Some questions

[turboscrew]

I have some basic questions:
Telescopes with small F-number are called fast. I guess that comes from AP meaning that with telescopes with small F-number you can use shorter exposure times. Is that because telescopes with bigger F-numbers make bigger images on the focal plane?

Are the optical limitations the main reason most AP is done prime focus?

In digiscoping, should you match the exit pupil of the telescope/eyepiece with the aperture of your camera? If the maximum aperture of the camera is 4 mm, it would be best if you could use an eyepiece that gives 4 mm exit pupil?

Is there a simple rule of thumb about how small an artificial star should be? I guess it's all about angular size. Bigger tubes (higher resolution) require smaller apparent size?

[Star Dad]

They are called fast from photography - that is they need shorter exposure time. Recall that also means less depth of field. In AP depth of field is irrelevant - since everything is at infinity any way. It also means wider field of view. I don't think it means that they are smaller images on the focal plane as compared to slower telescopes. They have narrower fields of view and to capture the same amount of light take longer than faster telescopes.

Afocal AP is difficult at best. I have done it on the sun, moon and Jupiter and Saturn. But it is hard to get a camera at the focal point of a lens. Prime focus is just so much easier. You can use barlows and focal reducers to change the field of view/magnification. Just think of a telescope as a long lens for camera... in my case a 1000mm lens with huge light gathering capability. A 2x barlow is the same thing as a "doubler" in regular photography.

I'm not expert enough on your other questions, so I'll leave others to explain more fully than I could.

[mikemarotta]

It is all beyond me and I will join you in waiting for the answers. For myself - and I look forward to an authoritative answer - larger F/numbers mean higher magnification with a narrower field of view. if you have a 100-mm objective with a 1200 mm focal point, your f/12 will give you 200x for a 6 mm ocular. But you are looking through a soda straw. With an Dobsonian 250 mm and f/4.8 you will get 80x with the same ocular but you will have much more light coming in and a wider field of view.

That said, I am sure that there are very many facets and aspects to this.

[turboscrew]

They are called fast from photography - that is they need shorter exposure time. Recall that also means less depth of field. In AP depth of field is irrelevant - since everything is at infinity any way. It also means wider field of view. I don't think it means that they are smaller images on the focal plane as compared to slower telescopes. They have narrower fields of view and to capture the same amount of light take longer than faster telescopes.

Afocal AP is difficult at best. I have done it on the sun, moon and Jupiter and Saturn. But it is hard to get a camera at the focal point of a lens. Prime focus is just so much easier. You can use barlows and focal reducers to change the field of view/magnification. Just think of a telescope as a long lens for camera... in my case a 1000mm lens with huge light gathering capability. A 2x barlow is the same thing as a "doubler" in regular photography.

I'm not expert enough on your other questions, so I'll leave others to explain more fully than I could.

I was thinking, if 150mm F/4 can use shorter exposure times than 150mm F/8... They have the same aperture, so they collect equal amount of light from the same target. Only bigger image would explain the difference in exposure times (on prime focus camera).

[turboscrew]

It is all beyond me and I will join you in waiting for the answers. For myself - and I look forward to an authoritative answer - larger F/numbers mean higher magnification with a narrower field of view. if you have a 100-mm objective with a 1200 mm focal point, your f/12 will give you 200x for a 6 mm ocular. But you are looking through a soda straw. With an Dobsonian 250 mm and f/4.8 you will get 80x with the same ocular but you will have much more light coming in and a wider field of view.

That said, I am sure that there are very many facets and aspects to this.

Magnification only depends on the focal lengths of objective and eyepiece. It has nothing to do with aperture, and thus very little to really do with F-number. But with the main mirror/lens alone, maybe the image distance from the lens/mirror causes the image to be bigger? In my head, these things are fuzzy.

[sdbodin]

All this stuff will make your head spin. Just go out and shoot, if it works, then OK, if not, try something else. More fun this way. A lot of that photographic carry-over from the film days has been overcome by the electronic 'film' of today. An interesting article is here:

http://www.stanmooreastro.com/f_ratio_myth.htm

Seems to match my own experience imaging.

Clear skies,
Steve

[turboscrew]

I was thinking along these lines (pun intended):

[turboscrew]

All this stuff will make your head spin. Just go out and shoot, if it works, then OK, if not, try something else. More fun this way. A lot of that photographic carry-over from the film days has been overcome by the electronic 'film' of today. An interesting article is here:

http://www.stanmooreastro.com/f_ratio_myth.htm

Seems to match my own experience imaging.

Clear skies,
Steve

I don't seem to have that luxury. If I get a cloudless couple of hours once a month...
The article is interesting, but at my level, still in the future. I'd just like to get something recognizable on the camera sensor.

[Baurice]

It's counter-intuitive a lot of the time! The F/number affects brightness, the focal length affects image size/field of view and aperture affects resolution (just as for visual). So a small aperture with a long focal length would result in an large dim image, with a small field view but no more detail on a large object, such as the Moon than a small aperture with a shorter field of view.

I can vouch for this because have found it in practice. Very short focal lengths might sound good but (as for visual use) there all sorts of distortion, such as chromatic aberration.

What I've also found is that there are lots of things that work if only you try. Can you record the phase of Venus with a 300mm lens? Er, yes!

[SkyHiker]

The less glass the better so prime focus is preferred.

Yes the F-number is only relevant for AP. For visual, the light flux is equally determined by the eyepiece.

Afocal is not difficult or bad. It is done by setting the camera focus to infinity then use the telescope to adjust. I used a small pocket camera (ELPH 100 HS) and lined up the lens by wrapping a piece of toilet roll around the eyepiece and the camera lens tube then attaching the camera with a gasket and a rubber band to the eyepiece. That way the alignment is pretty much guaranteed. The optical zoom can then be used to increase the magnification 10x or so.

Eyepiece projection is sometimes recommended but it's not easy at all, the reviews of that method are generally pretty bad in terms of usability.

The regular magnification of objective and eyepiece are clear from high school optics as per your diagram above, no mysteries there. When you talk about AP magnification there are no definitions. Personally I like to extend the notion of magnification for AP to be based on the smallest discernible angular features of an image, then divide that of the human eye by it.

[turboscrew]

I haven't found any way to figure out good starting points for camera settings (including eyepiece selection). I think it's easier with primary focus. It's hard to find enough data about a point-and-shoot camera. It would be nice to be able to estimate how big image you can get with an eyepiece and what kind of exposure time is needed.

And I think it's not worth trying to get a picture if the exit pupil the eyepiece gives, is about 1 mm. That would then be the "limiting aperture".

I guess it is the bigger image due to longer focal length that makes the image dimmer?
And maybe, because F-number is focal length (= image size) / aperture (= the amount of light), it is kind of simplification of "image area to be lit with unit light"?

[mikemarotta]

Magnification only depends on the focal lengths of objective and eyepiece. It has nothing to do with aperture, and thus very little to really do with F-number. But with the main mirror/lens alone, maybe the image distance from the lens/mirror causes the image to be bigger? In my head, these things are fuzzy.

Right. We agree on that. My point was that the aperture is the denominator and the focal length is the numerator and that ratio is the f/ number. That f/ number leads to some telescopes being labeled "fast" or "slow" by analogy to photography with film. Personally, I think that it is a misnomer when applied to telescopes. Your concern was specifically about using the telescope as a photographic lens.
You asked: "Is that because telescopes with bigger F-numbers make bigger images on the focal plane?" My respone was, "No." As you said here above, your concern is with the image distance from the lens or mirror.

I liked your drawing above. I downloaded it. Let me see if I can put some numbers in there.

[turboscrew]

Magnification only depends on the focal lengths of objective and eyepiece. It has nothing to do with aperture, and thus very little to really do with F-number. But with the main mirror/lens alone, maybe the image distance from the lens/mirror causes the image to be bigger? In my head, these things are fuzzy.

Right. We agree on that. My point was that the aperture is the denominator and the focal length is the numerator and that ratio is the f/ number. That f/ number leads to some telescopes being labeled "fast" or "slow" by analogy to photography with film. Personally, I think that it is a misnomer when applied to telescopes. Your concern was specifically about using the telescope as a photographic lens.
You asked: "Is that because telescopes with bigger F-numbers make bigger images on the focal plane?" My respone was, "No." As you said here above, your concern is with the image distance from the lens or mirror.

I liked your drawing above. I downloaded it. Let me see if I can put some numbers in there.

I'm actually not searching for a solution to any problem. I just want to know and understand. That knowledge I can then apply to all kinds of problems.

And in my little drawing, I tried to picture what happens with fast and slow telescope with same target and same aperture. It looks like due to longer focal length, the focal plane gets further and thus forms a bigger image.

[Star Dad]

I would not say it is a "bigger" image. On the same target, same aperture scope, you'll have a brighter image and more resolution of detail with a faster scope (low f number). I have only used a focal reducer a couple of times (0.5x) and I can attest that in the same scope the image taken of the same duration with and without it was very much brighter using the reducer, but the FOV was indeed much wider (ostensible twice as large). BUT the image on the focal plane of the camera is the same - that is in my case (roughly) 3K x 4K pixels in size. That doesn't change obviously. In my case I have determined that my normal f4.9, 1000mm fl, 203mm aperture, and camera can capture a maximum of 1 degree by .75 degree swath of sky. I can double that by using a 0.5x focal reducer. These I can actually plate solve. But if I use a 2x barlow I have yet to plate solve anything I've tried to image. It may be the narrower field of view or the dimmer image or both, I don't know. I have decided to plate solve first and then put the barlow in for imaging, once I have the scope tracking well on the guiding camera. That said, I have yet to actually be able to accomplish this - as the weather gods have been unhelpful. I tried using my 2x barlow on the Crab Nebula (M-1) without success on my last outing, and resorted to just imaging without the barlow.

I know it's kind of counter-intuitive this thing about the same size mirror collecting the same amount of light but different f ratios giving wildly different results - but I've come to accept it as actual experience has shown it to be true. Perhaps your concept of "It looks like due to longer focal length, the focal plane gets further and thus forms a bigger image." is true. I noticed that I had to crank my focuser all the way in to use the reducer and all the way out to use the barlow. (at least that is what I recall - I went through so many iterations of trying to achieve focus I may be confused). Not sure when I'll get my next chance to use the barlow - only the weather gods know for sure.

[turboscrew]

The weather gods have been pissed, obviously, here too. There hasn't been a few hours goodish weather window for about two months. No frigging chance to experiment with anything.

About the fast/slow scopes, I just wonder: The same amount of light "land" on two lenses/mirrors with equal aperture. One generates a brighter image than the other. So where did that light go with the other? Whatever the image, the amount of light hitting the lenses are the same. Something different must happen inside the tubes. Also longer focal length lenses tend to be thinner.

Hmm, a new thing to wonder: how does the TFOV form? How does the TFOV get restricted with longer focal length?

[mikemarotta]

... On the same target, same aperture scope, you'll have a brighter image and more resolution of detail with a faster scope (low f number). I have only used a focal reducer a couple of times (0.5x) and I can attest that in the same scope the image taken of the same duration with and without it was very much brighter using the reducer, but the FOV was indeed much wider (ostensible twice as large).
I know it's kind of counter-intuitive this thing about the same size mirror collecting the same amount of light but different f ratios giving wildly different results ...

Thanks, that helps a lot with a different problem entirely and I am now thinking of adding a focal reducer to my box of oculars. Also, I think that I grasp the intuitive perception. But there are so many other topics that I have to move my lips when I read. (:-)

For your tenacity with a problem you clearly demonstrate "telescope sisu."

Clear skies!
Mike M.

[JayTee]

Let me try to explain this the way it makes sense to me.

Let's assume we are talking about two telescopes of equal aperture, let's call them an f/5 system and an f/10 system. The aperture of the telescope determines how many photons we are going to collect at any given moment. In a low f number system (with a shorter focal length) we get this set quantity of photons illuminating a smaller area which to our human eye makes it appear brighter. With the longer focal length system that same set number of photons is spread out to make the bigger image and to our eye this appears to be dimmer. That is how I envision the term fast and slow.

Cheers,
JT

[turboscrew]

Let me try to explain this the way it makes sense to me.

Let's assume we are talking about two telescopes of equal aperture, let's call them an f/5 system and an f/10 system. The aperture of the telescope determines how many photons we are going to collect at any given moment. In a low f number system (with a shorter focal length) we get this set quantity of photons illuminating a smaller area which to our human eye makes it appear brighter. With the longer focal length system that same set number of photons is spread out to make the bigger image and to our eye this appears to be dimmer. That is how I envision the term fast and slow.

Cheers,
JT

And that's exactly what I suspected.