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Posts Made By: David Simons

May 25, 2005 06:38 PM Forum: Equipment Talk

RTMC Astronomy Expo Get together

Posted By David Simons


Post your picture !!

I will be a tall guy with beard surrounded by at least one kid. (see profile) I will be eating lunch at the hall at that time and will try to make it over ! (barring family emergencies)

We need Astromart hats !

Maybe somebody here can sell a few at RTMC ? Herb and Co. ?

May 26, 2005 09:21 AM Forum: Equipment Talk

RTMC Astronomy Expo Get together

Posted By David Simons

Hi Alex,

For what it's worth, I'll be bringing my Vixen 5" bino's and plan to set it up in the amphitheater area (past the pool) or if that does not work, on telescope ally.

June 10, 2005 06:51 PM Forum: Beginning Astronomy?

Is there a recommended/definitive sky chart?

Posted By David Simons

Bill Barrett said:

Can you recommend an easy to read sky chart or sky atlas? thank you, Bill

Hi Bill, I have started using the Orion Deepmap 600 folding chart and found it to be a very good starting place. Very easy to pack and hold at the scope. I also use the Cambridge Atlas which is a slightly more detailed. Much past that and I end up going to either the Uranometria or the computer.

July 16, 2005 01:35 PM Forum: Deep Sky Observing

Eyepiece Advice for DSO's

Posted By David Simons

Can anyone share from their experience what they like to use? I would appreciate anyone's input to help me make some good choices. It would save me some money/time on trial and error.


You might also post this in the "Eyepieces" Forum. Lots of opinions there ! wink

I can't go with too small an exit pupil or the floaters in my eye start to be more noticable than the object I'm trying to see. So I try to stay with 1.5-2mm exit pupil or larger. 3-5 is the sweet spot for me. If skies are really-really dark, I'll go up to 7mm, but I doubt my eye's pupil can open up to 7mm anymore.

Exit pupil = Mirror diameter / magnification, or:

Eyepiece focal length / f# of scope.

Example for Nagler ep's and your mirror:

31mm / 4.5 = 6.88 (needs really dark skies)
26 / 4.5 = 5.77 good low power combination
22 / 4.5 = 4.9
17 / 4.5 = 3.8 nice medium power
12/ 4.5 = 2.7
... etc. etc.
4.8 / 4.5 = 1.1 about my limit. Also 300X is about all you can hope to track in a dob anyway 8O

Do you have any issues with eyeglasses, as some of these have much longer eye relief so you can keep your eyeglasses on. I think the 31,22,17 and 12mm Naglers are better for that.

Maybe other folks can disuss the Panoptics, Radians, also the Pentax lineup. But for me, the Naglers seem to get a lot of use in my dobs. I would imagine the 17mm would be a really good all around eyepiece for your scope. 88X, 3.8mm Ex-p,~1 degree FOV. I think your ortho's might be a bit narrow FOV, and they might not like the wide f4.5 incoming light cone, while Naglers seem to put up with that much better. I have also tried using the Paracor, and it helps to sharpen the edge of the field quite a bit, but the stack of metal and glass hanging off the focuser can be a bit much. I also have the old Meade 18mm SWA and like it a lot also, good FOV, sharp, and lightweight being a 1.25" ep.

July 22, 2005 06:46 PM Forum: Deep Sky Observing

Nagler 31mm on a C11 for DSO ?

Posted By David Simons

jean bruno said:

I would like to know if I buy the Nagler 31 mm for a C11 is the better choice for Deep Sky Objects


I use a variety of wide field ep's on my C14, and almost all of them seem to work great. The f11 light cone of the C14 is very forgiving, the f10 of the C11 should be very similar.

You might try some of the cheaper ep's first, they might surprise you. One of the Naglers chief advantages, is they handle the short F# ratios of the big dobs, that is not an issue here. But if money is not a consideration :S , you can't go wrong with the 31Nag.

August 1, 2005 01:12 AM Forum: Telescope Making

cylindrical diagonals?

Posted By David Simons

jess tauber said:

The question I have (lacking a ray tracing program) is whether a simple cylindrical unit would suffice here, or would it have to be more like a toroidal mirror, to preserve the trajectories of rays laterally. Just on paper the reflection in the direction of translation away from the original axis looks ok. I have problems with 3D images without having an actual model in front of my face.

Thanks for input.
Jess Tauber


You might try getting some cylinder mirror from Edmund or Thorlabs and do some testing on a small scale.

August 3, 2005 06:24 PM Forum: Beginning Astronomy?

New to Astronomy

Posted By David Simons

Robert Bland said:

... I do not want to be disappointed in my first telescope nor do I want to regret not buying something that I will want to sell within a year or two. Any advice?

By the way I think this forum is great.

Hi Robert,

Welcome aboard ! This is exactly the right place to start.

There was a nice thread on this subject not too long ago.

Here is a link to one of the posts on that thread.

You might want to read through the whole thread if you have some time, as it will give you a lot of background to scopes and how they are used.

Bottom line: Dark skies, time, chair, star chart/map, wide angle binoculars, and most folks recommend an 8" Dob, the Orion ones should be fine.


August 9, 2005 04:22 AM Forum: Telescope Making

binocular eyepieces

Posted By David Simons

Dave budlong said:

I am sure this is a silly question, but I will ask anyway. Can the primary objective (50 mm) of a pair of bino's be made into any serviceable 2" EP? I know that even if it does work its not going to be great, but if it would be decent it would be worth the effort. I like to play around with silly projects so its just eating up "fiddle" time. I have a pair of Celestron Bino's I dropped in the parking lot after a birding hike and I fractured the prism, so other than looking at birds in the yard, they are shot.

Hi Dave,

Great question ! Maybe some interesting possibilities here. As Dan already mentioned, the FL of a symmetrical eyepiece using the objectives would be fairly long. Although you can measure this distance with the moons image, or sun if you are brave. If you use a 2" 2X barlow, that would cut the FL down to 50mm or so, making it somewhat more usable. Placing the barlow in front of a 2" diagonal or using a spacer would get you about 3X, so now we are down to 33mm or so. As Dan also mentioned, your ep wall diameter will most likely limit the apperant FOV. But you could easily do some experiments to see what works for you.

Another possibility is using the two objectives as an image erector. If the combined lenses are spaced at some distance from the primary image plane, the lenses will "relay" the image, but invert it also. If an eyepiece looks at this new relayed focal plane, it will see an upright correct image. Different spacings will allow you to get magnification or compression of the original image. To get 1:1 relay, I think you need to space the combined lense 2X the new focal length.

August 13, 2005 08:59 AM Forum: Star Parties

Some Photos from Stellafane 2005, Part 3

Posted By David Simons

Hi Dave,

Did you get a chance to look through or at the Porter Turret sccope ? W. S. Houston used to write up some of his observations using this scope. He mentioned it had very good contrast. Do you know any details about it ?

Thanks !

August 15, 2005 09:43 PM Forum: Equipment Talk

Favorite RFTs?

Posted By David Simons

Bill Byrd said:

[COLOR="Blue"]I love wide-field viewing! In light of the discussion about the definition of RFT, how about some discussion of your favorite RFT.

... My current favorite RFT is an 8" f/4 Meade SN used with a Paracorr and a 26mm Nagler giving 36X, with a 5.7mm exit pupil, a true field of 2.3 degrees and stars sharp to the edge of the field. Views of the MW are awesome!
Sooooo....what scope do you like for richest-field viewing, and why? Inquiring minds want to know! 8)
Bill in SA[/COLOR]

Hi Bill,

I agree about the Meade SN scopes, they give a very clean wide field. I'm currently testing a 10" triplet lens stopped down to 9" which gives an f4 system. I can use a 22 Nagler which gets a 5.6mm ex pup, 40X and 2 degree FOV. Stars are not pinpoint to the edge, but at the center FOV they become just pinpoints and filmy nebula and fine grained star fields resolve well.