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View Full Version : Telescope mods, colours in the sky



Evan
09-10-2010, 07:34 PM
I have added a couple of important mods to my scope and mount. One is motor drive in the declination axis which is the next step toward full remote control pointing capability. The other is a good size finder scope that is coupled to a web cam with software that automatically guides the telescope. This corrects any errors in the mechanical drives and will allow for much longer exposures. That in turn makes it possible to photograph very faint features not previously possible.

As luck would have it as soon as I had these mods ready the weather turned cloudy and it wasn't until Wednesday night that if finally had a chance to calibrate and test the autoguider. It works very well. I still have some items to sort out but the preliminary results are very positive.

These are the two changes to the scope. Bottom is the declination drive and the top shows the scope on the mount with the new finder scope. The finder is an LX-60 mm Meade refractor.

Thanks to Duffy of this board I now have both high quality gearmotors for the right ascension and the declination axes as well as the ascension worm provided by him in exchange for various considerations.

http://ixian.ca/pics7/scopemod1.jpg

These are real images of two stars that illustrate how much the colour may vary. They are accurately calibrated to display the proper visible spectrum colours.

http://ixian.ca/pics7/spectraltypes.jpg

This is one of my favorite targets, The Pleades.
This image capturs much more of the very faint nebulosity than I have done before.
http://ixian.ca/pics7/pleades2010.jpg

This is a stack of very long exposures taken on the drive with a 28 mm lens on the camera. The number of stars it reveals is staggering. This is a section of the Milky Way that is an old neighborhood so the colours are heavily shifted toward the Type K spectral range. Our Sun is a type G which is a bright slightly pink white.



http://ixian.ca/pics7/mw2010a.jpg


In the works for this winter is a new 10" scope. I have a line on a 10" mirror at a good price that is speced at 1/20 wavelength surface accuracy. That is accurate to 1/40th of a micron. <grin>

wierdscience
09-10-2010, 07:48 PM
That last photo is awesome,really cool!

What's the cap for on the DC gearmotor?

Tony Ennis
09-10-2010, 08:16 PM
That last picture is superb!

Evan
09-10-2010, 08:16 PM
It's to reduce noise that interferes with the USB lines. I am running the USB devices at up to the practical maximum distance of about 30 feet. There are several caps on the motor of various sizes that together make for a broad band ac cut filter.

Black_Moons
09-10-2010, 08:34 PM
Very nice photos evan.
In laymans terms of 'I don't know a telescope from a black hole in the ground', Why is the first axis (the one with the tiny worm gear) of the scope at 45 degrees almost?
Is there another motor or plan to motorise that axis?

Again, total clueless at astrophotography, but would'nt a 2 axis mount be able to point at any point in the sky? Or do you need to actualy be able to rotate the camera too?

Very strange axis arrangement. Must make for some intresting math to calculate how to make it move a certian way.

lynnl
09-10-2010, 08:59 PM
On one episode of a recent Science Channel astro physics program (of which there have been many lately), they were discussing the doppler shift, and how that had led to the understanding of the expanding universe.
Is that why we see the different colors in your third photo.

...also, I've been noticing a very bright star low in the eastern sky the last several nights. What star/planet is that?

(I claim zero knowledge of the celestial bodies, other than the moon and sun.
Oh yeah, and the big dipper.)

ftl
09-10-2010, 09:17 PM
Black_Moons:

The main axis is set to be exactly parallel with the Earth's rotational axis. That is equal to Even's latitude (He posted it once, but it is probably around 53-55*N). By doing that you only need to rotate on one axis to track the heavens. This is called an equatorial mount and is very standard in astronomy.

Too_Many_Tools
09-10-2010, 09:35 PM
Beautiful scope Evan.

Are there any discussions on HSM that discuss its construction?

TMT

Tony Ennis
09-10-2010, 09:47 PM
...also, I've been noticing a very bright star low in the eastern sky the last several nights. What star/planet is that?

It is Jupiter. (http://www.astronomy.com/asy/stardome/default.aspx)


Is [the doppler shift] why we see the different colors in your third photo.

No. The doppler shift is useful for determining the distance of galaxies. Stars in our own galaxy aren't moving away from us fast enough. The colors are due to energy output and imply the life phase of a star. Red giants are enormous, old, and relatively cool. Others are young blue and hot. Our sun is middle aged and will eventually turn into a red giant.

Evan
09-10-2010, 10:00 PM
I built the scope before I joined this board so the construction hasn't been documented here.

There is more here:

http://vts.bc.ca/astrophoto/scope.htm

A related project is here:

http://vts.bc.ca/astrophoto/drive.htm

lugnut
09-11-2010, 12:26 AM
Great photos Evan. You are luck to live in a place where you can see the stars. When I lived in Idaho before I retired, I could set out in the hot tub and watch the stars and enjoyed the heck out of it. Now here on the Oregon coast only a mile from the ocean I very seldom get to see the stars, I miss them.

dp
09-11-2010, 01:33 AM
The spider that holds your secondary appears to be the source of spokes in the point source objects in the images - do you suppose that is because of the two-layer/leg design? And I've wondered if taping the gaps would reduce that.

Evan
09-11-2010, 02:55 AM
I cut off one of the legs on each spider vane pair but it made no difference. The diffraction spikes are inevitable and cannot be eliminated except by placing the secondary on a glass plate. That then reduces the amount of light by at least several percent if the best possible coatings are used and over ten percent with no coatings. Even the Hubble shows diffraction spikes.

In this picture taken during the build you can see how little the spokes obstruct the primary. In order to eliminate the diffraction effect the spokes would have to be less than a wavelength of light thick. That would require a supply of unobtanium shim stock.

http://ixian.ca/pics7/tscopeb.jpg

Here is a view of the present configuration.

http://ixian.ca/pics7/scopespyder.jpg

lenord
09-11-2010, 03:32 AM
Evan,

A SPUR gear for the declination axis ? Driven by a spur gear head servo motor ? YOU ??? What, you forget how to make a wheel and worm gear ? :D


Lenord

Evan
09-11-2010, 03:56 AM
Declination isn't for tracking, just for slewing. I have to add a slew motor for the polar axis too. That declination motor is geared down a LOT so it works perfectly. It slews at about 1 degree per second.

dp
09-11-2010, 04:05 AM
In this picture taken during the build you can see how little the spokes obstruct the primary. In order to eliminate the diffraction effect the spokes would have to be less than a wavelength of light thick. That would require a supply of unobtanium shim stock.

I suspect it is the width, not the thinness that is the issue. Glancing light is bouncing off entire width the spiders onto the primary, no? That is, I suspect, what is generating the spokes. A test would be to split some hose and affix it to the primary facing edge of the spiders to prevent glancing light from carrying on to the primary. That creates a shadow, but would be interesting.

Another test is to stop down the lens with a disk over the top of the telescope with an offset hole that splits the spiders so they're not in frame. I know with large format reflectors (16" is large to me!), putting a cover over the telescope with a 4" hole centered at the half-radius point improves image sharpness at the expense of light gathering. Digital collectors have pretty much negated that problem relative to film.

Too_Many_Tools
09-11-2010, 04:14 AM
Evan, ever consider building an observatory for that beautiful telescope?

It deserves one. ;<)

TMT

Evan
09-11-2010, 04:23 AM
It is the thickness that produces the diffraction spikes. Vanes are standard practice for mounting the secondary. Some have tried using piano wire but it makes no difference. One approach is to use curved vanes which spreads out the spikes so that they don't show as much but that reduces image contrast generally and is not nearly as rigid. The main consideration is to keep the amount of obstruction as low as possible to maximize the light captured. The spider can add up to a surprising percentage of the primary mirror area.

Peter.
09-11-2010, 11:03 AM
Nice photos Evan, the quality of your astrophotos constantly amazes me.

Didn't I read recently that you were planning to make a larger mirror on your lathe?

Evan
09-11-2010, 11:43 AM
I am using the lathe to make metal mirrors but not of optical quality. I am interested in using metal mirrors to focus infrared light in the ten micron band. That is doable on my lathe, .1 micron is not.

Deja Vu
09-11-2010, 02:48 PM
Great photos Evan. You are luck to live in a place where you can see the stars.

Heh! I would say that Evan planned that.

Very inspiring, Evan.

Too_Many_Tools
09-11-2010, 03:15 PM
Evan..when you find the time could you discuss further the mount?

I am planning on building a mount for large reflector in the future and would like to hear the design details.

Thanks

TMT