John
I am seeing a 20" dob in your signature, is that correct?
If you are no good at star hopping you could fit out that dob with an Argo Navis system for a lot less $$$ than a go to SCT and you will be seeing galaxies all night long.
Get in touch with Gary if you are not sure about the fittings required, he will surely be able to help.
Cheers
Malcolm
Hey Malcolm, I shall look into that. I must confess to not having explored that idea. Argo Navis = push-to I think from memory, and when I finish this post I'll do a search.
Alternatively a 12 - 16" Skywatcher is not out of reach. There's probably a good argument for me to not go too over the top with expense. (I know I've got a couple of big dobs, but only relatively recent purchases, which were too inexpensive to ignore. With the 20" purchase I was travelling the South Island when it popped up on TradeMe here in NZ. The dob owner lived in an out of the way place in the Southern Alps and needed to move due to health reasons. Cue an urgency to sell and I had the ability to drive to him to get it pretty much there and then. So for less then the price of recoating the 23" dobs mirrors I struck a deal. The 23" also needs a focuser and replacement turntable base on the mount. The 20" was a good buy, I just finished collimating it tonight but my hired help (15 yr old son) to take it outside has gone to sleep! Looks a good viewing night too. I'll likely just move the 23" dob on once I'm happy with the performance of the 20".
Now it's me hijacking my own thread. I apologise to the contributers.
The joys of the internet. At least before the internet you had to buy books and read them and fortunately most authors had some idea what they were on about
I might add that the lower power limit (i.e. upper exit pupil limit) only applies to obstructed optics. Unobstructed, there is no penalty for going as wide TFOV and low power as your eyepieee case permits.
Aha, I see some peer review coming in. I hope there's not going to be a fight in the playground
Malcolm I went and had a look through that website after your note on Argo Navis, it and Servocat looks the thing. I hope to catch up with Dave from Hamilton who has a bit to do with mirror making, perhaps find out with his help how good the mirror is in my scope/s. (Hi Dave if you read this )
Love the comment Colin! By the way, it was easier to me to understandthan your first ��. . Would it be simply a better resolution of the same object then, it would also be brighter to look at? The tract in the book kind of threw me.
I understand how an increase in magnification makes the image dimmer, and how a focal ratio is calculated, how a imaging device builds up the light over time exposure, the very basics etc. Beyond that I'm wading through murky waters, so I thanks guys for the input. I also saw written recently that the way f ratio works in a telescope is different to how it works in a camera. I'll have to look that one up, as I'm pretty sure it's the same principle.? Both are effectively lenses.
Quote:
Originally Posted by AEAJR
Yes! That is the advantage of aperture, gathering more photons which gives you a brighter and potentially more detailed image.
An object at 200X looks about the the same in every scope regardless of FR or aperture.
The eyepiece you would use to achieve 200X would differ from scope to scope based on the Focal length of the scope but the image size would be the same.
Quote:
Originally Posted by ausastronomer
Absolute A grade crapola.
Cheers
John B
John,
You took exception to my post. I see you picked one sentence out of context of the entire post and took it out of the context of the post before it, which I was responding to.
But perhaps I am wrong. I am open to education. Please help me.
A 200X image will be the same size no matter what scope you look through. The brightness and possibly the detail will differ from scope to scope based on aperture but the size will be the same. In other words 200X is 200X
If I was not clear, I was referring to the size of the image or the apparent magnification.
I might add that the lower power limit (i.e. upper exit pupil limit) only applies to obstructed optics. Unobstructed, there is no penalty for going as wide TFOV and low power as your eyepieee case permits.
Quote:
Originally Posted by ausastronomer
Absolute A plus grade crapola
Cheers
John B
Thank you John for your comment, well reasoned as usual. Of course it should read "eyepiece case", not eyepieee case, my mistake. Thanks for pointing it out and apologies for any confusion.
You took exception to my post. I see you picked one sentence out of context of the entire post and took it out of the context of the post before it, which I was responding to.
But perhaps I am wrong. I am open to education. Please help me.
A 200X image will be the same size no matter what scope you look through. The brightness and possibly the detail will differ from scope to scope based on aperture but the size will be the same. In other words 200X is 200X
If I was not clear, I was referring to the size of the image or the apparent magnification.
Do you disagree?
Hi Ed,
The image size isn't "about" the same it's "exactly" the same at equal magnification, irrespective of aperture. But that's the only thing that's the same. To be honest for someone trying to get their head around something they don't understand, I didn't think you post was very clear, particularly when you put that single sentence in an isolated paragraph.
Thank you John for your comment, well reasoned as usual. Of course it should read "eyepiece case", not eyepieee case, my mistake. Thanks for pointing it out and apologies for any confusion.
Your spelling isn't the problem. The premise contains incorrect information.
Obstructed or unobstructed is irrelevant in this case as Steve already tried to point out to you. The links you provided are also irrelevant in the context of obstructed / unobstructed.
Your spelling isn't the problem. The premise contains incorrect information.
Obstructed or unobstructed is irrelevant in this case as Steve already tried to point out to you. The links you provided are also irrelevant in the context of obstructed / unobstructed.
Cheers
John B
You obviously did not read them before making your "crapola posts".
What people think does not matter. The laws of physics matter. In this case basic high school or first year uni physics depending on what country you're from and how much your government invests in science education (geometric optics is the main topic in any case). The problem with going too wide with a large aperture telescope is two fold. I'll try to illustrate by example.
Let's say one goes for 20x magnification with an 8" ~= 200mm (amongst friends) aperture telescope, with an f5 focal ratio (focal length = 1000mm). The eyepiece to use will have an FL of 50mm. The exit pupil will be 10mm, which will not fit into the human eye. What will fit into the average human eye is about 5-6mm. Allowing for little side-to-side head movement, let's say 5mm.
Issue #1. At 20x you will not be able to tell the difference between the views from a 4" scope and an 8" scope based on brightness alone. The views you get at 20x will be about the same as far as brightness goes. So you'd be wasting your money/effort with 8" of aperture if you just want to view the sky at 20x; and you'd be throwing ~75% of the light away (it'd be hitting your iris and not making it to the retina). But it's worse than that:
Issue #2 (more important). If the starlight comes out of your eyepiece as a 10mm wide beam, your eye's pupil (5mm wide) is acting as an aperture stop. The perimeter of your eye's pupil is not as nice near-perfect circle as a telescope lens or mirror, but has natural roughness to it (take a look). You will get diffraction artefacts from the light having to graze past the edge of your pupil. (Reduced contrast, unusual looking flares/spikes.) It'll be as if your telescope lens' or mirror's edge was badly chipped. And you will also get reflection artefacts from the light hitting your iris. So at low magnifications you will get much sharper and more contrasty views from a smaller scope that is better suited to your eye. For the present example: at 20x, a 4" apo will outperform an 8" apo (and probably even a 6" apo in practice).
And re @Finite, as an aside, that's where the practical limit on size of scopes for visual astronomy comes in. For a 1m or 40" aperture scope you need to be at roughly 200x to take full advantage of it. Arguably you could get more out of a 1.2 or 1.5m but it's kind of getting to the limits of what one can do; it's battling against two opposing "forces": 1) human biology that left us with two limited aperture cameras in our heads and 2) the atmosphere which rarely allows for more than 400x magnification without everything looking like a boiling mess.
References
Halliday & Resnick, Fundamentals of Physics 10th ed., Chapt. 34.
Young & Freedman, University Physics 13th ed., Chapt 33.
... still pays to buy books @ausastronomer --- for those who want to dig deeper:
Hecht, Optics.
Great post. Much more useful than the blind sarcasm posted by someone else.
Seeing your reference notes it reminded me that I have my freshman physics text book on the shelf, Halliday & Resnick, Fundamentals of Physics.
I should pull that out and add it to my current reference material. I wonder how much has changed in 45+ years. I imagine the basics are still the same.
Seeing your reference notes it reminded me that I have my freshman physics text book on the shelf, Halliday & Resnick, Fundamentals of Physics.
I should pull that out and add it to my current reference material. I wonder how much has changed in 45+ years. I imagine the basics are still the same.
It's all the same. Basic physics has not changed in 100 years (most of it in much longer than that). You can pretty much do all undergraduate physics with old textbooks. Newer editions have just more colourful illustrations that's all. I actually like the old texts better. Quantum field theory is the first point of departure and that does not feature till you've decided you're going to be a physicist. And even that's 50 y.o.
Btw. you're right it's still Fundamentals and not Principles of Physics. Fundamentals was what I first referenced but my Internet search seemed to show otherwise, so I made an erroneous correction to my post. Thanks to you I've restored it now.
I think it's fair use if I post a relevant excerpt from Halliday & Resnick from one of the latest texts. (Halliday & Resnick have both passed on but their excellent textbook lives on and continues to be referred to as H&R.) I bet you will find the equivalent passage in your old edition. If you do and can take a photo and post it here, I would be ever so grateful.
Thanks Steve, that was useful information for me,- good to know re the visual limit and why. And also the fact that the perimeter of the pupil is flawed. Is that then a fault of the lens in the eye, or a fault of the edge of the pupil, or something else do you know? eg Would that still hold true with an older pupil as the pupil no longer dilates to the same degree as when in ones youth?
(I suppose a relevant comparison to what I'm getting at could be to digital camera sensor size and lenses, - how a aps-c sensor does not utilise (does not 'see') the optical edge of a camera lens, whereas a larger full-frame sensor does, and so the camera with the smaller sensor can get away with using a lesser quality lens yet still produce a pleasing result.)
Just let me know if I'm being too annoying, I won't be offended.
I never thought of the exit pupil exceeding the pupil of the eye as actually causing interference. I realized I would be losing light but never expected it could actually degrade the image.
I use a 38 mm 70 degree AFOV eyepiece in my 203mm dob for 31.5X. If I am doing the math right that would be about a 6.4 exit pupil. I suspect that at my main observation site that that is probably exceeding my pupil size.
I have never noticed any distortion but I do find that as I move my eye around I can see more in the view. Kind of like looking through a window where you take different angles to see peripheral areas. I figured that was the result of the light cone exceeding the pupil.
Thanks for your post Steve. It helps illustrate my point. I'll try to explain.
Re issue 1: Assuming that the exit pupil of the 4" is at least that of the observer's eye, you are totally correct, you would not see a difference between the two scopes for brightness. That's exactly my point. If there was a penalty for allowing the exit pupil to become too large, the 8" should somehow suffer a loss in brightness, either compared to the 4" or compared to itself operating at higher power. It doesn't. Period. My arguments are being called irrelevant on here, when the only thing actually irrelevant is the light you are "throwing away". If one was to follow that logic, you'd also be "throwing resolution away" every time you use your scope below its highest practical power (seeing limits notwithstanding). You'd also be wasting all the light hitting the grass next to your scope, because you failed to bring a larger instrument. Of course you're not, it's just silly. As I said, "throwing light away" is a misleading argument, because it suggests you shouldn't be doing it*. That's what I take issue with.
Re issue 2. Yes, and that's why I said earlier it's of "approximately zero", not "zero" consequence. I acknowledge that the iris' edge is less than perfect and may introduce some diffraction artifacts. However, in practice the effect can be happily ignored. For example, when observing the Moon, the eye's pupil will be smaller than fully dilated, so it will encroach on the EP's exit pupil at a higher power than the telescope's "lowest practical power". If one was to follow your argument, full- disk, low power views of the Moon should be of mediocre quality at best in anything but small telescopes. They are not. They are actually quite good. Secondly, the iris stops down the light beam almost all of the time anyway. It's called life. If one was to follow your argument, people should be seeing said artefacts, and finding them obtrusive, whenever their pupils are the limiting factor to the amount of light their eyes receive, i.e. just about always. Yes, I do see some spikes on bright point like sources, but the are hardly obtrusive. Reduced contrast by using your eyes naturally? Gimme a break! For the avoidance of doubt, I'm not saying these artifacts don't exist, but I am saying that they are hardly a reason not to use as low a power as your eyepiece case permits. Importantly, we are talking about the effects of exceeding the maximum recommended exit pupil size here, not the difference between the optimum (2mm perhaps?) and say 10mm. Yes, that difference would be more than minor, especially for contrast and sharpness.
*The caveat to this, I think we all know and agree on, and it's what you appear to be wanting to avoid: Obstructed optics do have a low power limit. The effects introduced by the secondary absolutely dwarf anything I've seen myself having to discuss with you since my first post in this thread. I also note with interest that it has taken until now for anyone to produce the pupil-edge-diffraction argument. Not what I'd expect to see if it was the main, or even a significant, argument, sorry.
It's interesting how this discussion has evolved, considering my original argument. We've heard of digestion artifacts, been back to school, we've even gone travelling. However with the above in mind, I stand by every single word. Again, apologies to the OP for any unwanted diversion.
Further reading: Nagler, A. "Choosing Your Telescope's Magnification." Sky & Telescope (May 1991).
Thanks Steve, that was useful information for me,- good to know re the visual limit and why. And also the fact that the perimeter of the pupil is flawed. Is that then a fault of the lens in the eye, or a fault of the edge of the pupil, or something else do you know? eg Would that still hold true with an older pupil as the pupil no longer dilates to the same degree as when in ones youth?
I get diffraction and reflection artefacts from my own eyes if I view bright objects at excessively low powers. The most likely culprit for the diffraction flares/spikes is the somewhat rough edge of the pupil. But maybe some people have more astronomy friendly pupils than others.
If the lens in the eye is not near perfect, its performance degrades also with increasing exit pupil; e.g., astigmatism is much more of an issue at lower powers. OTOH, too small an exit pupil also shows defects in the eye like the "floaters" which start becoming annoying somewhere in the range of 0.5 to 0.9 mm.
Using a large exit pupil however will most likely not unravel the very fabric of the space-time continuum and destroy the entire universe. The destruction may in fact be localized merely to our own galaxy. It's a risk I'm sometimes willing to take with a finder EP and so far I've gotten away with it (35mm in an f/4.7). But if I wanted to push it much further, and/or make the most of wide TFOV observing, a smaller scope or binoculars would be preferable. I sometimes wish I had not sold my ED80. At 15-20x it would be great widefield to go between hand-held binos and the 10" Dob. I could never get the hang of larger binos (70-100mm).
I shall look into that. I must confess to not having explored that idea. Argo Navis = push-to I think from memory, and when I finish this post I'll do a search.
)
@ausastronomer --- for those who want to dig deeper:
It's a risk I'm sometimes willing to take with a finder EP and so far I've gotten away with it (35mm in an f/4.7). But if I wanted to push it much further, and/or make the most of wide TFOV observing, a smaller scope or binoculars would be preferable. I sometimes wish I had not sold my ED80. At 15-20x it would be great widefield to go between hand-held binos and the 10" Dob. I could never get the hang of larger binos (70-100mm).
