I've always been told to buy a lower F number telescope. It'll make astrophotography easier, faster etc.

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.

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.

I posted this on another forum so rather than repeating it I'll provide a link.

It may help others understand or if someone can disprove it I'd be grateful.


http://loresinger.com/FWIS/viewtopic.php?p=12925#12925

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MrObvious

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.

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.

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

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Observatorio de la Ballona

HI MrObvious, I concur with andy it depends on what you are trying to photograph. (Planets or Dso's) plus two other important accessories plays a major roll here, that is a reducer and a Barlows. Clear Skies

Hi Andy,

It seems we are pretty much agreeing but talking slightly apart.

What I'm saying is for the same size image of a particular object (on a CCD/film/eye) it makes no difference to the brightness whatever the native F ratio of the scope is.

If you were to require a barlow lens on a shorter F ratio scope because the object required magnification, then the longer F ratio scope will take less time to image (all else being equal) because now there is one more lens in the light path. Assuming the object size on the CCD in both cases is the same.

If the object to be imaged requires a focal reducer on a longer F ratio scope then the shorter F ratio scope will provide the shorter exposure time (all else being equal) because the longer F ratio scope now has an additional lens in the light path.

IOW, for the same size image to be seen on a CCD, it will require pretty much the same time for both scopes to form the same brightness image. Since the addition of a single lens won't impact the time by a huge percentage.

The shorter F ratio shouldn't be called brighter, it should be called wider.
Only the diameter of the main lens/mirror determines the exposure time given that the formed image will be the same size.

E.g. I noticed on your site where you described your experience at imaging at F10. The image wasn't as good as you'd wanted it, but, it should also be noted that it would also be larger than if you used F6, almost twice the size in fact.

The main reason I posted this is that there seems to be the misconception that a scope of lower F ratio will give brighter images. Sure it will, but it gives smaller ones too and thats only for the same set of eyepieces.

Most people I know choose a scope along these lines. As big a diameter as they can afford or is practical then buy eyepieces to get either a specific FOV or a specific magnification. Given that situation:

If one chooses the eyepieces to be the same magnification for both scopes then the images will be the same brightness for both scopes.

If one chooses eyepieces for the same FOV for both scopes, then both scopes will have the same brightness.

The only difference is that the longer FL scope will have longer length eyepieces. This also has huge advantages for people with glasses. It means their eyepieces will have more eye relief for the same price. The images will however appear identical in brightness.

So why say the lower F ratio gives a brighter image?

It's confusing to beginners and adversely affects people who wear glasses because the eyepieces are more expensive to produce with longer eye relief at shorter focal lengths.

All because there is a misconception that it will produce a brighter image.

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MrObvious

I think the problem is just talking about f-ratio confuses the issue because it reflects 2 things: The apeture and the focal length (f = focal length / apeture). If you keep your focal length constant, a lower f-ratio scope does produce a brighter image. That is why the image gets darker when you use a barlow lens -- you have increased the focal length while holding the apeture constant. The Takahashi Epsilon scope is f2.8, very fast. You get more data in less exposure time than other 500mm focal length scopes.

Addition of a lens will have little or no impact on the exposure. If you are creating the same size image then the focal length is the same. If the focal length is the same, the scope with the larger apeture (and therefore lower f-ratio) will have a shorter exposure time.

The apeture and the focal length determine exposure time and brightness. F-ratio encompasses both, so it confuses the issue to talk about it in isolation from the other specifications of a telescope.

Larger, but also harder to image. And the seeing problems are larger as well. That's why I will stick with f6.

Clear skies,

--Andy

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Observatorio de la Ballona

Thanks for responding Andy.

you said;
Thats correct. The key thing is that you keep the focal length constant. IOW, it works like a camera. A smaller F ratio will automatically imply a bigger aperture.

It's when people compare two scopes of the same diameter/aperture and then claim the one with the lower F ratio is a brighter scope, that is the problem I'm referring to.

This is what comes across confusing to beginners.
Claims that smaller F ratios make brighter images is wrong. Its larger main lens's that make brighter images.
If we buy a scope that has the same focal length as another scope then the F ratio will also determine the main lens size. Smaller ratios mean bigger lens.

If however most people are likely to buy a scope buy its lens diameter and compare two scopes of equal diameter main lens/mirror. Then claims of one being brighter than another because its F ratio is lower are just plain wrong.

To see what I mean your link states:
This is misleading. It's claiming its due to the F ratio, but its really a due to the lens diameter.
One can envision a smaller object requiring a barlow to get a reasonable size in this scope. If this is done so that the recorded image is the same size on both instruments the scopes will have the same exposure times. Likewise a F/5.6 instrument with a 2:1 reducer will give exactly the same image size, FOV and exposure times as this scope.

It is the lens diameter that determines the result. Not the F ratio.

I think the problem here is that they provide half the information. The rest is left to the reader.

In your case you know how it will all work out. A beginner will just assume that because the scope has a lower F ratio it is brighter simply by virtue that its the F ratio. I've had salesmen tell me that this 200mm scope is brighter than this 200mm scope because it has a faster mirror. This propagates the misconception.
If he said this scope provides wider views for the same size eyepiece, that would be correct and not misleading.



Summing up:
Two scopes of equal main lens size give exactly the same image brightness for the same magnification. (this is obvious)

Two scopes of the same focal length but different F ratios will have a different sized main lens. (this too is obvious)

Conclusion:
It's the size of the lens that is the key parameter, not the F ratio, when determining brightness.

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MrObvious

You are welcome.

I think we've hit the point of agreement. The confusion probably comes from the fact that camera lenses hold focal length constant while varying aperture (finally got that spelled right). My Nikkor 35mm f1.4 is a beautiful lens with a vary large aperture. Much nicer than say, the f3.5 model which is much smaller.

The comment about brightness only has meaning at a constant focal length.

Not misleading -- it is bright for a 500mm focal length scope. I know people who have been amazed at the data they collect from this scope in a small amount of time. But they are looking for a 500mm/f2.2 field of view, not high magnification. The more honest statement would be, "the great speed for its magnification."

Assuming 200mm is aperture, absolutely correct.

I agree. Light gathering power is ultimately determined by aperture. But the design decisions that go into f-ratio make the scopes different and therefore useful for different purposes. But as you say, not a priori better.



Clear skies,

--Andy

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Observatorio de la Ballona

Well, it seems we are in agreement, but quite it's ingrained in advertising and even in articles that attempt to explain it get it wrong.

E.g.

http://www.dur.ac.uk/astronomical.so...ction=articles

I put this in because it's a university teaching astronomy, yet they are misleading to the point that I would say they are plain wrong.


The second paragraph is correct, it also qualifies the conditions, i.e. the same eyepiece.

The first paragraph misleads more than it informs. It just makes a blanket assertion without qualifying what they mean.


I've found sites that specifically say that an 8inch F/6 is a brighter telescope than an 8inch F/10.
No qualifiers at all, just a blanket statement that implies lower F ratio means brighter image even on two scopes of the same aperture size.

Oh well.
I think this is one misconception that is too ingrained to be able to be changed.

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MrObvious

Ouch. It's that type of advertising that leads to disappointed buyers who turn away from the hobby (or avocation, if you prefer).

I guess we just have to keep at the guns and say, "It's not a simple decision."

Clear skies,

--Andy

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Observatorio de la Ballona