Hi guys,
I have an Orion 130mm f/5 reflector which I intend to use as a hand held [cradling] telescope to sweep the skies on low powers.
If I use a 0.5 focal reducer and a 25mm Plossl, my calculations show that I would get a magnification of 13x which would be ideal for my purpose.
Are there any issues I should know before I embark on my wide field celestial travels and maybe discover a comet or two?
Focal Reducer for 130mm f/5 reflector
Focal Reducer for 130mm f/5 reflector
Last edited by 10,000rpm on Wed Apr 07, 2010 10:33 pm, edited 2 times in total.
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weixing
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Hi,
Usually, the lowest useful magnification is using the formula = aperture (mm) / 7 (mm). At this magnification, the exit pupil produce by the eyepiece will match the maximum pupil size of our eye when fully dark adapted (assume that your eye pupil can open up to 7mm), so that all the lights gather by the scope will deliver to your eye.
For Refractor, you can go lower than this magnification without any issue, but not with reflector(newtonian), SCT and scope with central obstruction. This is because at this low magnification, you might see the central obstruction as a black spot in the middle of the field of view. You can easily see this black spot in the day when viewing through your scope at low power.
So what happen if you use 13x on your scope?? You'll get an exit pupil of 10mm. If your scope has a 30% central obstruction in diameter, the black spot will occupy around 3mm in diameter in the 10mm exit pupil. In Singapore light polluted sky, I don't think our eye pupil can open up to 7mm. If our eye pupil open up to 5mm, the 3mm black spot should be quite obvious against the light polluted sky background.
Anyway, other than the central obstruction issue at low magnification, you will have serious coma issue. A f5 reflector will had coma issue when viewing at low power and with the 0.5 reducer, I think the coma will be all over the view.
Just my S$0.02.
Have a nice day.
Usually, the lowest useful magnification is using the formula = aperture (mm) / 7 (mm). At this magnification, the exit pupil produce by the eyepiece will match the maximum pupil size of our eye when fully dark adapted (assume that your eye pupil can open up to 7mm), so that all the lights gather by the scope will deliver to your eye.
For Refractor, you can go lower than this magnification without any issue, but not with reflector(newtonian), SCT and scope with central obstruction. This is because at this low magnification, you might see the central obstruction as a black spot in the middle of the field of view. You can easily see this black spot in the day when viewing through your scope at low power.
So what happen if you use 13x on your scope?? You'll get an exit pupil of 10mm. If your scope has a 30% central obstruction in diameter, the black spot will occupy around 3mm in diameter in the 10mm exit pupil. In Singapore light polluted sky, I don't think our eye pupil can open up to 7mm. If our eye pupil open up to 5mm, the 3mm black spot should be quite obvious against the light polluted sky background.
Anyway, other than the central obstruction issue at low magnification, you will have serious coma issue. A f5 reflector will had coma issue when viewing at low power and with the 0.5 reducer, I think the coma will be all over the view.
Just my S$0.02.
Have a nice day.
Yang Weixing
"The universe is composed mainly of hydrogen and ignorance."
"The universe is composed mainly of hydrogen and ignorance."