No, depending on which of these tubes you use, an SCT can be converted to either an f/5 or an f/3 -- meaning the magnification and scale are reduced more or less. What makes the difference is the length of the tube, not the addition of a lens. It works with my SCT but I don't think it would with a refractor. -- Joe http://www.optcorp.com/os-fr-1-25inch-focal-reducer-dual-f-3-and-f-5.html ------------------------------ On Sun, Aug 11, 2013 9:39 AM MDT Brent Watson wrote:

Joe,

That would violate the laws of optics.  Any telescope has a fixed focal length.  If you move the film plane out beyond this distance then you are focusing closer than infinity.  You can find that distance using the lens maker's formula.  You cannot just place an extension tube in the path and get a larger image scale.  

Brent

________________________________ From: Joe Bauman <josephmbauman@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Saturday, August 10, 2013 5:22 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint)

In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube.  -- Joe

________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Saturday, August 10, 2013 5:06 PM Subject: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint)

Here are a few illustrations and tables from Sam Brown's classic "All About Telescopes" that are useful tools for making sure the object you're imaging will be the size you want it in the frame.

When the book was published, only film astrophotography was possible, but it's all directly applicable to digital imaging.  Your imaging chip just takes the place of the film.  The math is exactly the same.

The first shows the 4 primary methods of telescope imaging.  Note that a refractor is used in the illustrations, but the type of telescope is irrellevant.  You can imagine the light cone coming from the objective at left to be from any type of telescope desired, the camera doesn't care.

http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/Imagingsystemsill...

The next table will tell you the actual size of the object you want to image on your chip or film, knowing your effective focal length.  It's from the pre-metric days, so you'll have to convert inches to millimeters.  This will tell you if you have sufficient image scale to show the object at a satisfactory size.  You need to know the actual dimensions of your chip to know if the object will be too big or too small.

http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/imagesizetable_zp...

This final table gives you the formulae to determine spacing with various systems.  This will help you decide which commercial extenstion tubes, Barlows, adapters, etc., to order to suit your imaging system.

http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/projectionarithme...

Hope this is useful. _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy

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