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How tall is your secondary cage?

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#1 KerryR

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Posted 20 November 2012 - 04:56 PM

Hi all,

I'm re-designing my 16" f4.5 strut dob. I'm going to do a 'tradtional' truss design, this time.

I'm curious how tall those of you who did 'traditional' secondary cages made the cage. I want to nearly eliminate the ability of local lights to reach the focal plane. To that effect, I'm considering making the cage extend a foot above and below the focuser for a total height of around 2' for the cage.

I don't really want to have to attach a second baffle or shroud-- looking to accomplish as much as possible with the (traditional) cage alone...

Any thoughts?

#2 Meep_Esq

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Posted 20 November 2012 - 05:13 PM

I'm in the throes of building a new telescope, and have come to the same conclusion as you. My older Newt did suffer from stray light an awful lot because the 2ndry was fairly close to the end of the tube. I'm also going to put some low profile baffling in the tube opposite the eyepiece.

Vaughn

#3 killdabuddha

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Posted 20 November 2012 - 05:14 PM

Ours is 5.5" We wanted minimum weight and wind-resistance there and we use light baffles instead. (We have velcro-mounting patches to fill the gaps in the vinyl folder pockets that unfold at their creases to extend.)

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#4 Mirzam

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Posted 20 November 2012 - 05:45 PM

It's pretty easy to add a baffle at the focuser that takes care of most stray light. I recently unscewed the 2" snout from a spare 2" star diagonal. This was then screwed into the front end of my Paracorr, providing an effective "hood" to reduce stray light entry on my single ring dob.

JimC

#5 SACK

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Posted 21 November 2012 - 12:25 AM

Sounds smart Jim.
Got a pic to share?

#6 Dick Jacobson

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Posted 21 November 2012 - 08:43 AM

Experimentation showed that the height of the cage should equal its inside diameter for complete light blocking. I tested by holding a headlamp near the opposite side of the cage and moving it up and down until the stray light was completely blocked. Different eyepieces produce different results. My longest focal length eyepiece (34 mm) needed the greatest height, but it's not a simple relation between focal length and height. I did not have any kind of baffle at the bottom of the focuser; that might have helped.

#7 KerryR

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Posted 21 November 2012 - 10:04 AM

Thanks for the input, folks!

One thing that may have been obvious to others, but that I only learned last night is this: Because the most offending direct lights will tend to enter from the top of the tube, rather than centering the focuser in the middle of the cage, or making the cage 2x as tall (my original idea), you can put the focuser near the bottom of the cage. Telekits does this. This will help shave some height (and thereby weight) off my design.

My current scope has quite a tall focuser, this acts, sort of, like the 'snout' idea Jim mentions. I also have an anular baffle at the base of the focuser. This works well at dark sites, but my front driveway has some street lights that are about 100-200 yards away. Not close enough to make observing a no go, but often at just the right location to light up the inside of my focuser.

I want to start using my 16" more often, and this means getting it more usable under my predominant observing conditions... I'll also seize the opportunity to reduce the bulk of the components, so that it's easier to get in and out of the house and/or car.

#8 Mirzam

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Posted 21 November 2012 - 10:11 AM

Sorry, no picture as the scope is in its transport case getting ready to head to Chile. I'll document the entire scope build in a few weeks.

My focuser sits on top of the single ring, much like an Obsession Ultra Compact. The front end of the Paracorr is essentially flush with the front end of the focuser (which is a KineOptics 2"). The "snout" simply screws into the Paracorr where a filter might normally go. This reduces the acceptance angle of the Paracorr field lens sufficiently that I can use a smaller baffle opposite the focuser.

Building a tall UTA as proposed by the OP will add extra weight that must be counterbalanced. That can be a big disadvantage.

JimC

#9 Mirzam

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Posted 21 November 2012 - 10:22 AM

This is a really good book that provides a counterpoint to the Kriege and Berry approach:

http://www.willbell....nTelescopes.htm

JimC

#10 KerryR

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Posted 21 November 2012 - 11:11 AM

I'm not excited about the additional weight of the taller uta, but I am interested in it's ability to block stray light, while keeping setup quick, simple, and direct. I'll trade some weight and bulk... maybe. My scope never had the contrast in it's compact form as it did in it's original full-tube form, at least not under the kind of conditions (local lights) that I tend to observe most often under.

My current scope uses a flat ring and baffle uta. It needs more height forward of the top of the ring. I haven't found a clean solution to this as yet, because of the particular design of the uta. So, I'm looking at a re-design, with a premium placed on stray light control more than weight and bulk, though those things remain a concern. I believe I can beat my current scope on all fronts, and still solve the stray light issue effectively. This will, however, probably require counterweighting, in order to keep the height of the rocker down. I'll make the counterweight removeable, so that it can be handled separately, thus keeping the weight of individual components manageable.

I should mention that I like very simple and direct construction, with really simple tools and easily attainable cheap materials. (My current scope fits the bill extremely well). So, heavy engineering and metal work are largely out... which makes design somewhat limiting.

#11 JohnMurphyRN

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Posted 21 November 2012 - 01:58 PM

A pic from my 17.5" telekit, if this helps..

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#12 Pinbout

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Posted 21 November 2012 - 03:13 PM

Any thoughts?



you stated a 24" uta but what measurement you really need to know is how far above the focuser axis do you need to block out light.

in K&B they suggest a 7" minimum, so a 12" height above the focuser axis is still reasonable.

but to save on torque I would think about lowering the height below the focuser axis as much as possible.

#13 KerryR

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Posted 21 November 2012 - 10:10 PM

I'll base the height above the focuser on hand-ray tracing way-off-axis rays relative to the diameter of the opening of the uta and the diameter of the focus tube vs. how far into the tube I'll allow a ray to go (no where near the field lens of an ep), with a single baffle in the end (of the focuser tube), allowing for 1/2" of full illumination at the focal plane... I guess.


As posted by John (Thanks!), I won't put much extension below the focuser.

I get a little 'help' from the long profile focuser I use, courtesy of Meade's stock 4" diagonal which I recycled. I'm actually pretty shocked at the ability of local lights to corrupt the images in my current scope, considering the number of baffles in place...

#14 Mirzam

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Posted 21 November 2012 - 10:28 PM

Do you use a baffle behind your mirror?

JimC

#15 MitchAlsup

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Posted 22 November 2012 - 12:50 AM

I use a single upper ring 3/4" thick and a 29" circular baffle across from the focuser for my 20" DOB.

#16 Roel

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Posted 22 November 2012 - 01:37 AM

Instead of making a large and heavy secondary cage to block stray light, I chose to use a baffle made out of thin foam.

For my 16" Dob I used a 20" baffle: more than large enough.
The secondary cage has a height of 8" and weighs 4.4 pounds.

Connecting the baffle is not a lot of work, you can attach it with Velcro or something. It's also not an extra piece of equipement to carry around; when not in use the baffle is the dust cover for the primary mirror.

When you keep the upper tube assembly as lightweight as possible, the whole scope can be build a lot lighter. My 16" has a total weight of 65 pounds, that includes the heavy 28 pound mirror.

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#17 Mirzam

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Posted 22 November 2012 - 08:05 AM

Roel,

That is a nice idea to use the baffle for something else--in your case a dust cover. I was wondering if you also use a focuser baffle?

One thing about a Paracorr is that the field lens is rather exposed near the lower end of the focuser tube. By looking through the Paracorr (no eyepiece) with the scope pointed to a light-colored surface, it is possible to see whether the focuser baffle is effective-- A rim of off-axis stray light admitted by the Paracorr will disappear. Of course you also want to be sure that the entire secondary mirror remains visible.

JimC

#18 Roel

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Posted 22 November 2012 - 08:43 AM

I do use a focuser baffle. It's a single baffle at the end of the focuser tube with an inner diameter of 45mm.

To check if the baffle opposite to the focuser itself is sufficient, or if an internal focuser baffle is needed, just look wether from any point around the telescope the front lens of your eyepieces (or correctors/barlows etc) is visible. If not, the scope is properly baffled.

Also make sure EVERYTHING visible through the focuser tube is blackened telescope part. No patches of sky, shiny trussconnectors, etc. Even the edge of the secondary mirror should be flat black.

#19 allardster

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Posted 22 November 2012 - 09:09 AM

Another way to go about it is using an iris diaphragm at the end of the focuser tube. You can adjust it to get it the baffling as tight as possible for the eye piece you are using.
http://www.surplussh...item/m4182.html

#20 Dick Jacobson

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Posted 22 November 2012 - 09:22 AM

I used Protostar Flockboard as a light shield opposite from the focuser. I attached it to my single-ring UTA assembly at three points. When curved, the .030 or .040 inch thick flockboard keeps its shape very well without any additional support. See Flockboard

#21 KerryR

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Posted 22 November 2012 - 11:09 AM

My secondary assembly is currently a 3/4" thick flat ring. There's a full circumference thin aluminum baffle with a hole cut into it for the light path exit, it's 2.25 inches from this inner baffle to the end of the focuser draw tube, where theres an annular baffle, then a long focus draw tube. The full circumference baffle is just not tall enough when there's local lights-- I wanted the secondary assembly to store on top of the mirror box, so I was 'limited' by this design requirement. The assembly is a square shape with the circular aperture cut into it. While this made for a very convenient shape for the strut attachment (parallel), it dosn't make a convenient shape for attaching baffles opposite the focuser. On top of that, the spider is 4-vane, which complicates things further.

Overall, the scope is a VERY effective design-- perfectly balances, butter smooth, and handles everything from my 35mm Pan with a coma corrector to a 7mm Plossl alone.

My reason for a potential re-build is two-fold: Increase stray light control, and make the mirror box smaller and hopefully lighter. The lighter part creates some serious difficulties in designing the secondary assembly while addressing the stray light control.

#22 KerryR

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Posted 23 November 2012 - 01:27 PM

From the OP:

I may not need a re-build if:
1) I abandon the 'need' for a 1/2" fully illuminated field. Based on Gary Seronik's article, it looks like this isn't necessary for most observing requirements.
2) I abandon the 'need' to not vignette the %75 percent field. Again, based on Seronik's article, as long as edge of field doesn't drop below %50, things should be fine.

Based on hand ray tracing, Doing those things means I only need 2" more extension above or below my current uta if I make the first exit-ray baffle smaller. I can do this without cutting any of the %100 ray, and still keep over %60 at the very edge of the FOV.

Seems like a worthwhile pursuit considering I'm otherwise very pleased with my current scope's design... though weight and bulk reduction would be nice...

#23 David L

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Posted 23 November 2012 - 10:21 PM

I made a focuser baffle out a 3-inch PVC floor drain, grinding out the grating part and some of the flange to hold it in place. The flange fits under the mismatched curve of the GSO focuser on my 16-inch Lightbridge. I blackened the baffle with black sticky back felt. Dave






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