coincident with the OTA/UTA mechanical axis. However, if the scope happened to apply the "new mode" then you need to factor in the angle between the optical and mechanical axes.

Jason


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jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Secondary Sizing new [Re: Starman1]
      #5742302 - 03/19/13 05:31 AM

Quote:

You can do the trig if you want, but a rule of thumb is that the minimum size for the I.D. of the OTA at the front lip is the mirror diameter plus the field stop of the largest field stop eyepiece.
If that's a 31 Nagler, as an example, that would mean 25 + 1.654" (42mm ) = 26.654"
And that's only the minimum size, which depends a lot upon how deep inside the UTA the secondary mirror is.

The trig is simple:
the tangent of 1/2 the maximum field size in degrees = tube clearance distance/primary-to-opening distance
Example: 12.5" f/5. Max field =1.5 degrees with 31 Nagler.
Distance from primary to opening: 60"
So tangent of .75 degrees = clearance/primary-to-opening distance.
tangent of .75 x primary to opening distance = 0.785" clearance (1 side)
UTA = 12.5" + (2 x 0.785, or 1.57")
As you can see, the proper clearance comes very close to 1/2 the field stop of the largest eyepiece used. So an easier-to-remember rule is that UTA I.D. = mirror + largest field stop.
Calculated I.D. for the example: 14.1"
UTA I.D. from rule of thumb: 14.154"
[the scope was constructed with 14.25" I.D. My last 12.5" came with a 13.25" UTA I.D. I reconstructed the UTA to eliminate the error. It had a secondary properly sized for the larger opening. As I said, some commercial scopes aren't logically made.




Rob
Setting aside other considerations and focusing only on Vignetting, some scope builders (Obsession) do keep things a bit tight wrt secondary size and UTA inside diameter, it is important to remember, however, that the effects of each of these are not equivalent: Field illumination drops off much more rapidly when inadequate secondar size is the soutce of vignetting than when the source is the ID of the UTA.

To see why, imagine that both the UTA and secondary are both just large enough to match the size of the light cylinder/converging light cone on axis - with no additional diameter. Now consider in a scope the size of yours, the light path for an object one inch off axis roughly at the location of the Secondary and ID of the UTA. Assume for simplicity that the distances from the secondary to the primary, and the UTA to the primary are approximately the same. Now in such a case, the relative portion of the light cone missing the secondary by one inch results in a much larger loss of light (i.e. when subtracted from the total surface area of the secondary) and much larger loss of off-axis illumination, than when compared to the relative portion of the light cylinder missing (i.e. being cut off by) the UTA by one inch. As such, vignetting due to a tight UTA, results in a far less steep fall off in field illumination. Now I am not condoning this particular aspect of telescope design (and of course there are some rationale here, such as decreasing the central obstruction etc), but it is important to understand the difference.
Joe

Edited by jpcannavo (03/19/13 06:20 AM)


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jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Secondary Sizing new [Re: Jason D]
      #5742313 - 03/19/13 05:58 AM

Quote:

Quote:

Next question:

How do EPs' exit pupil related to the area of the focal plane? For instance, a 7mm exit pupil is only .275" so does this mean that as long as the center .275" is fully illuminated the whole FOV in that EP will be fully illuminated? I would think not or else why would anyone want a focal plane with .5" fully illuminated? Please explain.

Rob






They are very different.
Consider some star located in your EPs FOV. To see that star at maximum brightness, all of its photons that reflect off the primary mirror will have to also reflect off the secondary mirror. That is the first hurdle it needs to overcome. The second hurdle to overcome is to ensure all of its photons make it through your eyes pupil.
The first hurdle has to do with the 100% illumination calculation and the second hurdle has to do with the exit pupil calculation. They are different and somewhat independent.
It is possible for a star to survive the first hurdle and end up within the 100% illumination field then fails the second hurdle and end up having part of its light clipped by your eyes pupil.
100% illumiation tells you which stars survived the first hurdle. Exit pupil tells you if each star will survive the second hurdle.

Jason




Rob

All of above by Jason correct, but to emphasize the distinction between these two issues:

Field illumination - and its fall off due to vignetting - is realized (i.e. comes out in the wash) at the real image formed at the focal plane. One could photograph this real image and measure all the effects of vignetting being considered here: i.e. UTA, secondary size etc. The exit pupil, on the other hand, can be thought of as an optical aperture created by the eyepiece through which the eye "looks at" that real image. As such, (and to a first order approximation, i.e. a few fine points still lurking here) exit pupil issues/considerations - such as it being larger than the eye's anatomic pupil - are "after the fact" of field illumination fall-off (vignetting) at the focal plane, and will instead only bear on loss of light/brightness of the second real image (in this optical path) formed at the retina. Consequently, while the dynamics of size match and alignment, between the anatomic pupil and exit pupil do bear on brightness of the real image across the retina (again some fine points lurking here) they are independent of field illumination at the focal plane.

Hope this helps!

Joe

Edited by jpcannavo (03/19/13 06:32 AM)


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Secondary Sizing new [Re: jpcannavo]
      #5742383 - 03/19/13 07:55 AM

Thanks for the answers, but I think I need to rephrase my question better to get the answer I'm really looking for:

How much of the focal plane does an EP use? Is it dependent on the EP's field stop?

If so, then for some EP's of short focal length (and small field stop)such as an Ethos 8 used in a telescope with a fully illuminated focal plane diameter of .5" would get full illumination assuming they are centered in the focuser and the 100% illumination area is centered in the focuser, correct?

Others with large field stops (Nagler 31) won't be fully illuminated even with good design principles because the field stop is too large. Is this right?

Also, back on OTA vignetting, if the 75% vignetting point (using Newt of the Web) is outside of 2" using a 2" focuser (2" focal plane?), then this vignetting would not be noticeable, right? Only if I was using a 3" focuser and 3" EP would I notice this?

Thanks,

Rob


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Vic Menard
Post Laureate
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Secondary Sizing new [Re: robininni]
      #5742466 - 03/19/13 09:04 AM

Quote:

...How much of the focal plane does an EP use? Is it dependent on the EP's field stop?



Assuming there are no other lenses in between, yes.

Quote:

If so, then for some EP's of short focal length (and small field stop)such as an Ethos 8 used in a telescope with a fully illuminated focal plane diameter of .5" would get full illumination assuming they are centered in the focuser and the 100% illumination area is centered in the focuser, correct?



Since I use a Paracorr with my 8mm Ethos, I'm guessing it would be at least as good as without the Paracorr.

Quote:

Others with large field stops (Nagler 31) won't be fully illuminated even with good design principles because the field stop is too large. Is this right?



Before TeleVue redesigned the visual Paracorr (Type 1), the Paracorr itself was causing vignetting with the 31 Nagler. The new visual design seemed to correct the issue, although I'm not certain the issue was eliminated. The New Type 2 visual performance seems to be at least equal (if not better) than the corrected Type 1 with respect to field illumination.

Quote:

Also, back on OTA vignetting, if the 75% vignetting point (using Newt of the Web) is outside of 2" using a 2" focuser (2" focal plane?), then this vignetting would not be noticeable, right? Only if I was using a 3" focuser and 3" EP would I notice this?



I never could understand what they were trying to accomplish with this tidbit of information. As Joe has already pointed out, vignetting from the front aperture is inconsequential compared to light loss from an undersized secondary. Even when the front aperture is equal to the primary mirror diameter, the light loss is rarely more than a few percent. I'm usually more concerned with keeping the light path unobstructed, and tube wall currents managed.

With regards to matching the telescope's exit pupil to your anatomical pupil, I tend to favor keeping the exit pupil slightly smaller (perhaps by a millimeter or more, depending on your visual acuity). Using less of the edge of your anatomical pupil reduces scatter and keeps the image in the sweet spot, at least that's been my experience (with my vision).

Optimizing a Newtonian is all about trade-offs. Years ago, an f/4 Newtonian would primarily be used as a RFT unless you also had an interchangeable cassegrain secondary to push the focal ratio up to f/15 (one of my first project scopes many years ago). Modern f/4 Newtonians are much more versatile, which makes them much harder to optimize for the way they end up being used. The good news is that larger (visual) apertures with short focal ratios scale up easier, providing excellent performance across a wide range of magnifications.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Secondary Sizing new [Re: Jason D]
      #5742636 - 03/19/13 10:52 AM

Quote:

Quote:

You can do the trig if you want, but a rule of thumb is that the minimum size for the I.D. of the OTA at the front lip is the mirror diameter plus the field stop of the largest field stop eyepiece.
If that's a 31 Nagler, as an example, that would mean 25 + 1.654" (42mm ) = 26.654"
And that's only the minimum size, which depends a lot upon how deep inside the UTA the secondary mirror is.

The trig is simple:
the tangent of 1/2 the maximum field size in degrees = tube clearance distance/primary-to-opening distance
Example: 12.5" f/5. Max field =1.5 degrees with 31 Nagler.
Distance from primary to opening: 60"
So tangent of .75 degrees = clearance/primary-to-opening distance.
tangent of .75 x primary to opening distance = 0.785" clearance (1 side)
UTA = 12.5" + (2 x 0.785, or 1.57")
As you can see, the proper clearance comes very close to 1/2 the field stop of the largest eyepiece used. So an easier-to-remember rule is that UTA I.D. = mirror + largest field stop.
Calculated I.D. for the example: 14.1"
UTA I.D. from rule of thumb: 14.154"
[the scope was constructed with 14.25" I.D. My last 12.5" came with a 13.25" UTA I.D. I reconstructed the UTA to eliminate the error. It had a secondary properly sized for the larger opening. As I said, some commercial scopes aren't logically made.




You have made one major assumption. You assumed the optical axis is coincident with the OTA/UTA mechanical axis. However, if the scope happened to apply the "new mode" then you need to factor in the angle between the optical and mechanical axes.

Jason



You are correct. With the "tilt" of the optical axis in the "New Model" collimation, there is rationale for having the I.D. of the UTA be even larger than I calculate to account for that tilt.
You know, back in the '50s the standard was to have the I.D. of the tube be at least 2" larger than the mirror. Those guys may have known what they were doing.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Secondary Sizing new [Re: robininni]
      #5742651 - 03/19/13 10:59 AM

Quote:

Thanks for the answers, but I think I need to rephrase my question better to get the answer I'm really looking for:

How much of the focal plane does an EP use? Is it dependent on the EP's field stop?

If so, then for some EP's of short focal length (and small field stop)such as an Ethos 8 used in a telescope with a fully illuminated focal plane diameter of .5" would get full illumination assuming they are centered in the focuser and the 100% illumination area is centered in the focuser, correct?

Others with large field stops (Nagler 31) won't be fully illuminated even with good design principles because the field stop is too large. Is this right?

Also, back on OTA vignetting, if the 75% vignetting point (using Newt of the Web) is outside of 2" using a 2" focuser (2" focal plane?), then this vignetting would not be noticeable, right? Only if I was using a 3" focuser and 3" EP would I notice this?

Thanks,

Rob



Vic answered your questions, but I will add that it is useful to think of the telescope as having an image produced on its focal plane and the eyepiece merely a magnifying glass used to look at a portion of that focal plane.
Low power eyepieces see a larger portion of that focal plane and magnify less. High power eyepieces see a smaller portion of that focal plane and magnify more.
If the secondary size is chosen to provide 100% illumination over, say, the center 1/2" of the focal plane, then an eyepiece whose field stop was 1/2" or less would see a 100% illuminated field from edge-to-edge.
Of course, the exit pupil of that eyepiece might not fill your eye's pupil, so the image would be dimmer than a low power eyepiece with some edge-of-field illumination drop off.


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