Getting organized

My friend and neighbor Mel is going to help me make my first Dob.  He’s a retired tool-and-die guy, and knows how metal works.   I’ve been studying  David Kriege and Richard Berry’s excellent book on the topic, The Dobsonian Telescope.  Per their recommendations,  we are beginning with the mirror cell, and  between the odds and ends Mel has in his shop and a few items ordered from a surprisingly local metal shop, we may be about to begin.  Mel has cutters and welders, drills and taps and all manner of metal working tools, and the expertise born of 50 odd years of doing it.  If, between Mel’s place and my place, we can find a flat surface large enough to get stuff laid out.  Both of us seem to have problems with too-much-stuff.

using toilet paper rolls as wire organizers

Mel’s Central Organizing Principle

Mel at least has found a way of organizing the myriad of wired dinguses we all seem to accumulate.

Rainbows in the ice

We had all three grandkids this weekend, which is always a recipe for mayhem.  In the backyard, Job had managed to get Rose pretty wet so she was mad and went in.  Then he threw a wooden rubber band gun which hit Bri in the butt. He had to, he was out of ammo!  But she was kinda mad at that.  So I had to read Job the riot act, in the course of which I may have inverted him over the water trough.  In this position, he observed that the water had frozen in a thin clear sheet.  This distracted the both of us, and we proceeded to try to break the ice by punching it.  Which was a mistake because that ice was thicker than it looked, and felt like punching a brick wall.  A quick search found a small rock, and a few quick jabs poked a hole in the ice.  Some cracks formed, and Job said there were rainbows in the ice!

Iridescence in ice

To make sure there was no oily stuff in the water, we took off our gloves and started picking out chunks of ice.  Close up, we could see that the rainbows appear within the ice,  along cracks that went through its thickness. Job discovered optical interference!  Presumably the iridescent colors result from light bouncing around in  the cracks.  Interference makes the colors as light waves reinforce or cancel out each other in the resulting melee.  Wow, that water was cold!  Job and I beat a hasty retreat to get our hands under some warm water.

Newtons rings, interference patterns between glass slides

Inside we made our own rainbows (“Newton’s rings”) with two glass slides.  Like the ice, the two pieces of glass offer closely spaced reflective surfaces for light to bang around between.  In the picture, I’m putting some strain on the slides to give a little curvature and force them close enough together to get good interference.  It’s lit with a compact fluorescent bulb, so the colors are a bit weird, since fluorescents don’t give us a continuous spectrum.

 

 

In search of the flat field

I’ve been thinking about the problem of evaluating my flat field lighting and target, and two approaches came to mind. One is to make some flats, rotate the camera 180 degrees, make more flats, and then measure the difference.  I haven’t done that, but it seems like it should work.  The other method is to photograph the target  in a wide field with my dslr and look at color rendering and the light distribution over the field.

Here’s a stretched look at the master flat I had made.  It actually did a pretty good cosmetic job on the NGC80 image.

stretched maser flatIt shows the vignetting produced by the optical train.  The “donuts” are shadows of dust bunnies on the cover glass of the ccd chip; they’re shaped like that because my optical system has a secondary mirror that forms a central obstruction.  When this image is applied to the image of NGC80, all that crap is magically removed.  So flat fielding  removes artifacts of the optical system and ccd, but requires an evenly lit target to work.  Here’s the DSLR picture of the first try at a target:

first flat field target

First, it’s really red.  I’m using a 15 watt tungsten bulb to illuminate the target, which is bouncing off a white reflector before hitting the white target.  It looks like about a 2800K source, that gets even redder bouncing off the plywood interior of the hut.  Color matters because I will do photometry, and try to get very accurate measurements of star or minor planet brightnesses, and the sensitivity of the chip varies a bit with wavelength.  So I’m aiming at a more “daylight” balance on the theory that that’s a pretty average star color.  Being the color of our star.  Let’s see how even it is.

I converted the image to grey scale and used the horizontal box tool in Maxim to see average values across the image.  You can see that the illumination drops off on the right hand side.  It looks to me like if I can level out the profile, I should have a pretty good target.  The light is falling off because the scope is casting a shadow on the target.  It worked ok with my previous setup, but needs a fix with the new mount.  Here’s the way the setup should look.

I put some tough blue, a lighting gel used to convert tungsten to daylight, on the lamp, and fiddled with the reflectors.  The blue really helps the color balance.

The reflector and target are both white foamcore.  I need to come up with an intelligent mount for the cards, right now they are just taped and wired, and aren’t parallel. Also, I find that the sweet spot for the scope is right on the stud.

 

The illumination still looks pretty blotchy, so back to Maxim for more measurements.

At least the slope is in the opposite direction!  I still have about 10 percent variation across the target.  Next fix is to properly mount the target and reflector, which I think will make it pretty close.