ICEINSPACE
 Moon Phase CURRENT MOON Last Quarter40.1% Moon Phase...
 The Sun Now

 IceInSpace How do you estimate focal reduction?
 Register FAQ Calendar Search Today's Posts Mark Forums Read

#1
31-10-2007, 08:45 PM
 5ash Earthling Join Date: Mar 2005 Location: Hunter valley. nsw Posts: 1,000
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
Attached Thumbnails
 44.5 KB21 views
 60.2 KB19 views

Last edited by 5ash; 31-10-2007 at 08:45 PM. Reason: spelling
#2
31-10-2007, 09:42 PM
 Terry B Country living & viewing Join Date: Mar 2006 Location: Armidale Posts: 2,759
Quote:
 Originally Posted by 5ash 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
#3
31-10-2007, 09:44 PM
 ozstockman (Mike) Registered User Join Date: Mar 2007 Location: Kenmore Hills, Brisbane, QLD Posts: 392
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
Attached Thumbnails
 30.5 KB28 views
#4
31-10-2007, 10:07 PM
 rogerg (Roger) Registered User Join Date: Feb 2006 Location: Perth, Western Australia Posts: 4,540
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.
#5
01-11-2007, 08:30 AM
 Escortitis George Join Date: Feb 2007 Location: Greenwood, Perth WA Posts: 72
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
#6
01-11-2007, 10:35 AM
 5ash Earthling Join Date: Mar 2005 Location: Hunter valley. nsw Posts: 1,000
Quote:
 Originally Posted by ozstockman 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
#7
01-11-2007, 10:37 AM
 5ash Earthling Join Date: Mar 2005 Location: Hunter valley. nsw Posts: 1,000
Quote:
 Originally Posted by Escortitis 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
#8
01-11-2007, 10:38 AM
 5ash Earthling Join Date: Mar 2005 Location: Hunter valley. nsw Posts: 1,000
Quote:
 Originally Posted by rogerg 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
#9
01-11-2007, 08:10 PM
 ozstockman (Mike) Registered User Join Date: Mar 2007 Location: Kenmore Hills, Brisbane, QLD Posts: 392
Quote:
 Originally Posted by 5ash 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.
#10
01-11-2007, 08:27 PM
 [1ponders] (Paul) Retired, damn no pension Join Date: Nov 2004 Location: Obi Obi, Qld Posts: 18,778
Philip, what was the camera you used and can you describe the focal reducer you used?
#11
01-11-2007, 09:10 PM
 Terry B Country living & viewing Join Date: Mar 2006 Location: Armidale Posts: 2,759
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.
Attached Thumbnails
 25.6 KB12 views
#12
01-11-2007, 09:23 PM
 5ash Earthling Join Date: Mar 2005 Location: Hunter valley. nsw Posts: 1,000
Quote:
 Originally Posted by [1ponders] 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
#13
01-11-2007, 09:28 PM
 5ash Earthling Join Date: Mar 2005 Location: Hunter valley. nsw Posts: 1,000
Quote:
 Originally Posted by Terry B 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

 Bookmarks

 Thread Tools Rate This Thread Rate This Thread: 5 : Excellent 4 : Good 3 : Average 2 : Bad 1 : Terrible

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Beginners Start Here     Beginners Talk     Beginners Equipment Discussions     Beginners Astrophotography General Astronomy     General Chat     Astronomy Books and Media     Star Parties, Club and Community Events     Celestial and Astronomical Events     Observational and Visual Astronomy         Observation Reports     Astronomy and Amateur Science     Radio Astronomy and Spectroscopy Equipment     Equipment Discussions     Eyepieces, Barlows and Filters     ATM and DIY Projects     DIY Observatories     Software and Computers     Mobile Astronomy Apps     Astrophotography and Imaging Equipment and Discussions Images     Solar System     Deep Space     Nightscapes     Terrestrial Photography IceTrade Classifieds     Telescopes & Mounts     Eyepieces     Accessories     Cameras & Imaging Equipment     Non-Astronomy     Trade/Wanted     IceTrade Archives IceInSpace     IceInSpace Announcements     Website Feedback and FAQ

All times are GMT +10. The time is now 07:50 PM.