Quote:
Originally Posted by MrObvious
I've always been told to buy a lower F number telescope. It'll make astrophotography easier, faster etc.
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In buying a telescope, there is no magic number or criteria that will be best because different scopes have different uses. Generally, a faster (lower f-ratio) scope will make exposures shorter and guiding easier. But you should also match your focal length to what you are trying to do: wide field or high magnification.
Quote:
Originally Posted by MrObvious
This however is misleading. What it will also do is make the images smaller/and increase the FOV for the same eyepiece.
If I was photographing Jupiter for example with two different telescopes of the same diameter but different F ratio's people are claiming it'll be faster with the scope with the lower F ratio. Sure thats true if I want a smaller picture of Jupiter, but, if I want the same size picture they will be identical in brightness (all else being the same).
The photons collected are dependent on the diameter of the receiving lens not the ratio of of the lens.
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What you say is true, but it is not misleading. A different f-ratio on two scopes of the same apeture indicates different focal length. Longer focal length is greater magnification.
Jupiter and all the planets are very bright, so you do not need to struggle to get enough photons to get an image. I have taken many shots of Jupiter and Saturn at f30 and f40 using a 3x barlow and 4x Powermate on my C-11.
Quote:
Originally Posted by MrObvious
So if you want wide angle photos get a low F number scope or a longer eyepiece on a longer focal length scope.
If you want high magnification, it makes no difference in practice.
Here's the catch, DSO's.
If you want to photograph one of these it would really make little difference in practice what your focal length is. To get the same size image on film/CCD will require the exact same time on scopes with the same diameter
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For CCD imaging, you want to match the magnification to the size of the pixels on the CCD. I have a C-11 (2800mm FL f10) and and ST-10. The ST-10 has 6.8 micron pixels. At f10, that is 0.5 arc seconds per pixel. Imaging at this focal length is very difficult, and often pointless unless your seeing is excellent. If you are interested, I
wrote about this in detail on
my blog. I typically use a focal reducer that leads to an effective focal length of f5.95 and 0.85 arc sec per pixel. That is doable and I get good magnification on deep space objects with acceptable data collection times.
I also have an NP-101 (101mm FL f5.4) that give me 2.57 arc seconds per pixel. Very good for wide field nebula imaging, not good for high-magnification shots of either the planets or DSOs. There are shots with both on
my site.
Also, check out Ron Wodaski's
CCD Calculator. It will let you see what different scopes, focal lengths, and cameras produce.
HTH,
--Andy