I have this image by Mark D. Russell, Ph.D published on Sky & Telescope years ago about the effects of central obstruction. There's an obvious difference in image quality between a scope with 0% obstruction to one with 18% obstruction. I saw similar results based on my own objective observations with various telescopes since I often bought telescopes just for the sake of comparing them myself (sold them again at a loss soon after). In my opinion, central obstruction is like your vehicle's aerodynamics, the lower the better (in most cases); there's no definite cutoff to it.
There's an obvious difference in image quality between a scope with 0% obstruction to one with 18% obstruction.
Theres a subtle difference but its not enough to make one want to shell out for a non portable 10" apo refractor at 10 times the cost of a premium reflector. As a rough rule of thumb the equivelent contrast transfer function will be equalled by a quality reflector of diameter larger than the refracter by the diameter of its secondary.
I can vouch for this rule. Years ago ,we had a stock standard AstroOptics 10" F6 Newt beside a 7" F9 Astrophysics starfire on Jupiter. The view in the 10" Newt simply blew the 7" away as there was little color differentiation in the refractor and at magnifications enough to resolve small features the view had broken down in the 7" and was still lusterous in the 10". It was a poignant moment when I looked around and saw the 7" was layng idle after so much fuss had been made as it was set up. It was a $12K rig and that would have been 15 years ago.
I have this image by Mark D. Russell, Ph.D published on Sky & Telescope years ago about the effects of central obstruction. There's an obvious difference in image quality between a scope with 0% obstruction to one with 18% obstruction. I saw similar results based on my own objective observations with various telescopes since I often bought telescopes just for the sake of comparing them myself (sold them again at a loss soon after). In my opinion, central obstruction is like your vehicle's aerodynamics, the lower the better (in most cases); there's no definite cutoff to it.
That image is a simulation isn't it? The lack of rotation in Jupiter rather gives it away! I'd be interested to know how he got the 0% data - I can only assume he used refractors...
Actually, there is an easy test - make cut out circle of about 25% and adhere it to a bbq skewer. Get a mate to randomly hold it over/away (or just spinning it in situ will work) from your diagonal when you are looking at a subject and try to say when the aperture is in place.
Do this a statistically meaningful number of times and see if you were correct.
The theory is great, but reality is even better. Oh, and only try this in really steady seeing and on a dark night to get good data.
That image is a simulation isn't it? The lack of rotation in Jupiter rather gives it away! I'd be interested to know how he got the 0% data - I can only assume he used refractors...
You can measure 0% obstruction with a newtonian. make a cap for the end of it, with 4 circle cut outs. place it over the open aperture of the newt so that the spider vanes are situated between the circles, and the mirror is in the center... measure the area of the circles, do some math and figure out what the total unobstructed aperture of those 4 holes equals. If memory serves, you need a 16" newtonian to achieve 6" of unobstructed aperture... obviously its more weight/space efficient to have a 6" refractor, but it can be done...
If memory serves, you need a 16" newtonian to achieve 6" of unobstructed aperture... obviously its more weight/space efficient to have a 6" refractor, but it can be done...
That's not remotely even in the ballpark. Mark gave you the correct formula a few posts ago.
"As a rough rule of thumb the equivelent contrast transfer function will be equalled by a quality reflector of diameter larger than the refracter by the diameter of its secondary."
An 8"/F6 newtonian with a 1.52" secondary will perform at a level similar to a 6.5" refractor and that's what all the physics points to.
If, a 16" reflector is properly set up it will outperform a 6" refractor by so far its silly to even discuss them in the same sentence.
Despite its shortcomings in light-gathering I enjoy setting my 5" refractor up at a dark sky site and just panning the Milky Way with a long FL widefield EP. It produces such beautifully sharp views with tiny little stars.
That's not remotely even in the ballpark. Mark gave you the correct formula a few posts ago.
"As a rough rule of thumb the equivelent contrast transfer function will be equalled by a quality reflector of diameter larger than the refracter by the diameter of its secondary."
An 8"/F6 newtonian with a 1.52" secondary will perform at a level similar to a 6.5" refractor and that's what all the physics points to.
If, a 16" reflector is properly set up it will outperform a 6" refractor by so far its silly to even discuss them in the same sentence.
Cheers,
John B
Hey John, I think you misread what Alex was saying ...
Despite its shortcomings in light-gathering I enjoy setting my 5" refractor up at a dark sky site and just panning the Milky Way with a long FL widefield EP. It produces such beautifully sharp views with tiny little stars.
Same here
Everyone has their own preference, each side may say anything using all sorts of reasoning and arguments (whether true or slightly twisted to suit their opinions) to defend it. Brute force vs refinement, I'll take both in this case even though I'm inclined towards the latter.
Once again, keep in mind that fully multi-coated lenses transmit around 10-20% more light than combinations of mirrors (which surfaces deteriorate over time, not to mention that fine dust you decide to ignore since you don't want to clean that delicate surface too frequently). So direct aperture comparisons will not be accurate.
Despite its shortcomings in light-gathering I enjoy setting my 5" refractor up at a dark sky site and just panning the Milky Way with a long FL widefield EP. It produces such beautifully sharp views with tiny little stars.
Still not the same as panning the milky way with a 25" or 30" reflector.
And it would cost alot of pennys to get the quiv' in a refractor. So
cost/aperture v's contrast.... hell go with aperture, contrast is sure to
follow. I'd much rather try and view the bok globs in Carina with a 25"
reflector any day.
Hey John, I think you misread what Alex was saying ...
Maybe I did and if I did I apologise.
Is he talking about the aperture needed to get a 6" unobstructed view through an off axis mask? If so I got totally confused by the comment about "4 circular cutouts". You only need 1 circular cutout to make a 6" off axis mask for a 16" newtonian.
The formula for maximum diameter of an off axis mask is the aperture of the telescope multiplied by .413333. In the case of a 16" newtonian you can have a maximum size unobstructed off axis mask of 167mm or 6.6"
I have a 185mm off axis mask which I use very occasionally on my 18" Obsession. I mainly built it for experimental purposes. It gives excellent views for what it is. Namely, a 7.25"/F11 APO. It doesn't remotely come close to the full aperture 18" telescope.
For those that think I am somewhat biased against refractors, here is a photograph of me actually using a "serious" refractor.
Unlike some of the "toy" telescopes some people are commenting on and using as the basis of analysis, this is a custom designed and built 15"/F12 D&G refractor. It is worth well west of $US200K.
The eyepiece is a 31mm Nagler which gives 150X in that scope.
For those that think I am somewhat biased against refractors, here is a photograph of me actually using a "serious" refractor.
Unlike some of the "toy" telescopes some people are commenting on and using as the basis of analysis, this is a custom designed and built 15"/F12 D&G refractor. It is worth well west of $US200K.
The eyepiece is a 31mm Nagler which gives 150X in that scope.
Cheers,
John B
To give you some indication of the true size of this scope here is a photograph of it in its entirety, just prior to uncovering for the nights observing.
If you don't think it's all that big, consider that the ladder underneath it is a 6' mobile platform. The finderscope on the top is a 6"/F12 refractor. It also has a 5"/F10 refractor finder fitted to the far side which you can't see in this photograph.
For those that think I am somewhat biased against refractors, here is a photograph of me actually using a "serious" refractor.
Unlike some of the "toy" telescopes some people are commenting on and using as the basis of analysis, this is a custom designed and built 15"/F12 D&G refractor. It is worth well west of $US200K.
The eyepiece is a 31mm Nagler which gives 150X in that scope.
I was tempted to buy one of their scopes too just out of curiosity, but put off by the fact they are only achromatic refractors (CA affects contrast & sharpness too) and the tubes are way too long. Are you sure the custom 15" costs USD$200k? Because the 12" objective only costs USD$8.5k and the 8" tube assembly is only $3700.
I have a 185mm off axis mask which I use very occasionally on my 18" Obsession. I mainly built it for experimental purposes. It gives excellent views for what it is. Namely, a 7.25"/F11 APO. It doesn't remotely come close to the full aperture 18" telescope.
Ditto for my 20" - In my experimentation, aperture won every time, in every condition. In bad seeing the gap was smaller, but there was still a noticeable gap!
I put 4.5" off-axis aperture stops on a 12" dob and a 12" SCT (I know a 12" f/10 SCT with a mask is not ideal) and compared them against 4" & 5" Apo's, and to be fair I brought the budget Chinese 127mm Apo too. Even the cheap 127mm Chinese Apo blew both reflectors away by far when used with the 4.5" aperture stops. The reflectors showed much darker backgrounds & tighter stars with the aperture stops, but the images were greatly dimmed too; while the refractors showed brighter & crisper images with dark backgrounds, fitting the definition of contrast better.
As mentioned before, direct aperture comparisons between lenses and mirrors will not be accurate. In general, 6" FMC lenses will transmit significantly more light than 6" mirrors can reflect. In my humble opinion, if the mirror of a 10" f/5 dob is still spotlessly clean and freshly re-surfaced, it should perform more or less on par with a 6"-6.5" Apo in terms of light-gathering ability ONLY, the image quality will not be in the same league. Most of my observing is done from my fairly light-polluted backyard.
Don't get me wrong, I like the big dobs too. I'm even considering getting one of those Obsession dobs (can someone tell me how easy/painful it is to set up an 18"/20" dob by yourself?). Just my 2c.
Are you sure the custom 15" costs USD$200k? Because the 12" objective only costs USD$8.5k and the 8" tube assembly is only $3700.
As you would be aware the cost of a refractor increases logarithmically as the aperture increases. That cost also includes the pier, the mount and the mount foundations. Be rest assured you don't mount a 15 foot long refractor on an aluminium legged tripod and an EQ6 head.
To give you some idea how the cost increases logarithmically a 6" Takahashi APO costs $18K mounted
I put 4.5" off-axis aperture stops on a 12" dob and a 12" SCT (I know a 12" f/10 SCT with a mask is not ideal) and compared them against 4" & 5" Apo's. Even the cheap 127mm Chinese Apo blew both reflectors away by far when used with the 4.5" aperture stops.
What you fail to understand is that you continue to compare refractors against poor quality newtonians, or newtonians that are not properly optimised and or set up. It is possible to make a horrible mess of a newtonian. It is a lot harder to make a horrible mess with a refractor.
You actually asked "what's wrong with my newtonian". I told you what was likely wrong with it. I also told you how to fix it. You didn't want to listen and have now formed an opinion based on a very small low end sample of newtonians. Comparing a GSO dob/newt to a premium newt is like comparing a Lada to a Lexus. You fail to realise you were only driving a Lada and a broken one at that.
I happily concede that a refractor will always have greater contrast when compared to a newtonian, but when compared to a high grade properly optimised newtonian, there isn't much in it. No where near the difference you are claiming. A properly set up high grade 10" newt will in fact smash any 6" APO and by quite a margin on anything, inluding lunar/planetary. A medium quality poorly setup 10" newtonian probably won't equal a 6" APO for lunar/planetary.
A 12" SCT isn't even worth comparing against a good newtonian or a refractor. They are a convenience scope that does a fair job of everything and a good job of nothing. Further, they take about 3 years to cool down properly to deliver 1/2 decent views and they only ever get to 1/2 decent views.
I had already got rid of that dob you mentioned , I've bought 4 different Newtonians, 2 SCT's & 1 Maksutov and even more refractors since then. Sold again at a loss soon after. I admit with Newtonians I only bought the ones that were readily available at the time (easier). What was the process you had to go through to get an Obsession dob?
PS. Lexus is one of my fav daily cars, but the handling isn't very exciting, you'll need another car for fun.
Even the cheap 127mm Chinese Apo blew both reflectors away by far when used with the 4.5" aperture stops. The reflectors showed much darker backgrounds & tighter stars with the aperture stops, but the images were greatly dimmed too; while the refractors showed brighter & crisper images with dark backgrounds, fitting the definition of contrast better.
The images in the refelectors were probably dimmer because you didn't bother to fit suitable eyepieces to make each combination give the *same exit pupil / magnification* in your comparison evaluation. In a quality reflector with enhanced secondary it is unlikely that there would be any significant differnce in light transmission, not anyway that would lead to the images looking `significantly dimmed'.
I used to demonstrate years ago a 70mm stop on my 8" F8 Newt against against my Vixen 70mm Flourite refractor. With both instruments running at 65X ( the 8" scopes lowest mag) the views were essentially the same. It was so demonstrative to remove the stop and show how much more can be seen with decent aperture.
