Image of the day

Captured by
David Marks

Lightning in the desert

My Account

New to Astromart?

Register an account...

Need Help?

Posts Made By: David Simons

September 6, 2006 06:26 PM Forum: Reflectors

Looking for a lens maker

Posted By David Simons

Hi Keith,

You might try Surplus Shed (dot com)

Send an email, and they say they can get a lense made. They quoted me one once, although I eventually got it cheaper through other channels.

[COLOR="Blue"][SIZE="Small"]
Keith Lawson said:

Ok, before you start telling me I'm in the wrong forum, let me explain wink There are some glass pushers out there that have made a lot of mirrors but have made lenses also. I'm in need of a custom made barlow for a planetary Newtonian project. The final instrument will be 10" F/15. This is a purely visual lunar/planetary instrument. The primary will be F/6. The barlow will be ahead of the diagonal and the secondary obstruction will be only 18% including the housing for the lens/diagonal combination.

The barlow is a flint forward design. At final focus the 100% illuminated field will be 1/2 degree and the star images will be diffraction limited to a 1/4 degree field of view. The glass components for the barlow are BK7/N-SF5.

Any mirror makers out there that are small lens makers too?

Keith
[/SIZE][/COLOR]

September 15, 2006 06:50 PM Forum: Telescope Making

Perkin Elmer mirrors- cylindrical, man!

Posted By David Simons

Jess,

Any pictures ?

cylindrical- sagitta about 7 inches

Cylindrical ??

At one inch, they seem very thin.

[COLOR="Blue"][SIZE="Small"]
jess tauber said:

I have the opportunity to purchase two identical large front surface mirrors originally manufactured by Perkin Elmer. They are 30 x 40 inches, and curved, about 1 inch thick. Unfortunately they are on the other side of the country and I don't have any way of determining whether they are of any use for an ATM. So far as I can tell they don't appear to be conic sections though I could be wrong- they may be cylindrical- sagitta about 7 inches, so would be a close focus at any rate.

I also don't know what kind of surface smoothness and figural accuracy they have. Not exactly the kind of gamble one wants to plunk down a couple of hundred dollars for, but it they do focus images well that would be a great price. I'm waiting to hear back from the seller to see whether he can follow my instructions to find what kind of focus they do have.

My inquiries to the current Perkin Elmer outfit have gone nowhere- and the seller has no idea how old these are or what their original application was. At least the coatings are relatively new.

Has any of you ever seen one of these things, and can you say what they were used for?? Thanks.

Jess Tauber
[/SIZE][/COLOR]

September 26, 2006 12:50 AM Forum: Telescope Making

Temperature-controlled figures?

Posted By David Simons

>>
... Maybe I need a new supply of mushrooms ...
<<

Hi Jess,

No mushrooms needed. wink

I do recall an article in an old 70's? 80's? Sky and Telescope about attaching resistors to the back of a spherical mirror and adjusting the current to warp the mirror into a parabolic shape.

The fellow that tried this needed to adjust the current every now and then to tweek it back into shape. I suppose with a combination of TEC's mounted to the mirror and maybe a CCD pickoff camera, an actively controlled system could be built. However getting a glass material that would do this on it's own is probably a bit more challanging smile

October 7, 2006 02:52 AM Forum: Reflectors

What is the best F ratio for a large newt?

Posted By David Simons

Hi Gary,

You might want to build the scope in a smaller scale, say 12" mirrors, using the same f-ratio as the larger design to try to work out some of the basic design bugs. Although scaling to larger size scopes presents it's own problems, at least the basic optical layout and eyepiece placements can tested.

The common configuration for the Bino-Newtonion/Dob is the JMI style, but with some relay optics, I think much smaller configurations can be made with the same size mirrors. another possibility of the relay optics is to tailor the F-ratio for your eyepieces. F5 does seem to be near optimum.

BTW how is the 50" Cassegrain going ?

October 9, 2006 07:47 PM Forum: Telescope Making

Refractor objective.

Posted By David Simons

A few years back right here on Astromart, I was able to pick up a "slightly scratched" FS128 Takahashi fluorite objective for a very reasonable price. I was just about to order the tube and focuser from Texas Nautical, but later found myself in the possesion of Chinese made 127mm f8 OTA. A little turning on the lathe to get the extra 1mm on the lense cell and I was in business. I now have a simple looking 5" OTA, that gives knock you socks off views.

Re-building a refractor can be lots of fun, provided the lenses are available !! Another possibility is getting a nice 7" achromat lens in the size you need installed in your lens cell. There is a fellow I am working with now in China who is making an 8" f6 lense for me. The first one I received had a small chip in it, so I am waiting to get the replacement second lense and cannot give any optical qualifications yet. I believe they also make 7" lenses as well. Another more distant possibility is to get these folks to do a 7" f9 ED doublet which although costing (much?) more, may give a much better visual image than the Achromat, but may have the same centering/spacing/collimation problems of the Meade.

A true 7" apo triplet lens will be a thing of awe both in perfomance and price !

December 3, 2006 12:56 AM Forum: Equipment Talk

Refiguring smaller mirrors.

Posted By David Simons

Hi Dave,

Check Alan Raycraft. His info is near the bottom of my cluttered webpage:

www.planetatm.com

As Floyd suggested though, the cost of this will likely put you way past the entire price of the original scope. But you will have a keeper smile

[COLOR="Blue"]
Dave Sewell said:

Hello all, Does anyone know who provides refiguring services for those who would like to get their smaller sized mirrors refigured? - (Besides Woden.) I've got one of those 5.1 inch f/5 chinese reflectors that provides very nice views, except that the star test reveals some turned down edge....... Thanks, Dave Sewell
[/COLOR]

December 8, 2006 12:35 AM Forum: Refractors

Ceragioli Refractor

Posted By David Simons

Here is one of Rogers original ads for a triplet semi-apo. (same lense ??)

http://www.astromart.com/classifieds/details.asp?classified_id=362150

I saw one of Rogers 8" achromats at RTMC, and it gave a very sweet image. If I didn't already have an R.E. Brandt 8" objective, I very likely would have have bought his.


[COLOR="Blue"]
Danny Vettoretti said:

Interesting footnote on the Ceragioli refractor ad......


8O
[/COLOR]

December 16, 2006 10:06 AM Forum: Refractors

Possible new Petzval F/5 Apos from APM/TMB/LZOS

Posted By David Simons

Hi Markus,

Any update on the Petzval scopes ? smile

[COLOR="Blue"]
>> ... So our Plan for 2007/2008 is to introduce 3 new modells of fast imaging system apos , optimised for the larrger CCD Cameras.

We are happy to talk here we you first which modells could be the most welcome once.

From our point of view, all shall be transportable , so our Ideas are

- 160 mm F/5 with 60 mm diffraction limited image circle
- 200 mm F/5 with 60 mm diffraction limited image circle
- 240 mm F/5 with 60 mm diffraction limited image circle

each of such telescope will be equipped as standart with the new Starlight Instruments Motofocuse with PC control Boxes and representing the state of the art of Apo Production for CCD Imaging.

If you are a serious imager and you have serious wishes we shall consider in such new production, your Ideas are more then welcome

many thanks
Markus Ludes
APM-Telescopes ... <<[/COLOR]

January 12, 2007 04:02 PM Forum: Deep Sky Observing

It is good a 6" f/5 refractor for DSO?

Posted By David Simons

Hi Paulo,

For large dim objects, they are great. I have reasonably dark skies, and low power wide field views of the Milky Way are very nice. Small planetaries that require high magnification will not work out as well, as these type of short refractors tend to not hold up to high power views. I use a 31mm Nagler with mine, but it taxes the focuser a bit.

Here's my initial review:

http://www.astromart.com/forums/viewpost.asp?forum_post_id=231758&poll_id=&news_id=&page=

[COLOR="Blue"]
Paulo martin said:

Hello,

This is my question.A 6" f/5 refractor is good for DSO wide field?
For nebulas, star cluster..

Paulo
[/COLOR]

January 12, 2007 06:39 PM Forum: Telescope Making

Largest privately owned objective?

Posted By David Simons

Jess,

It sounds like you are referring to an achromat or other objective lense scope ?? A fellow Al George in Washington had a 15" achromat IIRC.

Otherwise the group-70 project in the south San Francisco Bay is the largest I am aware of. I did not realize it at the time, but I used to work a few blocks away from the shop they were renting to figure the mirror.

Now up here in the North Bay, we are working on a 40" scope, but other private parties also have such scopes these days.


[COLOR="Blue"]
jess tauber said:

I'm curious as to what the biggest objectives are in privately-owned scopes you folks have ever heard of- any type of scope. And recent things, not the famous ones of bygone years. Does anyone own something at 40 in., 50? Bigger?

Jess Tauber
[/COLOR]