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The term "Prime Focus"

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#1 Alex McConahay

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Posted 26 January 2013 - 10:59 AM

Hi all,

Every now and then I see a thread about "prime focus" imaging. I thought it might be good to point out that what imagers do at "Prime Focus" is not actually done at "Prime Focus" of the telescope. Now, I realize that this is just a little quibble, but imagers have their own special meaning for this term that is not shared by the larger world of telescope designers and makers, and astronomy in general.

Rarely do imagers use newts or sct's at "Prime focus." In the strict sense of the word, the prime focus of any telescope is where the rays of light from the main objective come to focus. In a newt, this is directly in front of the primary mirror--someplace further out than where we put the secondary. In a Cassegrain it is also someplace in FRONT of the primary mirror---not behind it, where we usually put an eyepiece or camera to collect the light bouncing off a secondary. And in a refractor, it is right at the end of the tube before we put in a diagonal or any other optic. Of course, putting your camera or eyepiece in this position in a newt or SCT causes problems.....your head (or camera) is in the light path!!!!! So, newts have a secondary mirror that reflects the light out the side. Cassegrains (like SCT's) have a secondary that reflects the light back and down through a hole in the primary. When you put a sensor there to catch the light off the secondary, you are no longer at "prime focus." (At least to the Amateur telescope maker or the fanciest of telescope designers. You are at "prime focus" only for the imager---who has taken the term over from the original meaning.) Many of the large telescopes in the world have several different "Focus" points---- Newtonian, Cassegrain, Nasmyth, Coude and other places where they can put a camera and collect the light. The important thing about "prime focus" is that there is no optic, including secondary mirrors, besides the primary objective (lens or mirror) affecting the light.

Now, before we start a long thread on the meaning of "prime focus." I must say-----Imagers have their own meaning for the term. They simply mean there is no eyepiece, Barlow, focal reducer, field flattener, para-corr, or other optic in the way. Some go even further and use the term even when there is another optic in the way (such as an internal field flattener or other optic as you find in some astrographs, or a Televue NP 101, etc.....). For these, as long as the camera is just sitting at the end of the focuser tube it is called "Prime Focus."

I'm not trying to start a long disputation about the term "Prime Focus." Words are what they become to mean by usage. And in imaging circles the phrase has been misused so many times it has become a standard (like "Kleenex" for the word Tissue, or "Google" for the term "search out on the internet)." It is just that it is a cloudy, rainy morning, and I have not had my scope out for two nights, and I was bored, and ......

Alex

#2 zerro1

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Posted 26 January 2013 - 11:50 AM

As in "schmitd camera"??

#3 austin.grant

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Posted 26 January 2013 - 12:45 PM

I'm not trying to start a long disputation about the term "Prime Focus."


The best way I know of to not start "long disputations" is to not create a post on the topic at Cloudy Nights! ;)

I agree that the SCT/RC and similar scopes don't exactly meet the prime focus definiton, as they have multiple optics that are each altering the focal point. However, in a standard Newtonian, the flat secondary mirror simply redirects the light. The camera is still at the true Prime Focus point of the primary mirror.



#4 Alex McConahay

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Posted 26 January 2013 - 03:55 PM

Yes, a Schmidt Camera has a sensor (usually slightly bent film) at the Prime Focus.

It also has a corrector plate BEFORE the Prime Objective, a mirror in this case. The definition of "Prime Focus" relates to the Primary, or Prime Objective.

Alex

#5 John Carruthers

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Posted 27 January 2013 - 05:19 AM

True enough, most imaging is done at Newtonian, Cassegrain, or prime for a refractor, focus.

#6 Footbag

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Posted 27 January 2013 - 10:32 AM

As someone who has been misusing the term, I wonder what the practical aspect of the true definition is. Is it the point in which you would test a telescopes optics? A secondary would flip the image, so I could understand if that became the distinction. With a newt, the focus point is the same distance, its just picked off.

For those of us misusing the term, it is a very handy way to say no FR, barlow, etc... Or operating the telescope at it's designated f ratio.

For an SCT, is prime focus at f2? Thats where the primary comes to focus, correct?

#7 Alex McConahay

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Posted 27 January 2013 - 11:13 AM

>>>>>>I wonder what the practical aspect of the true definition is.

The term as we (mis)use it is quite useful to us imagers as long as we are with imagers only. You just have to realize that when you are talking to the larger world of astronomers, that you are misusing the term.

>>>>> Is it the point in which you would test a telescopes optics?

Yes, one can do some mirror tests at "Prime Focus." (Usually, though, amateur mirror makers test at the "radius of cuvature" which is twice the focal length. But that is not its main use. In fact, the Prime Focus position is used relatively rarely in reflecting telescopes unless they are very large. This is because the sensor (Camera or eyeball in a human head) robs the lght path of a significant amount of light in a small telescope. In fact, in very large telescopes, astronomers used to sit in a cage at the "prime focus" with an eyepiece. But that has been replaced by cameras largely. And larger cameras fit and balance more easily at the "Cassegrain" focus at the bottom of the tube.

>>>>> A secondary would flip the image, so I could understand if that became the distinction. With a newt, the focus point is the same distance, its just picked off.

True, but a secondary, also robs some small percentage of the light and probably has some small wave front error.

>>>>>For those of us misusing the term, it is a very handy way to say no FR, barlow, etc... Or operating the telescope at it's designated f ratio.

Yes, and this is why I say it is still quite useful to us imagers.

>>>>>For an SCT, is prime focus at f2? Thats where the primary comes to focus, correct?

It varies from scope to scope, but yes, generally. Usually this is just a few inches in front of the corrector plate. And a newt's is just a few inches beyond where the secondary goes. On an SCT this is where the Hyperstar is located (although the Hyperstar has correcting lenses so, it cannot be considered Prime Focus imaging).

I wish I could come up with a convenient word or phrase to attach to "imaging without additional optics past the basic telescope construction".....but I cannot. And besides, words are what we use them to mean.

But if you are ever touring Mt. Wilson, or Palomar, and they point out the "Prime Focus" cage on the big telescope, don't look to the side (or back) of the tube....It is up in front of the mirror, smack in the middle of the tube.

Alex

#8 Jeff2011

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Posted 28 January 2013 - 11:22 AM

Thanks Alex you just ruined my day :grin:. Just kidding. Thanks for enlightening us. I just modified my Dob last month for "Prime Focus" AP by moving my primary mirror up to extend the focal point. I guess I can't call it that anymore. What would be a more appropriate term? Perhaps Secondary Focus?

#9 Alex McConahay

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Posted 28 January 2013 - 01:19 PM

>>>>> What would be a more appropriate term? Perhaps Secondary Focus?

It is actually called the "newtonian" focus by those who care about these things. (As I said earlier, there is also a Coude Focus, Cassegrain Focus, and others.)

But, as I said in another post----I wish I could think of a good term for "At the newtonian focus without barlow, paracorr, flattener, .....or other corrector in the path!!!!"

I don't expect the world of astroimagers to start using the term in its original sense. I was just thinking it might be nice to point out that we have taken over a term that already has another meaning in the larger world.

Alex

#10 michael hester

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Posted 28 January 2013 - 01:40 PM

By the purest definition of the term only a singlet refractor will be capable of "Prime Focus". This means nobody images at prime focus as most astronomical refractors are doublets (2 lenses). I don't use the term often but if I did I would use it to refer to attaching the camera directly to the scope's rear thread cell or focuser.

#11 Dave Mitsky

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Posted 28 January 2013 - 01:56 PM

Wouldn't the term be appropriate for a Herschelian reflector?

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#12 Jeff2011

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Posted 28 January 2013 - 02:03 PM

Not that I would want to do this, but I suppose that one could achieve true prime focus in a newtonian by removing the secondary and spider vanes, exending the tube and placing a minature camera in the tube extension further up from were the secondary was. That would render the scope useless for visual observing. I wonder if one of the ATM guys have done this? I guess there would be no real benefit to doing this.

#13 Alex McConahay

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Posted 28 January 2013 - 03:11 PM

>>>>>I suppose that one could achieve true prime focus in a newtonian by removing the secondary

Sure, but then it would no longer be a "Newtonian," which is defined as a reflector where the image is bounced out the side by a secondary placed in just the right position to do so.

It would just be a reflecting telescope. But not a Newt!

Alex

#14 Alex McConahay

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Posted 28 January 2013 - 03:15 PM

>>>>Herschelian....

Yeah, I should think so.....since you are standing where the object comes to focus from the primary optic.

But it does not need to be.....you could, theoretically, stick your head in the middle of a large reflector's lightpath. But if you aren't using that secondary, you are not using a newt at that point.

Alex

#15 ccs_hello

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Posted 28 January 2013 - 08:27 PM

Prime focus in my book is:

There is an optical system that focus light into the primary nodal plane where the image is focused. If you place a white paper at that plane (the image plane) and point the optical system at far-away objects during day-light, a reverse image will form and sharply focused.

BTW, if moving the white paper beyond that point, the light starts diverging, which means passed the focal plane.

Note that I only use the term optical system, not limiting to refractive lens or just mirrors, not limited to lens type/build (acho, apo, singlet, doublet, triplet, complex lens system), not limited to mirror design (Newt, Cat, MCT, etc.), not limited lens primary use (astro OTA, camera lens), not limited to add corrective optics (e.g., Petzval lens, focal reducer lens, Barlow lens which all positioned "before" the primary nodal).

Primary focus imaging is instead of putting the white paper and using eyes to watch what's being projected onto it, you put a camera film, an image intensifier tube input (photo-cathode) surface, or image sensor in that image plane.

Hope it helps!

Clear Skies!

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#16 CounterWeight

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Posted 29 January 2013 - 12:56 AM

prime focus = primary (of the optical system, or better put at focal length of) focal plane imaging.






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