How tall is your secondary cage?
Posted 20 November 2012 - 04:56 PM
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...
Posted 20 November 2012 - 05:13 PM
Posted 20 November 2012 - 05:14 PM
Posted 20 November 2012 - 05:45 PM
Posted 21 November 2012 - 08:43 AM
Posted 21 November 2012 - 10:04 AM
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.
Posted 21 November 2012 - 10:11 AM
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.
Posted 21 November 2012 - 11:11 AM
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.
Posted 21 November 2012 - 01:58 PM
Posted 21 November 2012 - 03:13 PM
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.
Posted 21 November 2012 - 10:10 PM
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...
Posted 22 November 2012 - 12:50 AM
Posted 22 November 2012 - 01:37 AM
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.
Posted 22 November 2012 - 08:05 AM
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.
Posted 22 November 2012 - 08:43 AM
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.
Posted 22 November 2012 - 09:22 AM
Posted 22 November 2012 - 11:09 AM
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.
Posted 23 November 2012 - 01:27 PM
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...
Posted 23 November 2012 - 10:21 PM