Posts Made By: Keith Lawson

Looking for a lens maker so I can produce this telescope:

10 inch F/15 Planetary Newtonian with built-in barlow/corrector. 11.8% obstruction, cemented barlow, diffraction limited to 1/4 degree, 100% field illumination to 1/2 degree. 10" F/6 primary. Anybody out there willing to make a small cemented lens for me. BK7 and SF5 glasses. Mild curves. I'm willing to pay a reasonable cost for it. The "big boys" want too much to produce the lens.

Also: Any of you telescope experts willing to give the design the "once over"? The scope was designed in Atmos.

Keith

I just had to share Celestron CPC 1100 awaiting the darkness.

Hi All,

I just came in from another observing session with my new CPC 1100. I'm posting this thread to give Celestron some credit for the very good optics in this C-11. I just observed the moon Tethys (400x)only 1 arc second below the ring of Saturn (11:00 PDT). The moon was seperate from the ring. The seeing was average and I had to confirm it multiple times when the seeing settled for about a 1/4 to a 1/2 second but it was there. Not bad for an SCT. At lower magnifications more in tune with the seeing (140x to 190x) Saturn is crisp and well defined. No vagueness of focus. Nice. The XLT coatings work as advertised. Minimal scatter and excellent light transmission. I'm impressed. I didn't expect it to perform this well. This thing has enough nerve to have snap focusing at 800x. To say I'm happy is an understatement 8)

Setup and ease of use is like falling off a log. I don't miss German Equatorials. No polar alignment. No counterweights. No odd sky positions etc.. For the visual observer this scope's ease of use is a dream (note: setup is not for the physically challenged. 65 lbs fork & OTA)

Keith

Mr. Christen,

As I was reviewing some old threads I came across this statement by you:

"Optically, Maks have a sweetness that is impossible to match with any SCT design presently made, even if the optics of those SCTs could somehow be made perfect. All you have to do is run some of these common SCT designs through a design program like ATMOS to see the results. And it shows up on the night sky at your eyepiece."

Pardon my ignorance but could you please elaborate a little. Forgive me if this has been answered before but what is wrong with the current design of SCT's? The SCT's I found in ATMOS are pretty good. The one with the aspheric secondary (like what Celestron does) looks near perfect with strehl ratios in the high 9's. I have owned high-end Maks (up to 8") as well as SCT's (up to 14").

The C-11 I currently own is excellent in my estimation. (Thanks Robert Piekiel for your book. Everyone does not have access to a interferometer.) It defines the airy disk to "textbook" standards. A glass 250 line grating at 4 bands outside focus are straight to the edge, are of high contrast and easily seen. The knife edge image is flat. What is it that I do not understand about SCT's?

I'm asking this from a visual observers standpoint. I couldn't give a baby gnat's butt about astrophotographic/off axis performance.

Keith

Hi All,

I just thought I would share my simple solution for SCT image shift. Since I refuse to pay the high price for some aftermarket focusers I made my own. It's a simple helical focuser. It drops into my existing 2" diagonal. It was made for 1 1/4" eyepieces. The threads were precision machined for a nice smooth fit. There is no play even when extended to 1 1/2". I use the main focus knob on the OTA for rough focus and the helical for fine focus (high power planetary observing). For visual use it works quite well. I tried to find something like this in the aftermarket arena but found nothing.

I didn't spend the time to knurl the focuser since the eyepiece thumbscrew acts as a fine focus knob.

I also use the helical focuser when I want to mimic the waves of defocus in Aberrator. I can do it easily now since I know the thread pitch (12). One turn is .083" of defocus and if my calculations are right this comes out to 4.79 waves.

Keith

I believe everyone has a dream instrument that is either beyond their bank account or is no longer made or a combination of other factors. So I ask: What is your dream scope and why? No professional observatories please.

Here is mine: A Questar 12. Optical and mechanical perfection! No GOTO but man what an instrument 8) As a lunar/planetary observer this would be an ultimate instrument. Now if I could have it and good seeing to match...

Anybody got any more info on this company :S http://www.istar-optical.com/

Sure looks promising

Keith

This is my first telescope project. It's a 6" F/5 (Meade mirror) from 1976 when I was in the Air Force stationed in Alaska. The EQ head was from a scavenged and highly modified Jason refractor mount. Man, money was tight in those days. Anyone else willing to share their first TM project?

Keith

All,

With the seemingly large choice of import lenses available I decided to do some software experiments and see what the difference would be in the chromatic aberration error in the different lenses offered. These are general numbers but for those who are thinking of making a large refractor using some import optics (me) I thought I would post my results.

Please understand that these are general numbers. A general guideline. There too many factors to be precise but this should give a very basic understanding of how much color (visual) one can expect from achromatic lenses of different diameters and focal lengths.

A 127mm or 152mm F/15 used to be a good standard (3X Airy disk) but with the proliferation of 150mm F/8's out there I decided to use it as the standard. The numbers represent the size of the red and blue (combined) blur circles as compared to the airy disk diameter. The color crossings of the red and blue are between the 70% & 100% zones. The current standard for decent color correction in an standard achromatic lens is 3X the airy disk. Also remember that, as in all scopes, the airy disk shrinks with aperture so this is part of the equation as well.

I am sharing this for someone like me that is thinking of building a large refractor using an import lens. The prices for such large lenses are hard to resist. The quality is still open for debate. Anyway, here goes:

150mm F750 16X
150mm F1200 7.6X (standard)
150mm F1500 6X
150mm F1800 5X
150mm F2250 4X
204mm F1200 17X
204mm F1800 9.6X
204mm F2400 6.5X
204mm F3000 5.5X
216mm F2700 6.6X
220mm F3000 6X
228mm F1800 12X
250mm F1375 25X
250mm F1800 15X
300mm F3000 11.8X

Simi-Apos or any other "special" corrections were not part of these equations. Leaning the correction torward the red or blue were not used as a factor as well.

To the professional opticians, if I am in error please let me know. I don't want to be a source of misinformation. I will pull this thread if so.

Keith

Just purchased a 200mm/F1800 lens from Istar Optical. Good communication with Ales of Istar. Will let you know what transpires...

The TM itch can be scratched but it never goes away

Keith