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Equipment Discussions >> Reflectors

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FastMike
sage


Reged: 09/22/10

Loc: Florida 27° 15' N
Re: Webster F3 24" new [Re: gatorengineer]
      #5856351 - 05/12/13 09:11 PM

Quote:

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.

Quote:

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.

Quote:

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

Quote:

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.

Quote:

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.


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Aperturefever
super member


Reged: 12/12/10

Loc: Lake Macquarie, Australia
Re: Webster F3 24" new [Re: FastMike]
      #5856901 - 05/13/13 07:16 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!

Edited by Aperturefever (05/13/13 07:21 AM)


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maire
member


Reged: 09/23/11

Loc: France
Re: Webster F3 24" new [Re: Pinbout]
      #5856930 - 05/13/13 07:58 AM

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

http://www.astrosurf.com/voute/T600/secLum/T600sec.jpg

Edited by maire (05/13/13 07:59 AM)


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: Webster F3 24" new [Re: Mike Lockwood]
      #5857157 - 05/13/13 10:29 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....


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Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Webster F3 24" new [Re: gatorengineer]
      #5857314 - 05/13/13 11:57 AM

Quote:

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.

Quote:

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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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: Webster F3 24" new [Re: Mike Lockwood]
      #5857799 - 05/13/13 03:42 PM

Quote:

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.




Mike, thanks for fully illuminating this thread.

Best,
Mark


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Eric Webster
Vendor (Webster Telescopes)


Reged: 10/04/06

Loc: Detroit Michigan
Re: Webster F3 24" new [Re: mark cowan]
      #5858392 - 05/13/13 07:13 PM

This picture was deleted earlier in the thread:

Edited by okieav8r (05/13/13 11:30 PM)


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Eric Webster
Vendor (Webster Telescopes)


Reged: 10/04/06

Loc: Detroit Michigan
Re: Webster F3 24" new [Re: Eric Webster]
      #5858396 - 05/13/13 07:14 PM

As was this one:

Edited by okieav8r (05/13/13 11:30 PM)


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Eric Webster
Vendor (Webster Telescopes)


Reged: 10/04/06

Loc: Detroit Michigan
Re: Webster F3 24" new [Re: Eric Webster]
      #5858403 - 05/13/13 07:18 PM

Sorry about the delay for you guys who emailed us about the pictures being deleted by the CN Moderators.

I could not remember what email address or password we used for these forums, as we have not posted here in many years.

Hopefully everything is now straightened out as far as using pictures from our site/forums.

Keep up the lively discussions!


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Project Galileo
scholastic sledgehammer
*****

Reged: 11/14/07

Loc: Jefferson County, Colorado
Re: Webster F3 24" new [Re: Eric Webster]
      #5858416 - 05/13/13 07:29 PM

Every time my wife walks by my computer monitor and these pictures are on it she laughs at me and says, "are you looking at telescope porn again?"

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okieav8rAdministrator
I'd rather be flying!
*****

Reged: 03/01/09

Loc: Oklahoma!
Re: Webster F3 24" new [Re: Project Galileo]
      #5858974 - 05/13/13 11:57 PM

There have been some issues in this thread over the last few days with regard to the posting of photographs on Cloudy Nights forums from other websites. Please note that with regard to the Cloudy Nights TOS, a blanket permission statement on another website allowing the use of photos does not meet CN's requirement for the posting of photos on these forums. The Cloudy Nights TOS prefers that users instead post a link to the website where the photograph resides. If a photo is posted by a member who does not own the photgraph, a permission statement that specifically states that permission is granted to post the photo on the CN website must also be posted with the photo. Please refer to the Cloudy Nights TOS, or feel free to contact an administrator (red user) for further clarification.

The thread is now reinstated.


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: Webster F3 24" new [Re: okieav8r]
      #5859238 - 05/14/13 07:20 AM Attachment (16 downloads)

Here is running the dimensions that Mike provided in Web Newt.

Edited by gatorengineer (05/14/13 07:24 AM)


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: Webster F3 24" new [Re: gatorengineer]
      #5859248 - 05/14/13 07:25 AM Attachment (12 downloads)

Here are the results, perhaps I got a parameter wrong but he scope vignettes at the secondary cage. The focuser to front of tube distance is also likely short.

Edited by gatorengineer (05/14/13 07:26 AM)


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: Webster F3 24" new [Re: gatorengineer]
      #5859263 - 05/14/13 07:38 AM Attachment (10 downloads)

Heres how I might play with it..... Extend the cage forward a bit, and give a little more backfocus. As it sits I am going to get alot of light from above the cage on the field lens of the Eyepiece.... and If I put a flockboard up the scope will again vignette. so backing out the focal plane with a 3" focuser and a paracorr 2 is imho a better solution....

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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: Webster F3 24" new [Re: gatorengineer]
      #5859265 - 05/14/13 07:39 AM Attachment (12 downloads)

the corresponding ray trace.....

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FastMike
sage


Reged: 09/22/10

Loc: Florida 27° 15' N
Re: Webster F3 24" new [Re: gatorengineer]
      #5859569 - 05/14/13 10:47 AM Attachment (9 downloads)

Quote:

Heres how I might play with it..... Extend the cage forward a bit, and give a little more backfocus. As it sits I am going to get alot of light from above the cage on the field lens of the Eyepiece.... and If I put a flockboard up the scope will again vignette. so backing out the focal plane with a 3" focuser and a paracorr 2 is imho a better solution....




What your calculations didn't to take into account is the use of a Paracorr. The Paracorr (SIPS) is at a fixed distance with respect to the primary mirror.

The Paracorr will be at a fixed point in space regardless of how large the UTA is. So as you make the UTA larger the Paracorr will just protrude further and further inside the UTA (see picture).

The UTA could be lowered on the truss poles to place the Paracorr farther outside the UTA. But then a larger secondary mirror would be needed to fully illuminate the focal plane. I think this is what you were trying to accomplish with a 31.5" UTA?


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FastMike
sage


Reged: 09/22/10

Loc: Florida 27° 15' N
Re: Webster F3 24" new [Re: gatorengineer]
      #5859595 - 05/14/13 10:58 AM Attachment (11 downloads)

Quote:

Here are the results, perhaps I got a parameter wrong but he scope vignettes at the secondary cage. The focuser to front of tube distance is also likely short.




Here is a picture of the Paracorr 2 (SIPS) from inside the UTA. As you can see the lens is flush with the inside of the UTA. Having a 3" focuser will accomplish nothing.

It's very possible the UTA is vignetting the light cone.

Answer this -> what good is having a larger UTA if the Paracorr 2 vignetts the light cone *more* than the UTA? Having a larger UTA will accomplish nothing if the Paracorr 2 vignetts more than the UTA does.


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FastMike
sage


Reged: 09/22/10

Loc: Florida 27° 15' N
Re: Webster F3 24" new [Re: gatorengineer]
      #5859616 - 05/14/13 11:09 AM

Quote:

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....




Sorry, but I trust Televue more than you to design optics.

Televue states the Paracorr 2 is designed to work with f/3.0 telescopes. I take them at their word. I trust they would properly design the Paracorr 2 so it would provide 100% illumination with their own Ethos eyepieces.

Plus I can tell you this from experience. The 8mm Ethos is 100% illuminated in the center with a f/2.75 telescope.


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: Webster F3 24" new [Re: FastMike]
      #5859627 - 05/14/13 11:12 AM

Mike,

You and Mike L have an entirely different explanation of the functioning of the sips device. I have a paracorr 2, not a SIPS, so I do not know. Mike L says the SIPS field lens Moves... if I read your post correctly, you say it is fixed relative to the primary?

You need roughly 30MM of focus travel so if the field lens of the SIPS is fixed, then you are right, 3" does nothing. If it goes "up the focuser" like a Paracorr 2 would, then 3" definitely gets you something...

If it is fixed, its a completly different optical formula than a paracorr 2..... and a 3" focuser would get you nothing....

Edited by gatorengineer (05/14/13 11:19 AM)


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gatorengineer
Carpal Tunnel
*****

Reged: 02/28/05

Loc: Hellertown, PA
Re: Webster F3 24" new [Re: gatorengineer]
      #5859636 - 05/14/13 11:18 AM

Mike,

Televue designs the Paracorr, and has a done a great job of it no doubt.... Mike L does an outstanding job of figuring the optics..... These two items get integrated by the scope manufacturer.... You have a correctly sized secondary, some fast scopes (at least early ones), didnt have correctly sized secondaries....... Secondary cage size, is a mix of a bunch of factors, its a compromise ( diameter, weight secondary size, and the need to baffle above the cage to keep stray light off of the field, are just a few of the factors)


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