Question regarding focal lengths. - Page 2

mswhin63 (Malcolm) · #21 26-05-2009, 12:29 AM

Thanks for that info, I was part of a info night at ASWA with a talk on Square Kilometre Radio Telescope.

At the beginning of the talk it was mentioned that if you get light to hit 2 area of a mirror equally you would still receive an image. Hence the design of the sq km radio telescope placing dot of radar dishes over a sq km to give an effective narrow angle or a bigger scope effectively.

I believe this principle is being used with World Biggest telescope where 4 large scope and a few others combine in accurate alignment could provide a much larger effective scope. (still learning)

So it was just curiosity whether 4 very accurate hole in a lid could provide a similar focal length effect. I wasn't important though I would consider a single off axis hole of 4 to 8" on my DOB. Be a cheap on destructive experiment. I tried the single 2" holes but found the magnitude level severely degraded.

M54 (Molly) · #22 26-05-2009, 01:35 PM

Quote:

Originally Posted by Alchemy

400-750mm short
750-1250mm medium
1250-2000 plus long

f5 and below fast
f5 to f8 midrange
f8 to f15 plus slow

Quote:

There is still some confusion over the terms fast and slow as against short and long. I prefer Troy's use of the terms fast and slow with focal ratios. An f/5 scope with large aperture can have a longer focal length than a f/10 scope of small aperture.
Focal lengths can be shorter or longer, but the absence of a defined scale makes this is an even more contentious area. Just where does a short focal length range end ... at 750mm or 900mm or ? And where does a long focal length start?... the 200-inch (5.1m, f/3.3) Hale Telescope on Mt Palomar has a 16.8m focal length.

Regards, Rob

Am I beginning to suspect that whether a focal length is short or long is entirely dependent on the relationship to its aperture?
So the 5.1M Hale Telescope would have a short focal length when related to its mirror?

Robh (Rob) · #23 26-05-2009, 03:48 PM

Quote:

Originally Posted by M54

Am I beginning to suspect that whether a focal length is short or long is entirely dependent on the relationship to its aperture?
So the 5.1M Hale Telescope would have a short focal length when related to its mirror?

Molly,
Great insight. And it's the terminology that's causing the confusion.
A relatively short focal length in comparison to the aperture size is often referred to as a short focal ratio. This is where the confusion comes in. As, for example, the Hale Telescope has a short focal ratio but a long focal length. That's why I like Troy's point of using fast for a short focal ratio to save confusion with focal length. A relatively long focal length in comparison to the mirror size is often referred to as a long focal ratio but the term slow would again save confusion with long focal length. For example, an 80mm aperture, f/10 telescope only has an 800mm focal length.

Regards, Rob

Robh (Rob) · #24 26-05-2009, 04:07 PM

Quote:

Originally Posted by Wavytone

You you could cut a 4" hole in the lid, off-axis, and turn it into a nice 4" f/15 scope (same focal length as the 12" was). Would be even better than a refractor as its achromatic.

I'm assuming the 4" off-axis "aperture" would require a different telescope collimation. Yes or no?
If so, how would you collimate the scope with respect to the new off-axis "aperture"?

Rob.

mswhin63 (Malcolm) · #25 26-05-2009, 11:26 PM

Not totally sure but from opinions of other I have spoken to re-colimination is not needed. The light does not spread when is enters the 4" hole and it reflects of only the portion of the mirror in line with the hole.
As the mirror is curved within collimination it will arrive at the secondary mirror at the right point.
The only real affect is the amount of light collected.

Wavytone · #26 26-05-2009, 11:47 PM

No recollimation.

With four holes, when its out of focus you'll see four sets of out-of-focus images, these will all converge to a single point as you move the eyepiece through focus.

Try it some time, if you haven't.

BTW there was a point with sub-diameter aperture masks if you are looking at close doubles - sometimes the secondary star is invisible if its buried in the second or third bright airy rings around the primary. By reducing your scopes aperture the right amount, you change the spacing of the rings (wider) putting the secondary in the dark space between the rings.

Robh (Rob) · #27 27-05-2009, 12:06 AM

Quote:

Originally Posted by mswhin63

Not totally sure but from opinions of other I have spoken to re-colimination is not needed. The light does not spread when is enters the 4" hole and it reflects of only the portion of the mirror in line with the hole.
As the mirror is curved within collimination it will arrive at the secondary mirror at the right point.
The only real affect is the amount of light collected.

I have no problem at the secondary but what happens at the eyepiece?
A reflection at the primary mirror centre will end up at the eyepiece centre. A reflection near the primary mirror edge will end up towards the eyepiece edge. So, if you have a 4" circle of reflection away from the primary mirror centre, won't this end up as a smaller field away from the eyepiece centre? Which means the scope is off-axis alignment.
Rob.

mswhin63 (Malcolm) · #28 27-05-2009, 01:00 AM

Good point, the cap on the main cover I use for bright moon viewing when the moon is really bright (save time fitting a filter), but I still seem to get a good picture. Never really took much notice but maybe the Field or View is reduced.

I was using my elcheapo scope when I tried that BTW

Robh (Rob) · #29 27-05-2009, 02:12 PM

Quote:

Originally Posted by mswhin63

Good point, the cap on the main cover I use for bright moon viewing when the moon is really bright (save time fitting a filter), but I still seem to get a good picture. Never really took much notice but maybe the Field or View is reduced.

I was using my elcheapo scope when I tried that BTW

I'm still intrigued by this idea ... reducing the 12" aperture size to say 4" and increasing the f ratio.
I did some more thinking on this. As the aperture focal length is still the same, the true FOV for whatever eyepiece you're using must still be the same. However, this same field of view will be appear in a smaller exit pupil of lower brightness (as exit pupil depends on aperture size).
As the aperture hole is 4" and off-centre with respect to the primary mirror, the reduced exit pupil will, I think, appear off-centre in the eyepiece. You probably won't notice this as your eye will shift slightly from the eyepiece centre to the centre of the exit pupil circle. This probably won't cause too many optical nasties providing its not too far off the eyepiece centre. I'm not sure how you could re-collimate the exit pupil to the eyepiece centre (star testing?).

Regards, Rob

Wavytone · #30 27-05-2009, 07:50 PM

Using a small aperture off axis the scope effectively becomes an unobstructed Herschelian of the same focal length, and reduced aperture.

There's no real gain by doing this unless you really want a light loss, eg to observe the moon which is otherwise painfully bright in a 12".

Robh - you're right re the reduced exit pupil appearing off-centre in the eyepiece, it does, having done this with a 60 mm aperture on my C8. Not that it was noticeable as in my case it becomes 60mm f/30.

Robh (Rob) · #31 27-05-2009, 11:52 PM

Quote:

Originally Posted by Wavytone

Robh - you're right re the reduced exit pupil appearing off-centre in the eyepiece, it does, having done this with a 60 mm aperture on my C8. Not that it was noticeable as in my case it becomes 60mm f/30.

Thanks for the confirmation. I was pretty curious about the effect.

Regards, Rob