[LVAS] Finder Scopes And a Formula

Thu Feb 18 18:59:35 PST 2010

All,

This is very good info to know when you are trying to scan the sky. By knowing the field of view of your finder and/or eyepieces, you can move a representative circle around your star charts to give you an idea of what to look for once you are out in the field. It can cement your perception of what you are supposed to be seeing.

Fred

From: drivester at hotmail.com
To: lvas at lvlug.org
Date: Thu, 18 Feb 2010 14:16:10 +0000
Subject: [LVAS] Finder Scopes And a Formula

Fred and All,

The reason that I asked you about your finder field was that I wanted to send to you a brass ring to match your new "Pocket Sky Atlas". Your finder is 5°degrees. Hey, I know you probably already have a 1-inch ring or washer. 

This would require a 25 mm ring or almost exactly 1-inch to match the Pocket Atlas. I just don't like the wires that are bend into a circle as we have all seen over the years. I seem to like things that are a bit more finished. 

Most times I seem to always have a brass ring in my pocket with my change. See the attachment of some of my star Atlas and a variety of rings to match each atlas and some of my wider angle low power EP's. The small Edmund 5 atlas was my first that I purchased in the early 70's. The $1.25 cost tag is still on the front 

A standard straight through Meade 50 mm finder has a 5° "true" FOV. That is what the Meade specs say. 

That is the finder that my 10-inch came with. I replaced it with a 90° Antares (correct image). If you have never used a correct image 90° finder you don't know what you are missing. My very low pedestal caused me to have to get into all kinds of contorted positions to see through a straight finder. However, with a large and taller scope such as the 16-inch Lightbridge, a 90° finder might be difficult. I don't know. 

I also like looking at an atlas without having to turn it upside down. 

As we all know a 90° mirror image finder is pretty worthless if you are attempting to star hop to an object. If you have one, please consider a change. Your life and observing will become much more enjoyable.

 It is my opinion that there should be a ban on 90° finders that are not "amici" or correct image. Hey, just my opinion. It is pretty difficult to turn an atlas opposite side and shine a flashlight through the back. This has to have confused a many budding young astronomers. 

If anyone would like to check and get the exact and true FOV of their finder or EP's just use the following formula.

Find a bright star near the celestial equator. The telescope will point at the equator when the tube is at a right angle to the polar axis. (Hey, just pick Betelgeuse). I have put it on the perfect and correct star and it does not make that much difference as long as you are within 10° or so. 

Put the star on the edge of the field and time how long it takes to go to the other side. Divide the time in minutes by 4 to get field in degree's.

Example: It takes 20 minutes to cross   

20/4 = 5° (true field)

Do it with your eyepieces. 

I just found a note card with all of my  EP's timed and calculated. Below is my 12 mm UO Konig.

Actual timing 2.65 minutes/4 = 0.66° or 40 arc minutes (true field). Remember 60 minutes equal 1°  

Another way to check your EP without timing. The 12 mm says that it has a 60° apparent field. 

Focal length of my scope is 1143 which gives me a magnification of 95 x.

60°/95x = 0.63 which is not much different from UO specs. I have found that by divided the MFG stated apparent field by magnification it is normally very close. I have checked and recorded all of my EP's and they match up pretty well with the specs. 

Example:

100° AP (Televue) When used with your scope it gives you 80 x. Then 100°/80 = 1.25° true field. 

If you are not working with dimensions of objects or the true field please give it a try. It will open up a whole new world for understanding how much sky or field that you are looking at and the cataloged size of an object will become much more meaningful. 

If you don't want to know your EP true fields, that is ok. Check your finder and then measure your atlas and make a ring to better star hop on your next outing.

Hope you enjoyed and will consider doing some calculations and will begin to slide a ring around on your atlas. This will then show you how much sky you are seeing with your finder and where that deep-sky object is in comparison.

Hope you enjoyed...

Roger

From: rayworth1969 at hotmail.com
To: lvas at lvlug.org
Date: Wed, 17 Feb 2010 23:07:11 -0600
Subject: Re: [LVAS] Turned Down Edge

Roger,

Thanks for the kind words. 

I have no idea what the FOV of my finder is. It is a Meade 50mm finder given to me by Dave Blanchette. It is their standard finder for the SCT line. All I know is that it works great when I'm not using my laser. It used to work even greater BEFORE I got my laser.

The other night, I used it for a full hour before it got dark enough to see the laser.

Fred

From: drivester at hotmail.com
To: lvas at lvlug.org
Date: Thu, 18 Feb 2010 02:36:20 +0000
Subject: Re: [LVAS] Turned Down Edge

Fred,

Thank you for sharing this information. Not being a mirror maker I was not sure exactly what caused the problem. Your description is almost verbatim of some information that I have read in the past. If there is a turned down edge (the article said) you are not really losing anything by masking off 1/8 to 1/4 inch, as it was only scattering light.

Maybe you could share some more mirror making essays. I have always wanted to make a mirror. However, at this point I am more interested in learning more about it, and the problems that are encountered and how they affect the mirror, etc. 

By the way, what is the FOV in degree's (°) is your finder on your 16-inch? 

Good job in explaining a turned down edge...Roger    

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