Mitchell
member
Reged: 05/19/08
Posts: 82
Loc: USA
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James, quick question, maybe I'm missing it, but is there a dial indicator on your tester for taking readings? Perhaps a later add-on?
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Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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The bear
professor emeritus
Reged: 02/11/08
Posts: 727
Loc: rushville, indiana
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that is what i did for the rod i used a steel rod i got a while back for a light holder works great. used some things i got from walmart the things for moving furniture i cut them up for the Teflon stuff. used some dowels for the rod to hole it up.used some switches i had over from my RC stuff and a AA battery case i had left over, i hope i have a microscope stage coming not sure yet and on and on. i make everything i can by hand. dial indicators i got off the bay and the wood i had scraps of i do not throw anything away that is wood. etc i am a pack rat per se.
doc
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Longitude -85.42786 Latitude 39.59153
when all else fails use duct tape "works for me"
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The bear
professor emeritus
Reged: 02/11/08
Posts: 727
Loc: rushville, indiana
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man that second link was the bomb let me tell you i added that to my favorites so fast it wasn't even funny.  leds i used what i had on hand and made it so if it goes out i can change it out with a plug off a old telephone battery...cool
doc
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Longitude -85.42786 Latitude 39.59153
when all else fails use duct tape "works for me"
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2454
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cool idea making the LED socketed with your old phone battery plug !
- a note to everyone else: many times you have to change the resistor when you change the LED
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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I always thought that the light source had to be a pinpoint source (pin hole or a slit). Why is the led behind only one knife edge? I've seen some plans where there was a second knife edge in front of the led to make a slit source.
Also you can get some small halogen lamps that run off of 12v in wattages from 4-50 watts that ought to make a good light source. Some of these are about the size of AG1 flash bulbs. Also available are small low voltage lamps used in Malibu outdoor lighting (4-7 watts), and grain-of-wheat lamps that are even smaller than LED's made for model RR projects. Granted LED's run cooler, but might be a bit dim for long throws.
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Darren Drake
Pooh-Bah
Reged: 10/09/02
Posts: 1041
Loc: Illinois
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Here is another take on testing your mirror in a completed scope.
http://www.cloudynights.com/item.php?item_id=1637
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Astronomy educator
Sidewalk astronomer
18 inch f4.42 dob on eq platform w ST120 f/5 finder
8 inch f/6 dob
8 inch f/8 eq planetkiller
William Optics red 10th Anniversary 80mm FD
24lb eyepiece box
Cernan Space Center astronomer
Member of Northwest Suburban Astronomers
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Mitchell
member
Reged: 05/19/08
Posts: 82
Loc: USA
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The LED behind one half of the knife edge acts as a slit. The light is cut once on the way out and the second time on the return trip. This has become the most popular type of tester. A "Slitless" tester. A pinhole is used by some for various reasons, but it will work with or without. Mine is just a green LED half covered by the knife edge.
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Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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jcham21
super member
Reged: 12/02/07
Posts: 137
Loc: Baton Rouge, Louisiana
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Mitchell- Yep I have a dial indicator for the tester, but it's not attached at the moment. I also need to add a pad for the tip of the dial indicator stem to contact on the bottom of the stage.
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James
Canon XSi
Celestar 8
Check out my astrophotography gallery:
http://www.flickr.com/photos/23148441@N03/sets/72157603787621660/
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greenglass
sage
Reged: 01/22/06
Posts: 434
Loc: Ontario,Canada
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My slitless tester
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7x50 Tasco binoculars
4" f/9 reflector
12.75" f/5.4 reflector
Edited by greenglass (05/29/08 10:17 AM)
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Don W
Postmaster
Reged: 05/19/03
Posts: 13230
Loc: Wisconsin, USA
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Since this is such an important topic, we are making this a sticky thread.
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
William Optics Megrez 90
Coronado PST
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1952
Loc: Boechout, Belgium
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Quote:
The LED behind one half of the knife edge acts as a slit. The light is cut once on the way out and the second time on the return trip. This has become the most popular type of tester. A "Slitless" tester. A pinhole is used by some for various reasons, but it will work with or without. Mine is just a green LED half covered by the knife edge.
Correct but not necessary to partially cover up the LED at all, we always use a bare LED, works just fine. We did some thourough testing using a bare LED compared to old fasion pinhole, nope no difference at all exept for lots brighter using a LED. (Focograms..!)
Another idea.
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Chris
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 616
Loc: Easton, PA, USA
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Walt wrote Quote:
Thanks for the link Gary, Just what I needed!
doc wrote Quote:
yes gary that is the one i read all the time helped me quite a bit
Walt, doc - Thanks!
Mitchell wrote Quote:
Hey Gary, (Long time no see!)
Over two months now!
Mitchell wrote Quote:
For everyone, Dave also has published this into a book with further info. I lost the link but it is a 120 page paperback for a very reasonable price.
It's titled the same as the article and available on the big book selling site that's named after the river in South America.
(Nice candid portrait by the way!)
Gary
(Incidentally, if you haven't read "Understanding Foucault", or did and didn't get it, it's not strictly necessary, but helps to have seen "Forbidden Planet"...)
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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I think that if you use a clear led it wouldn't need a slit or a pin hole in front of it since the led chip is nearly a pinpoint source as it is. An led with a translucent diffused dome is no longer a pinpoint source and might need a slit in front of it. Most surface mount leds ARE clear, maybe I should look for one of those.
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2454
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by the emails, it looks like more than a few of you guys are having trouble with the "dropping resistor" choices for your LEDs.
we need the resistors so the LEDs dont burn out.
lets take the radio shack #276-316 blue LED (ill use it because someone asked about it).
voltage 3.7vdc (thats Volts, DC)
MCD: 2600 (bright!)
20mA (draw in milliamp)
lets say we are going to power this LED from 3 "AA" batteries. each "AA" is 1.5 volts for a total of 4.5 volts.
the formula we use is: (Source voltage - LED voltage)/ Amps
so we take (4.5V - 3.7V) / .02 = 40
that means we need a 40 ohm resistor.
Wait a minute, i hear you saying, where the he11 did you get the .02 for the Amps??? (this is where everyone is screwing up) remember that we are using MILLIAMPs, but the formula called for AMPs. 20mA is .02 Amps. Ahhhhh......
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Actually you need a current regulator (but resistors are cheaper and good enough). You could use a series FET with a trimpot connected to the gate to adjust the current (which will be independent on the supply voltage over a wide range) but that's over kill (the led would maintain constant brightness over a wide range of battery voltage though).
OK enough geek stuff.
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2454
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LEDs only light up with voltage connected one way.
the positive (+) side has the longer lead.
but what if both leads are the same length (like you snipped it off a broken meade telescope)? the Cathode (-) side of the plastic body will have a flat spot.
but what if you found some strange custom LED that has neither? try voltage both ways quickly. when it lights up, mark the (-) lead with a black sharpie marker.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 616
Loc: Easton, PA, USA
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There are several online calculators. This LED calculator also gives the resistor colors in case you have some around and don't want to figure that out. The specs needed can be found by clicking on an LED here for example.
The 12000mcd Superbright LEDs are very bright. Fairly narrow spread and painful to look at directly. Mine arrived in a couple of days.
Gary
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 5959
Loc: Bremerton Washington
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Quote:
Quote:
The LED behind one half of the knife edge acts as a slit. The light is cut once on the way out and the second time on the return trip. This has become the most popular type of tester. A "Slitless" tester. A pinhole is used by some for various reasons, but it will work with or without. Mine is just a green LED half covered by the knife edge.
Correct but not necessary to partially cover up the LED at all, we always use a bare LED, works just fine. We did some thourough testing using a bare LED compared to old fasion pinhole, nope no difference at all exept for lots brighter using a LED. (Focograms..!)
Another idea.
Chris..My white LED is as bright as a small headlight..so I mask it with a pinhole. Then I filter it.
It works great.
Rob
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www.goldmtobservingcenter.com
Providing a great place for amateur astronomers, and ATM's to come and enjoy their hobby.
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The bear
professor emeritus
Reged: 02/11/08
Posts: 727
Loc: rushville, indiana
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oops forgot the resistors but i have a problem i have no idea where , what, specs are for this led guess i better rerig this puppy. if i were to use a pot how would i gook it up i am clueless in rushville and just learning electronics and it is so much fun...
doc
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Longitude -85.42786 Latitude 39.59153
when all else fails use duct tape "works for me"
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Mitchell
member
Reged: 05/19/08
Posts: 82
Loc: USA
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Perhaps I was unclear. I understand you dont need a pinhole, I meant the LED must be roughly half covered (cut off) by the Knife edge. I was indeed emphasizing both will work well!
--------------------
Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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