One Cave done, and another in progress
Posted 10 May 2014 - 12:18 PM
Posted 10 May 2014 - 12:27 PM
Because of budget cuts it not used for about 10 years. Things weren't in real bad shape but both the scope and dome need restoration.
I've already restored the OTA and I'm in the process of doing the the dome and teh telescope mount.
Here is a shot of the mount when I brought it home. Not in bad shape but it could use some work.
Posted 10 May 2014 - 12:30 PM
Posted 10 May 2014 - 12:38 PM
Here is a picture of two of the ones I repaired along with the original wheels. These will be installed in the next few day and hopefully solve the dome rotation problems
Posted 10 May 2014 - 12:54 PM
Did you have to refigure the DK optics?
Posted 10 May 2014 - 01:57 PM
Beautiful work on the old caves!
Did you have to refigure the DK optics?
Thanks Tim. The optics are, to be kind not very good and I hope to refigure them this Summer. It is a shame since all those kids over all those years including the retired science teacher had used that scope and thought that Jupiter would only show three bands and on Mars you might be able to see a polar cap. The retired science teacher was amazed at the detail that was visible in my home made Dob. The other issue was the lack of the proper size baffle tube on the primary. Until I installed one the image was washed out from sky flooding and it made deepsky observing next to impossible.
Posted 10 May 2014 - 03:08 PM
I haven't been out with mine again in a long time, but the new baffle tube really upped the contrast. The Jupiter pics I've taken with it were soft, but the seeing that night wasn't great either, and the 10" f/6 that we made in class didn't do a whole lot better that night.
Not to get too far off topic here, but when I do get back to testing the optics, is it possible to get so-so inside and outside focus stars and still be off several turns of the secondary hub screw? I wonder if I'm just way off still?
Posted 23 May 2014 - 10:04 PM
I rewired the mount so both the DEC and RA drive are wired to one power cord vs the two original ones. I installed a vintage red neon indicator light, that was used on other Cave mounts. The hole used for the indicator on other Cave mounts was used on this mount for were the two power cords entered the drive housing. The resistor and capacitor used in the hand paddle that allows the DEC slow motion motor to to be reversible were replaced along with all new wiring of the correct gage. I also installed a vintage plug on the DEC motor so it could be unplugged if needed. A number of grounds were also installed to improve the electrical safety of the scope.
Posted 24 May 2014 - 07:38 AM
Posted 24 May 2014 - 09:00 AM
Posted 24 May 2014 - 10:34 AM
Magnificent! Did Cave ship the scope with a bad mirror, or will your figuring repair someone else's hack?
The optics have never been touched since the scope was purchased by the school district back in the early 60's. I have heard more cases of bad images then good with these Cave cassegrains. Cave didn't make the optics but contracted them out. From what I can tell who ever was making them didn't understand how to calculate the knife edge reads to test for the needed ellipse so the figure is way off from were it should be. Cave also didn't understand how to build a Cassegrain from the lack of proper baffling.
If someone who has never looked thru a telescope and uses this Cave to look at the Moon, Jupiter and Saturn, it does show you craters and three bands on Jupiter and the rings around Saturn but that is it. You'll never get more then three bands on Jupiter with it. When I set up my homemade 8" f/7 Dob near it, the kids and the retired science teacher that used it for 20 years were amazed at the difference in the image quality. Three bands in the Cave and a space fly-by image in mine. I'm not try to boost or brag about my homemade scope. The mirror is not some "1/50" wave master piece. It just has a TRUE 1/8 wave figure.
The poor optical quality in the Cave saddens me in a number of ways. The school paid good money for this scope and it wasn't made like it should have been. How many kids were turned off by the image instead of being turned on to astronomy over the last 50 years it has been in use. I've heard the kids say a couple of times when we used it,
" That's Jupiter ?" vs "OMG that is so cool" when using my scope.
The bottom line continues to be that many people are comparing poor images in one telescope to poor images in other telescopes and believe their image is great since it matches everyone else. Learning to test optics is not hard. I learned how back when I was in high school by using Sam Brown's "All About Telescopes". People in this Forum spend huges amounts of energy in mechanically restoring their scopes but most don't take any time in learning to test their optics which I don't understand. Many times an optical problem is a simple fix, like a flipped element in lens system or a setting up the correct spacing between elements. So why not learn to test your optics and be sure all is OK ?
So hopefully this Summer I'll have the time to pull the optics and refigure them.
Posted 24 May 2014 - 11:29 AM
Thank you again for reminding us that looking at the instrument is only part of the equation.
I'll never reach your level of expertise, but you have instilled pushing glass and testing to classics and not just ATM optical work.
Posted 25 May 2014 - 10:11 AM
its a shame the optics/figures in these Cassegrain`s were a poor quality.
Posted 25 May 2014 - 11:32 AM
Now that I think of it, I've heard before that Cave's Casssegrain were wanting. Cool thought, that after you refigure the optics, yours will be better than new.
We have a skilled optician in our club who likes Newtonians for just this reason, their simple optics. One good primary, one flat secondary, a mechanical alignment, and he's done.
Have to wonder whether this forum's beloved small refractor used to be seen as magical against larger reflectors for this reason. They held their colimation, whereas the Newts and Casses did not.
Testing optics may be something anyone could learn, but doesn't it require a lot of equipment and the space to leave it set up?
Posted 25 May 2014 - 01:07 PM
Optical testing requires very simple equipment that one can easily build. The kinds of errors I typically seeing in commerical optics are large ones. So a simple look with a knife edge and or Ronchi screen easily shows the problems. Your not going to be worrying if your optics are between 1/16 and 1/20 wave it will be more like if they even close to 1/2 wave or worse.
What amateurs don't understand is that 1/2 wave optics do produce an image that at low to medium power isn't that bad. It's not a blurry mess that won't come to focus but when the seeing is good you'll never see that "Spacecraft fly-by" image that a set of TRUE 1/8 wave will produce. When you do have a blurry mess that won't come to focus your looking at a couple waves of error. So when I hear people say that they mixed and matched optical parts and say the image is just fine, I'm sorry I don't believe them.
An excellent book is Sam Brown's "All About Telescope" since it show drawing of just about every type of defect a mirror can have.
As for were testing and fabrication can be done, I've done a fair amount in my kitchen and my living room, setting up the tester on a kitchen chair at one end of the room and the mirror stand holding the mirror on another chair at the other end of the room. So nothing special is required and within minutes the room is back to normal.
I do have a setup in my basement for more complex testing but again that is not needed for the typical stuff that an amateur is looking to determine.
A while back someone that posts here sent me two Edmund mirrors. One was a 6" f/6 that came out of a red tube fork mounted reflector sold in the 80's. He knows enough about optics and star testing to know something wasn't right with this mirror. The image on the left is this mirror. You can see that it has ring zones, a small hill in the middle and the shadows are very asymeterical because the mirror has a large amount of astigmatism. The quality of the optics was stated to be 1/8 wave. It's not, it more like 2 waves !The mirror on the right is what a parabola should look like. So within 30 seconds of me having the Edmund mirror on the test stand I knew it was junk and this is very typical of many commerical optics I have tested.
Posted 25 May 2014 - 03:06 PM
Old Bomber Bob learned to do the knife-edge test over 30 years ago, so anyone can do it. Built my own test stand, too. Ground my own mirror, but I've never been motivated enough to attempt a lens (it's good to know your limits!).
I think it was Terra who said, "Cave is the Unitron of reflectors." I agree with that - they are wonderful!
Posted 26 May 2014 - 11:50 AM
Keep up the good work!
Posted 05 June 2014 - 09:36 AM
That is two Caves now done.
Posted 05 June 2014 - 01:58 PM
That thing looks just like mine!