Re: ATM Relay lense

George,

I don't think the relay lens is a good idea. A number of sources, and lots of experience, says that any diagonal less than 0.2 X the diameter of the primary doesn't buy you any increased performance. You won't see any difference between using the 1.52" and the 2.6" diagonal. In fact, its likely that any small improvement you might get from the smaller diagonal is overwhelmed by the distortions introduced by your relay lens.

Albert

----- Original Message -----

From: George Blahun Jr.

To: charvell ; ATM

Sent: Wednesday, October 24, 2001 3:54 PM

Subject: Re: ATM Relay lense

Herb:

     Hello. First a disclaimer, I am not an expert, in fact I'm currently building my first scope, a 12.5 inch f/4. I plan to use a relay lens also and have done some research and received some advice from people on this list as well as members of the local astronomy club. The size of the diagonal seems appropriate, although instead of keeping the original focal plane 2 inches from the diagonal I am going to try 3 inches. Therefore, my diagonal will be 1.52 inches (minor axis) giving about 12% obstruction and the relay lenses will be about 2 1/2 inches up from the tube top. This seemed like a good compromise from the original 2.6 inch minor axis diagonal which "Newt" suggested, and the potential to use a 1.3 inch diagonal. I haven't decided how to deal with the increased distance required from the top of the tube to the new image plane yet. The coma corrector should go in the focuser as it normally does (mine arrived today). I am using, at least initially a couple of 52mm x 185mm coated achromats which hopefully will not cause too much false color. I believe it is important to use good quality lenses here with as long a focal length as is practical.

     In short, I hope everything you have done is correct, because it is very similar to what I have been working on. Good luck and please keep me/us posted as I will.

George

----- Original Message -----

From: charvell

To: ATM

Sent: Wednesday, October 24, 2001 6:25 PM

Subject: ATM Relay lense

I've been doing some reading on relay lenses and have some questions. There's not much available for reading or maybe I just don't know where to look.

    Primary 16" f/4.5 (72")

    Secondary 1.83" (11%)

    Upper tube dia. 18"

    Focal plane (70") 2" from center of secondary towards focuser (7" from tube wall).

    2 cemented+coated planoconvex acromats (Sky&Telescope Oct. 2001)

    Lense dia. 80mm (app. 71mm clear or 2.8").

    Lense, combined focal length f/2.25 (6.3")

    F.O.V. 100%=1.14  (1.42") 36mm

    F.O.V. 75%=1.23 (1.54")

    Lowest power eyepiece 2" 32mm 70 deg. true field 1.224

The spot size is large enough to accomodate a good size CCD chip but I'm not sure about film photography.

Questions:

1)    Using a coma corrector, where is the safest place to install it and will it still do its job?

2)    What ammount of false color will I be seeing.

3)    What other effects will I be seeing that aren't aparent?

4)    Did I design this correctly or did I make a mess out of things?

5)    Is there anything else I should be aware of?

Thanks again,

Herb (can't leave well enough alone) Watson