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Old 31-10-2007, 08:45 PM
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How do you estimate focal reduction?

I have taken two photos of a powerpole , the first at prime focus (F10) and the other using a focal reducer.How can i estimate the equivalent focal length using the reducer by comparing the photos. The photos are shown below.
I'd be grateful for some help . regards philip
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Last edited by 5ash; 31-10-2007 at 08:45 PM. Reason: spelling
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  #2  
Old 31-10-2007, 09:42 PM
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Quote:
Originally Posted by 5ash View Post
I have taken two photos of a powerpole , the first at prime focus (F10) and the other using a focal reducer.How can i estimate the equivalent focal length using the reducer by comparing the photos. The photos are shown below.
I'd be grateful for some help . regards philip
Firstly you need to stand on you head to orient the pole.
To be honest the pics will be a bit difficult to use. You need to take an image of something with well defined marking that you can measure on both images. You then measure a reproducible distance on the photos in either pixels or mm and divide one number into the other. This gives you the reducing power. Then multiply this by the prime focal length.
Simple really
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Old 31-10-2007, 09:44 PM
ozstockman (Mike)
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Hi,

I have no idea if there is any tools to do it automatically so I did it in PS manually :-) The second image seems to be taken at f/2.4. You can see both images compared below

cheers,

Michael
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Old 31-10-2007, 10:07 PM
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I use a starfield and Astrometrica to calculate the exact focal length (and hence focal ratio) of my telescope. It takes a little work first time to work out how to do it, but then gets easier.

Roger.
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Old 01-11-2007, 08:30 AM
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I found a good bit of software that may help, do a search for 'New astronomy CCDcalc 1.4.15'. It's a small program thats very useful for what your trying to do, both graphically and with all the numbers.

George
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Old 01-11-2007, 10:35 AM
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Originally Posted by ozstockman View Post
Hi,

I have no idea if there is any tools to do it automatically so I did it in PS manually :-) The second image seems to be taken at f/2.4. You can see both images compared below

cheers,

Michael
Thank s for your help michael . However is it really possible to get F2.4 on an LX90 i wonder.Image still looks good.I think with a spacer i can move the camera back a bit more and achieve further reduction.
regards philip
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Old 01-11-2007, 10:37 AM
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Originally Posted by Escortitis View Post
I found a good bit of software that may help, do a search for 'New astronomy CCDcalc 1.4.15'. It's a small program thats very useful for what your trying to do, both graphically and with all the numbers.

George
Thanks George,
I do have that program but my camera is not listed and i dont have all the info necessary to add it.
regards philip
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Old 01-11-2007, 10:38 AM
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Quote:
Originally Posted by rogerg View Post
I use a starfield and Astrometrica to calculate the exact focal length (and hence focal ratio) of my telescope. It takes a little work first time to work out how to do it, but then gets easier.

Roger.
Thanks Roger,
I was trying to get a simple estimate based on the pics.
regards philip
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Old 01-11-2007, 08:10 PM
ozstockman (Mike)
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Thank s for your help michael . However is it really possible to get F2.4 on an LX90 i wonder.Image still looks good.I think with a spacer i can move the camera back a bit more and achieve further reduction.
regards philip
sorry Philip, I can't help here. I haven't had any experience with focal reducers yet. I bought a 6.3 focal reducer two days ago but it hasn't not arrived yet.
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Old 01-11-2007, 08:27 PM
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Philip, what was the camera you used and can you describe the focal reducer you used?
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Old 01-11-2007, 09:10 PM
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I superimposed the little image on the big one using the gimp and got a rough measurement of 150 pixels wide compared to the big image of 720 pixels. This gives you a focal reduction of ~0.21x
I don't know the focal length of your scope but just multiply it by ~0.2 to give a pretty close estimate.
To get a much more accurate estimate you need to plate solve a star field through the focal reducer. I have an old version of AIP for windows that I would use to do this. I'm sure there are other programs that can do the same.
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Old 01-11-2007, 09:23 PM
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Originally Posted by [1ponders] View Post
Philip, what was the camera you used and can you describe the focal reducer you used?
Hi paul,
The focal reducer is incorporated with a 1.25" diagonal (called a rich field adapter for SCT telescopes) . The unit was supposed to be F6.3 for visual use .I'm using it with a G star-ex for the pic shown. I found that by withdrawing the camera from the diagonal and refocusing i gained more and more reduction.
I believe i can probably get slightly more by moving it a little further out. Hence my request in the wanted adds for a broken barlow lens to gain me a bit more distance.I know from experiment that too far and the image changes into the black of the secondary obstruction.So thats the setup.
regards philip
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Old 01-11-2007, 09:28 PM
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Quote:
Originally Posted by Terry B View Post
I superimposed the little image on the big one using the gimp and got a rough measurement of 150 pixels wide compared to the big image of 720 pixels. This gives you a focal reduction of ~0.21x
I don't know the focal length of your scope but just multiply it by ~0.2 to give a pretty close estimate.
To get a much more accurate estimate you need to plate solve a star field through the focal reducer. I have an old version of AIP for windows that I would use to do this. I'm sure there are other programs that can do the same.
Hi terry thanks for the help. The focal length of the LX90 is 2000cm , so 0.2 of that is 400cm, F2 .The image looks very good for f2 , didnt think that was possible with an LX90 , cant wait to see how astropics come out.
regards philip
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