Posts Made By: Dan Newman

In going up in aperture, this is not an uncommon observation: Saturn seeming whiter (and brighter).
In my cases, it was going from 4 - 6" refractors over to my 18" newt or between my 6.5" off axis newt
and my 18" newt. There's a big difference in light grasp and I've always figured that that was how my
eyes were responding to the significant change in brightness. The loss of some detail (brown bands)
could either be a collimation issue or too much light washing that detail out?

While the change may not seem too significant between a C14 and the 15" Obsession, my back of the
envelope calculations suggest a 1.24x increase at the focuser going from the C14 to the 15" Obsession.
(C14: 14.25" primary, 4.5" obstruction, one additional mirror [diagonal] that I'll assume to be giving 0.96
transmission; 15" Obsession: 15" primary, 2.6" obstruction; assume similar reflectivity of remaining two
mirrors in both scopes; ignore corrector plate.) So lets call it 20% more light grasp. [Of course, your
primary and secondary mirrors are likely cleaner and hence more reflective than those on a newt.] Question
is, is that enough to explain the difference? Searching around on the web, I've found reports of Saturn
appearing whiter going from 5 to 6 inch refractors (40% increase). Also ran across a review by Ed Ting
where he mentioned this as just a matter of fact consequence of going up in aperture (7" MakCass vs.
a FS102). Both of those cases are much more than 20% (or even 24%). So, while my knee jerk reaction
is that it's the light grasp, I have to wonder if there may not be some secondary effect also.

However, this doesn't explain why it seems more pronounced to you from your C14 to the 15" newt
vs. from the FS128 to your C14. Would be interesting for you to try it with the FS128 and C14
on the same night. Or, to go from your C14 to a smaller newt (e.g., 12.5, 10, 8 ) and see if your
C14 now seems whiter.

Dan

Greg Nowell said:
3. I'm having a hard time imagining how "adding violet back in" would make planet images whiter. To my untutored way of thinking, adding a tad of violet to a very slightly creamy colored palette would make for a rosy hue. So clearly the physics at work here are not the same as mixing colors on a palette, but I'm hoping to get a better grasp....

Yes, mixing light is different from mixing paints. The example most folks should be familiar with is if you
mix red, green, and blue light beams together you get white. Obviously, if you did that with paints you'd
get some sort of brown. Another example with light is mixing red and green light to get yellow. Google
"mixing light beams" to find examples. There's interactive java applets which let you move the "spotlights"
around.

And, if you've never seen some of the 2-color color perception experiments which Dr. Land did in the 50's,
then those are interesting to read about as well. They have less to do with the physics of light and more to
do with the human physiology of color perception.

So, back to "adding violet back in", with light you want to add back in the missing blue to get to white,
not take it away.

Dan

It's been my impression that Al and crew arrives at the recommended settings for TV EPs through
1/2 part calculations (eg, field stop considerations) and 1/2 visual observations. Other folks have
done it similarly for other EPs. Unfortunately, some manufacturers do not publish field stop info
for their EPs. Anyhow, if you google "Paracorr settings for all brands of EPs", you will
find a 2007 thread on this topic at another website.

Dan

Having had fixed and modified a few Telrads, here's my list of suspects,

1. Batteries
2. Battery terminals
3. The off/intensity potentiometer has gone bad
4. A wire has come loose (fixed a couple of those in the past)
5. It was left in the daylight pointing towards the sun and the reticle
got done in. (Anacortes sells replacements.)

And, there's

6. You're looking through the wrong end

A loose wire is the problem I've fixed in the past for others. I've heard and read
others reporting their off/intensity potentiometer going bad, but I myself have
not encountered that. The remaining electronics -- a resistor and an LED --
are unlikely to go bad. But pots, yeah they go bad.

If you have a voltmeter, you can check to see if voltage is being sent to the LED.
To get to the pot, use a small screwdriver or whatever to pop the lever/knob off
of the pot post. It's tight but will come off. Then remove the 3/8th nut and push
the post through the hole into the inside of the Telrad. I'd be careful about trying
to short across the pot to see if things work: you don't want to do it wrong and
allow full battery current to the LED. That's what that resistor is there for: to put
an upper limit on the current which the LED can draw.

If current is making it to the LED, but you don't see the reticle illuminate then the
reticle is shot. (It's unlikely to be the LED which is shot: after all current is flowing
and most LEDs, when the fail, fail open not closed.)

Dan

And a shot of two grazing.

I recall encountering an 8 or 10" newt whose primary had a known bad TDE.
The owner had a removable edge mask and could demonstrate the TDE by
removing the mask. With the mask present, you could discern an appropriately
picked faint target near a brighter object. Remove the mask and it became
significantly more difficult. With the mask in place, the scope performed
nicely, just with a reduced aperture. With the mask gone, it was not unlike
a fairly dusty mirror.

Dan

Looks like fire crews have remained in place for the time being, as per the
8:07 PM update at

http://www.mtwilson.edu/fire.php

Sunday, 30 Aug 09, 8:07 pm PDT - A critical aspect to the survivability of the Observatory should the fire sweep across it is whether or not fire fighters will be on site during such an event. The U.S. Forest Service continually assesses the danger to fire fighters in any scenario and will withdraw fire crews in situations that are particularly precarious. Such an evaluation took place on Mount Wilson in the last half hour with the decision for the fire crews to remain in place tonight. That's very good news.