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Question on Field Flattners

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#1 taki53

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Posted 16 February 2013 - 09:42 AM

I'm just getting into astrophotography and was wondering about the use of a field flattner. I have a vixen 100ed sf f.9 900mm apo scope. I plan to use the canon 60D on a celestrom cg5 mount. Would a field flattner benefit? I have some idea how they work, but I would like to get a better understanding. Also what type/model would I need for my set up? Thanks!

#2 jrcrilly

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Posted 16 February 2013 - 10:06 AM

At that focal length, a refractor has relatively mild curvature. Across a chip that size you would probably find it tolerable. I did, when I used similar rigs.

#3 taki53

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Posted 16 February 2013 - 12:42 PM

Thanks! Please forgive me but I'm not quite sure what you mean.

#4 jrcrilly

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Posted 16 February 2013 - 12:52 PM

That telescope will have field curvature (every telescope has field curvature). The issue is how much curvature there is and how much of it will affect the image. How much there is, in a refractor, depends on the focal length of the objective. At 1000mm it isn't severe. The effect on an image depends on imaging chip size - for a given degree of field curvature the focus difference between the center and the edge depends on how far apart they are.

Your proposed setup involves a degree of field curvature across a chip size that most would consider acceptable. A shorter refractor or a larger chip would change that.

#5 zerro1

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Posted 16 February 2013 - 12:52 PM

I think he means that the flattener may or may not be completely needed.

The problem is that they aren't all equal. some are not just flatteners but also focal reducers. I have an AT2FF (AstroTech) that is just a flattener. I also have a Televue TRF2008 wich is a focal reducer/field flattener. Unfortunately, you can't turn off the focal reduction in the Televue so if you "don't want" to have the reduction sometimes" then you have to get two different pieces of equipment to meet that desire.

#6 orlyandico

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Posted 16 February 2013 - 03:57 PM

a reducer would be useful though because f9 is rather slow and a reducer would take it down to f7.2 which is almost a full stop. this would allow you to halve your exposure times at the expense of a shorter focal length and some tolerable vignetting.

i used a william-optics 0.8X flattener/reducer for a long time on an orion 100ED (same as the Vixen 100ED Sf) and QHY8 camera (APS-C sensor).

#7 taki53

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Posted 16 February 2013 - 04:23 PM

Thanks! How did your photo's come out?

#8 CounterWeight

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Posted 17 February 2013 - 10:05 AM

I would add that unless you can find a flattener or reducer/flattener made for your specific scope by the same manufacturer, it's critical to read and understand the spec's of any other flattener or reducer/flattener, make sure it will work at you focal ratio. All are not created equal. Even after finding one that may work, it's likely if not certain that you will need to play around with the spacing between the optic and your CCD to get best results. For this you'll need some spacer rings of various sizes.






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