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Webster F3 24"

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#1 Aperturefever

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Posted 09 May 2013 - 01:17 AM

Ok. Here's my aperture fever cure. Webster have recently completed this F3 24". It has a Kennedy mirror and a zenith eyepiece height of 67".
That's the good news ... the bad news I wasn't the customer! Time to start saving.
But seriously - I think I have found my dream scope. I've been pondering a really short 24" for a while now. This is exactly what I pictured. What an awesome bit of gear.

http://webstertelesc...c.php?f=8&t=634

#2 Pinbout

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Posted 09 May 2013 - 06:47 AM

nice picture, thanks for the tease, it's not yours! :foreheadslap:

:lol:

#3 tigerroach

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Posted 09 May 2013 - 07:49 AM

Coolness!

#4 mak17

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Posted 09 May 2013 - 08:46 AM

Lovely indeed. I've looked thru a 28 f3 Webster. Views were amazing.

#5 Danagoo

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Posted 09 May 2013 - 09:42 AM

That scope gets a Tim "the Tool-man" Taylor: "Rrrr, Rrrr, Rrrr" :lol:

Neat looking cart mount too.

#6 JMW

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Posted 09 May 2013 - 04:36 PM

It's the perfect BIG mirror with feet on the ground dob. I personally am saving up for the 18 f/4. I want a scope I can grow old with. I would have to have a personal assistant to help me with the weight of a scope this big when I got older 70+.

An f/3 (f/3.5 with Paracorr) scope would force me to totally rethink my eyepieces. My Ethos 21mm would would have an exit pupil of 5.99. I would be limited to Ethos 17mm for matching my pupil dilation and typically limited to 7mm or longer do to typical seeing.

#7 UmaDog

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Posted 10 May 2013 - 01:02 AM

An f/3 (f/3.5 with Paracorr) scope would force me to totally rethink my eyepieces. My Ethos 21mm would would have an exit pupil of 5.99. I would be limited to Ethos 17mm for matching my pupil dilation and typically limited to 7mm or longer do to typical seeing.


If the exit pupil is a little larger than your own pupil it really is no big deal at all. No reason that I can see to avoid using the 21 mm Ethos on such a scope.

#8 gatorengineer

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Posted 10 May 2013 - 07:34 PM


An f/3 (f/3.5 with Paracorr) scope would force me to totally rethink my eyepieces. My Ethos 21mm would would have an exit pupil of 5.99. I would be limited to Ethos 17mm for matching my pupil dilation and typically limited to 7mm or longer do to typical seeing.


If the exit pupil is a little larger than your own pupil it really is no big deal at all. No reason that I can see to avoid using the 21 mm Ethos on such a scope.



It will perform just like a 16 F4.5 with a 31 Nagler in it... Way too much wasted light.... as you say with a smaller EP, you wont waste, light on the exit pupil, but it will vignette badly... your looking at least a 6" secondary not to vignette terribly, and now you are back to 25 percent CO. Compared to 14% in my 25 F5 so contrast should suffer over a traditional dob.

Shipping it with a 2" focuser shows they don't understand light cones.....

Put a Wynn 3" corrector in a 3" focuser, and a 17 E in it an it would make some sense...

Other than that its a scope for someone with lots of money that doesn't understand optics, Exit Pupil, and vignetting. That wants big and cool, but not necessarily better. I have never read about a shoot out of superfast versus F4.5ish of comparable quality optics.......

#9 Wozzel

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Posted 11 May 2013 - 01:02 AM


Put a Wynn 3" corrector in a 3" focuser, and a 17 E in it an it would make some sense...


Make none sense how?

Wynn 3" make only photography. No insert eyepiece ever still.

Paracor#2 good use to still f/2.2 make nice, make for eyepiece use.

#10 Wozzel

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Posted 11 May 2013 - 01:06 AM

Make small for woman is nice, yes.

They make detail about:

http://webstertelesc...c.php?f=8&t=634

#11 UmaDog

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Posted 11 May 2013 - 04:00 AM

It will perform just like a 16 F4.5 with a 31 Nagler in it... Way too much wasted light.... as you say with a smaller EP, you wont waste, light on the exit pupil,


All depends how you look at it: with the lower power you get a wider field of view. So that's a gain. The image won't be any dimmer when the exit pupil is over-sized, it will just cease to get brighter. Is that so bad? What's the alternative? To also set up a 16" and put the 31 Nagler in it so as not to waste light? :) Might as well just bung the 31 in the larger scope and forget about it, no?

As you say, you could go for a large secondary and a 3" focuser. But then you're looking at some pricey gear. Also, I know Webster don't like overly large secondaries. To use a 3" focuser I imagine you'd also need a pretty large secondary and that's something they want to avoid. TBH, at these large apertures I don't think the small secondary obsession makes that much sense.

#12 dscarpa

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Posted 11 May 2013 - 11:04 AM

Quite a scope! I would want to get good on collimation with my some day 11" F/5 before taking that on. David

#13 careysub

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Posted 11 May 2013 - 09:27 PM

... your looking at least a 6" secondary not to vignette terribly, and now you are back to 25 percent CO. Compared to 14% in my 25 F5 so contrast should suffer over a traditional dob.


Hmm.. plugging the specs into Mel Bartel's calculator, and assuming a 15" intercept distance, it looks like a 5" diagonal gives very good field illumination for the widest EP this scope can profitably use (the 21" Ethos with the essential Paracorr). This is only 21% CO, usually thought to be a very good ratio.

(I have no idea what size they are actually using.)

The case for significant visible improvement much below 21% seems in doubt (only small MTF curve improvements occur).

If a low-rider (tilted secondary-focuser axis) design were used you could get bragging rights of 26" or so at the same ZEPH.

#14 mark cowan

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Posted 11 May 2013 - 10:19 PM

Yes, the geometry works out very well re. CO, so long as the secondary is sized correctly.

Best,
Mark

#15 turtle86

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Posted 11 May 2013 - 10:57 PM


An f/3 (f/3.5 with Paracorr) scope would force me to totally rethink my eyepieces. My Ethos 21mm would would have an exit pupil of 5.99. I would be limited to Ethos 17mm for matching my pupil dilation and typically limited to 7mm or longer do to typical seeing.


If the exit pupil is a little larger than your own pupil it really is no big deal at all. No reason that I can see to avoid using the 21 mm Ethos on such a scope.


Totally agree. Sometimes it's worth wasting a few photons to get a larger TFOV...

#16 gatorengineer

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Posted 12 May 2013 - 11:57 AM

The New Explore 30 100 gives me a little more field (and better correction) in my 25F5, and leaves me with roughly $20,000 in the bank....

Lets look at illumination for a second... the 21 Ethos has a 36mm field stop. Your well illuminated is roughly 0.25" or about 6mm, or you could say at the EP, where you are desing to have a wide field of view, less than 5% of the field is fully illuminated. At F3, I would guess your edges are at best 60% illuminated.... the drop off at F3 is tough.

The scope has its niche, its feet on the ground, no falling off a ladder, weighs a boat load less than my 25 F5 (probably 100 lbs or more). and a reasonable van could carry it assembled. Yes there are pluss certainly, its a beaut, its bleeding edge of the technology... But with that type of stuff there are buts....

By the way, I havent even checked Newt to see if the upper cage vignettes at that fast (it might have to be bigger to take the incomming light cone).... this would ripple through secondary sizing etc...

#17 FastMike

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Posted 12 May 2013 - 01:06 PM

Shipping it with a 2" focuser shows they don't understand light cones.....


Using a Paracorr 2 there is no need for a focuser larger than 2". The lens on the Paracorr will be approximately flush with the inside of the UTA. A larger focuser will accomplish nothing, which is why they ship with a 2" focuser.

Put a Wynn 3" corrector in a 3" focuser, and a 17 E in it an it would make some sense...


That makes no sense, a 2" focuser and a Paracorr 2 is all that's needed.

Other than that its a scope for someone with lots of money that doesn't understand optics, Exit Pupil, and vignetting.


You're sure about that?

- Using a 21E my 28" f/2.75 (with Paracorr 2) has a 6.6mm exit pupil 94% edge illumination and .92 degree TFOV.
- Using a 31N your 25" f/5 (without a Paracorr) has a 6.2mm exit pupil ~80% edge illumination and .76 degree TFOV.

Lets look at illumination for a second... the 21 Ethos has a 36mm field stop. At F3, I would guess your edges are at best 60% illuminated.... the drop off at F3 is tough.


My 28" f/2.75 has the following illumination:
0.00 in 100.0%
0.10 in 100.0%
0.20 in 100.0%
0.30 in 99.59%
0.40 in 98.51%
0.50 in 97.23%
0.60 in 95.88%
0.70 in 94.49% <-- 21 Ethos (your guess was wrong)
0.80 in 93.07%
0.90 in 91.63%
1.00 in 90.18%

I havent even checked Newt to see if the upper cage vignettes at that fast


It doesn't. The internal baffling of the eyepieces is the limiting factor in off axis illumination.

#18 gatorengineer

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Posted 12 May 2013 - 06:42 PM

Look at Mike Lockwoods, own website, for secondary illuminations. Your figures unless you have around a 7+" secondary are not possible. 7" Secondary is 25% CO....

If you are getting 0.4" at the SIPS, what happens in the distance between the SIPS field lens and the Eyepiece field lens? Answer The light cone drops as a function of distance between is what happens. If you have 1.5" or so between the SIPS exit lens and the field lens of the 21E you actually have well, 0 fully illuminated. Hence the need for the Wynn and a 3 inch focuser, and an even Bigger Secondary. There is at least one person who uses it visually and that I believe was posted here a while ago.

Also check out this post

http://www.cloudynig...4360360/page...

It shows in the Ethos family the range of focal locations is -9 +19.5MM or 28.5MM, So you are almost to 0 fully illuminated IF there were NO distance betwen the SIPs exit and the field lens of the 21 E. Lastly this also doesnt account for the SIPS' other shortcomming which is that it can be optimized for the distance from the EP to focuser, like a normal Paracorr 2 (over that range the SIPS effectiveness would vary significantly)

If you let me know your secondary size, cage ID, I would be interested in running the numbers.

Also my analogy is for the yet to be delivered 30 Explore which will give me a degree. Granted your 21E does give you a 0.1 degree more than my 25 with the 100E, and yes its quite an improvement over the 31 Nagler I currently use. And Yes I am concerned about Illumination at F5 with a 3.5" secondary

I take it you also havent read the recent post on Paracorr 2 performance at F2.8 in Mikes group? check it out makes for a good read.....

Lastly Think through what you are saying? Why do they make 3 and 4 inch focusers and Wynn correctors? To illuminate a full frame 35 Millimeter chip without clipping in the focuser.... How big is a 35MM chip compared to a 21 E Field lens?

You have a great cutting edge scope. But I stand by what it says cutting edge has limits, and Kudos to you for pushing the hobby.

#19 gatorengineer

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Posted 12 May 2013 - 06:52 PM

oh, and here is another reference that contradicts Seroniks work on secondary sizes.

http://www.brayebroo...nings/diagon...

Go to the 3rd set of computations and if you look at the F4 calculation......

#20 Mike Lockwood

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Posted 12 May 2013 - 08:09 PM

Look at Mike Lockwoods, own website, for secondary illuminations. Your figures unless you have around a 7+" secondary are not possible. 7" Secondary is 25% CO....

If you are getting 0.4" at the SIPS, what happens in the distance between the SIPS field lens and the Eyepiece field lens? Answer The light cone drops as a function of distance between is what happens. If you have 1.5" or so between the SIPS exit lens and the field lens of the 21E you actually have well, 0 fully illuminated. Hence the need for the Wynn and a 3 inch focuser, and an even Bigger Secondary. There is at least one person who uses it visually and that I believe was posted here a while ago.

I don't follow your reasoning. By my calculations, assuming no internal vignetting, at F/3 the SIPS has about a 13mm diameter fully illuminated spot.

Here's the dimensioned diagram that I made up to show the relevant SIPS dimensions:
Posted Image

First, note the focus position without the SIPS - 88mm from the bottom. Next, note that the focus position with the SIPS, when set properly, is 47mm higher.

Therefore, the light is not a simple converging cone as it passes through the lenses in the Paracorr lenses - it follows a more complex path that we can't know.

So, we calculate illumination based on the focal plane location without the SIPS. I measured my Paracorr 2 lens (can't get to my SIPS right now) opening at 42mm. At f/3, a 29mm diameter light cone will focus 88mm from the bottom of the SIPS. This is obviously smaller than the 42mm lens, so the central spot is easily fully illuminated at f/3.

Additionally, there is a 6.5mm margin around the edge of the lens, so we can estimate about a 13 mm-diameter fully illuminated field at f/3 with the SIPS.

Using these assumptions, we can also calculate the fastest f/# that will have a fully illuminated central spot with the SIPS: 88/42 = f/2.09

I take it you also havent read the recent post on Paracorr 2 performance at F2.8 in Mikes group? check it out makes for a good read.....

Not sure what you're referring to... I said it worked fine, but I didn't recommend going faster. F/3 works great.

As for a 6" secondary, it sounds about right to me.

#21 FastMike

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Posted 12 May 2013 - 08:11 PM

If you are getting 0.4" at the SIPS, what happens in the distance between the SIPS field lens and the Eyepiece field lens? Answer The light cone drops as a function of distance between is what happens. If you have 1.5" or so between the SIPS exit lens and the field lens of the 21E you actually have well, 0 fully illuminated.


I believe this is where your calculations went wrong. You don't add the distance between the Paracorr lens and the eyepiece lens when figuring illumination. I don't know how it's calculated maybe someone else knows (TV rep?). The Paracorr *moves* the focal plane. So all you need to do is illuminate the telescope focal plane without a Paracorr to get a fully illuminated eyepiece with a Paracorr.

Lastly this also doesnt account for the SIPS' other shortcomming which is that it can be optimized for the distance from the EP to focuser, like a normal Paracorr 2 (over that range the SIPS effectiveness would vary significantly)


I don't understand what you are trying to say? Think of it this way: The SIPS is a Paracorr 2 with a tunable top, it's just the tunable top on the SIPS is a Feather Touch focuser rather than the twist top like a regular Paracorr 2. Other than that there is no difference between the SIPS and the regular Paracorr 2.

If you let me know your secondary size, cage ID, I would be interested in running the numbers.


29" inside diameter of the UTA
28" primary
7" secondary
Secondary to focal plane 18.25"
Focal length 77.1" f/2.75

I take it you also havent read the recent post on Paracorr 2 performance at F2.8 in Mikes group? check it out makes for a good read.....


Link or quote? I have not seen it.

Why do they make 3 and 4 inch focusers and Wynn correctors? To illuminate a full frame 35 Millimeter chip without clipping in the focuser.... How big is a 35MM chip compared to a 21 E Field lens?


The eye does not work like a camera. You can't just take what is used for photography and apply it for visual.

I have tested my scope to see what was vignetting the edge of field by placing post-it-notes on various components. What I found was the internal baffles of the eyepieces are the limiting factor. So you might calculate the UTA is too small or the Paracorr field lens is too small but neither of those matter because the internal baffles of the eyepiece clip the light cone first before anything else does.

Some other food for thought. My f/2.75 is better corrected than your f/5 everywhere except possibly in the center few millimeters of the focal plane. I have used f/5 telescopes without a Paracorr and they had far too much coma for me. I didn't like the view. I would use a Paracorr in your f/5 scope if it was mine. The edge of my f/2.75 has so little coma you have to look hard to see it. Your f/5 is loaded with coma (relatively speaking compared to mine). Plus your scope has a narrower field of view, less edge of field illumination, not to mention the ladder. :)

#22 Aperturefever

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Posted 13 May 2013 - 06:16 AM

Thanks to the two Mikes for your education on this cutting-edge design. Given I see this seriously as a very usable big-aperture dobsonian your hands-on experience and comments are invaluable. I guess there is always something to learn when others are at the forefront of this pursuit. To me this looks to be such a useable telescope, and light years away from the solid-tube Meade DS-16 I used to wrestle onto an equatorial mount in the late 1980s!

#23 maire

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Posted 13 May 2013 - 06:58 AM

My aperture fever is just closed... because of my new homemade scope:

http://www.astrosurf...Lum/T600sec.jpg

#24 gatorengineer

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Posted 13 May 2013 - 09:29 AM

Mike at F3 you say the sips has a fully illuminated spot of 13 MM at the focal plane (which is what i think you are saying), I agree with you IF the field lens of the SIPS is fully 100% illuminated. This would require a MONSTER Secondary to accomplish.

In The adjustable 20-40MM distance the light cone is still doing its thing (e.g. coning, so 13MM would like correspond to the 20MM side of the adjustment, less so at the 40mm side), What I am trying to get at is that the focal distance of various ethos eyepieces require over a 1" movement of the eyepiece relative to the focal plane, the light cone is again their coning.....

all of this put together is at F3, I do not beleive an Ethos 8 would have any areas 100% illuminated, even with a fully illuminated SIPS....

#25 Mike Lockwood

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Posted 13 May 2013 - 10:57 AM

Mike at F3 you say the sips has a fully illuminated spot of 13 MM at the focal plane (which is what i think you are saying), I agree with you IF the field lens of the SIPS is fully 100% illuminated. This would require a MONSTER Secondary to accomplish.

Let's assume 16" from the secondary to the focal plane. 16"/3 = 5.33" means that a 5.33" secondary is required to fully illuminate the center spot. So, add 13mm which is just over 1/2", that's 5.83", and a 6" secondary comfortably covers it. At 25% obstruction, that's not a monster by my definition.

In The adjustable 20-40MM distance the light cone is still doing its thing (e.g. coning, so 13MM would like correspond to the 20MM side of the adjustment, less so at the 40mm side), What I am trying to get at is that the focal distance of various ethos eyepieces require over a 1" movement of the eyepiece relative to the focal plane, the light cone is again their coning.....

No. The adjustability that I indicate is that the entire SIPS unit moves up and down, with a range of 20mm of movement, within the mounting plate, and the locking ring screws down against the plate. This is how the focal plane is positioned properly using the spacer that is included.

Think about it like a helical focuser - the entire unit screws in and out of the plate to properly position it. Then the locking ring simply holds it in place.

One set, the primary to SIPS lens and SIPS lens to focal plane distances do not change (unless the primary is moved significantly closer or farther from the SIPS by using the collimation adjustments). The focuser merely positions the eyepiece with respect to the stationary focal plane.






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