Are red dot finders still too bright?

[Rogering]

Hi everyone,
I'm on the hunt for a Red dot finder for my Orion ED 80, however after researching I see that many people still complain about the excessive brightness of RDFs.
I currently use my Telrad (on minimal brightness setting) on my dob so I know what they are complaining about.

Are there any RDF units that address this brightness issue?
or maybe it's time for a DYI cut down Telrad.
Any thoughts?

Thanks

[bojan]

It is easy to turn down the brightness of RDF... one just needs to change the values of series resistors (I think am using up to 300k with one of mine (http://www.ebay.com.au/itm/171007011997?_trksid=p2060353.m1438 .l2649&ssPageName=STRK%3AMEBIDX%3AIT)).

[julianh72]

My RDFs all have infinitely adjustable brightness, so that at minimum brightness (just past the On / Off click), the dot is not visible. I assumed this was typical of all RDFs - I guess this is not the case for some RDFs?

[Rogering]

Hi Julian, which brand and version of RDF's do you have?

Many thanks!

[Rogering]

Thanks Bojan, that's a good idea, swapping out the resistor, simple enough and I have a soldering iron!

Many thanks

[Wavytone]

If you are electronically minded, there's an even better solution using a 555 timer and a couple of components from JAYCAR - make the LED blink ON for 0.1 sec, then OFF for 0.5 sec.

The longer dark space lets you see faint things without being blinded, 0.1 sec means it can blip quite brightly without affecting your dark adaption.

Much better IMHO than simply dimming it.

[AstralTraveller]

Years ago some friends modified Telrads to allow them to blink on and off. [They also added heaters around the window to prevent dew.] I liked using them a lot. If you are interested I can ask for details.

PS. Looks like Wavytone beat me to it.

[julianh72]

I just use the cheap generic RDFs - as sold by Celestron, Orion, etc. I've had 4 or 5 over the years with several different telescopes, and all have had infinitely adjustable brightness.

If you're going to hack an existing RDF, just adding an adjustable resistor in series with the LED should do the trick.

[Steffen]

I wonder whether there is a second-hand market for almost depleted silver-oxide batteries...

[ChrisV]

Anyone got a circuit? A quick n dirty for the DIY thread !

I didn't think a 555 could run off less than 5V. And there's usually only about a 3V battery in RDFs and telrads.

[Wavytone]

Mmm... Use a 7555 - works with a supply 2...18 volts. I'll find a circuit, it's a real beginner thing...

Here http://www.intersil.com/content/dam/Intersil/documents/icm7/icm7555-56.pdf

Use the circuit in figure 5. Use equations 1,2,3 to find the resistor and capacitor values. The output from pin 3 should drive your LED with a resistor in series, with the LED, possibly 220-470 ohms depending what voltage you're supplying.

For this to work properly the capacitor C should be ceramic, 0.1 microfarad, it won't work with electrolytics. RA and RB will be of the order of 470k - 4.7megohm.

BTW I saw a nice idea for a finder the other day - instead of using a red dot it had two pieces of Perspex, each with a small circle engraved about 1 cm diameter. The Perspex pieces were about 5cm apart, with circles aligned along to optical axis. Basically looking through these, the circles appear concentric when you are on-axis, alternatively if you are off axis one appears displaced accordingly. There was a red LED on the side which illuminates the circles using internal reflection within the Perspex. Would be a really easy project to make one like this !

[ChrisV]

Thanks. That'll do. I didn't know there was a CMOS version.
And I like that idea for a finder you found.

Edit: and yes, RDFs (at least mine) are too bright.

[bojan]

Perhaps a monostable (one shot, figure 6) circuit is better option... I almost always forget to switch my RDF off...
And, any CMOS (Schmitt trigger) (http://www.zen22142.zen.co.uk/Design/debounce.htm) inverter will do the job, with only a cap, a pair of resistors (one in series of LED) and switch (for trigger).

[Renato1]

I thought the same as you.
Regards,
Renato