Nope, I think you will find that a big dob is like a KEG of 1979 Penfolds Grange Hermitage . It makes me really very happy indeed!
(I love this debate )
So many of the opinions on refractor vs reflector seem to be based in the past, and on the unfair comparisons of finely crafted megabuck refractors costing thousands of dollars, versus newtonians built cheaply.
Refractorphiles seem willing to spend a mint to get the very best, but in the main, newtonians are expected to be cheap by comparison. When a newt is made with care and precision its another story. I would back Dave47Tuc's Takahashi M160 newt against any refractor costing up to 4x as much.
You cant buy a tak newt anymore, and I'm not aware of anyone at all producing premium newtonians in smaller sizes unless they are short astrographs.
So many of the opinions on refractor vs reflector seem to be based in the past, and on the unfair comparisons of finely crafted megabuck refractors costing thousands of dollars, versus newtonians built cheaply.
Refractorphiles seem willing to spend a mint to get the very best, but in the main, newtonians are expected to be cheap by comparison. When a newt is made with care and precision its another story. I would back Dave47Tuc's Takahashi M160 newt against any refractor costing up to 4x as much.
You cant buy a tak newt anymore, and I'm not aware of anyone at all producing premium newtonians in smaller sizes unless they are short astrographs.
Parallax (USA) , Parks (USA) , Vixen (Jap) and Orion Optical (UK) do high quality newtonians in the 6"-16" range. I am sure there are others in Japan, and EU who also do high quality newts but their names escape me at the moment.
Forget about Meade, Orion TS, Konus, Celestron and most others for high quality in newtonians.
Indeed, a Tak M180 is perhaps the finest reflector that I've ever seen... Custom built 10~12" newtonians can also shake most cases of refractorholism...
Forget about Meade, Orion TS, Konus, Celestron and most others for high quality in newtonians.
The sad thing is most people that express an opinion on the lack of contrast in a newtonian, compared to a refractor, have only ever used these lower end newtonians, which are poor examples on which to form an opinion on the full capabilities of the newtonian design. Out of the box the standard of baffling on these scopes is pretty horendous. Further, whilst the mirrors generally have a good figure and correction, they don't have the same surface smoothness as a premium newtonian mirror and accordingly they lose a good bit in contrast.
Comparing one of these newts to a premium newtonian is a bit like comparing a keg of beer to a keg of 1979 Penfolds Grange Hermitage
Indeed, a Tak M180 is perhaps the finest reflector that I've ever seen... Custom built 10~12" newtonians can also shake most cases of refractorholism...
I am currently in the process of building a 10"/F5.3 truss newtonian with a Mark Suchting mirror. The scope will have a premium everything, including a Protostar quartz secondary, a Feathertouch dual speed focuser, Argo Navis and Servocat jnr. When I am finished building it, I will happily let anyone throw their 6" refractor of any manufacture up against it. In actual fact, I haven't yet seen a 6" refractor that gets close to my present 10" tubed newtonian. Which has a very good mirror, but most importantly, has been carefully baffled. FWIW I have used 6" APOS from Takahashi, Astrophysics and TMB.
That all having been said, at some point in the future I am likely to try my hand at "digital" astrophotography. My previous attempt at astrophotography (prior to the introduction of running water and electricity) ended up with countless wasted rolls of Kodak Ektachrome and a very big bunch of frustration. For my introduction to digital astrophotography I will be purchasing a premium 4" or 5" Takahashi APO. Will I put an eyepiece in it? Not very often
to have a scope, especially a high end imaging scope and not use it for visual work also, seems to be a waste of a lot of it's potential especially considering the money they cost. maybe just looking at things is a bit old hat nowadays.
I am currently in the process of building a 10"/F5.3 truss newtonian with a Mark Suchting mirror. The scope will have a premium everything, including a Protostar quartz secondary, a Feathertouch dual speed focuser, Argo Navis and Servocat jnr. When I am finished building it, I will happily let anyone throw their 6" refractor of any manufacture up against it. In actual fact, I haven't yet seen a 6" refractor that gets close to my present 10" tubed newtonian. Which has a very good mirror, but most importantly, has been carefully baffled. FWIW I have used 6" APOS from Takahashi, Astrophysics and TMB.
That all having been said, at some point in the future I am likely to try my hand at "digital" astrophotography. My previous attempt at astrophotography (prior to the introduction of running water and electricity) ended up with countless wasted rolls of Kodak Ektachrome and a very big bunch of frustration. For my introduction to digital astrophotography I will be purchasing a premium 4" or 5" Takahashi APO. Will I put an eyepiece in it? Not very often
Cheers,
John B
Do you think the truss rather than a baffled tube will loose a fair bit of the contrast you would have otherwise ?
That's the reason why I've not opted at this stage in my 10" f4.66 newt OTA rebuild not to go to a truss. Even though the potential to save on OTA mass is attractive , but not as important with my new GEM , the Atlux will hand the 10" OTA handily I think with capacity to spare no matter how I load up the 10".
I was bit shocked when I took the mirror out of the storage / shipping box last night and examined it with a magnifying glass and saw what looked like a lot of minute pin holes all over the surface .... never noticed them before and they are too small to see without a magnifying glass.
I don't think they have had any effect on the images I was able to see before I demolished the old OTA it was in. I am however wondering now if I should ship the mirror off for a recoat - I'd like one of those superduper Ti02 overcoated dialectric ultra high reflectivity coatings (would be like adding an inch diam to the mirror) .... what do you think ?
Last edited by Ian Robinson; 17-08-2008 at 04:17 PM.
Realistically for most people, that's the reason why we have several different scopes. The big reflector (cheaper per aperture) when we need that extra aperture and the nice Apo when we want that supreme unobstructed image quality including contrast, crispness & fast cool-down time. Appreciating a fine Apo probably involves more subjectivity, just like enjoying that nice Grange
Do you think the truss rather than a baffled tube will loose a fair bit of the contrast you would have otherwise ?
Nope, a premium truss dob has excellent contrast. The only reason tubed newtonians need baffling is because they have a tube there for the light to bounce around off.
The truss design with a good fabric light shroud has nothing there to facilitate internal reflections affecting contrast.
Another plus for the truss design is that you can use a smaller secondary mirror size than is possible with the tube design. With the tendency to go to faster and faster newtonians, this aids contrast as it is easier to get the secondary obstruction under 20% with the truss design. The MTF curves on a scope with a <20% obstruction are essentially the same as for an unobstructed scope.
I want that telescope that Angelina Jolie was using in Tomb Raider. Was that a refractor? (Sorry, I wasn't paying much attention to the equipment at the time.)
And here are more plusses for trusses:
- it can be easier to cool your mirror to ambient temp if it's in an open cell
- they are lighter & more portable. This is not a trivial point. I believe that light & portable scopes get used more.
Nope, a premium truss dob has excellent contrast. The only reason tubed newtonians need baffling is because they have a tube there for the light to bounce around off.
The truss design with a good fabric light shroud has nothing there to facilitate internal reflections affecting contrast.
Another plus for the truss design is that you can use a smaller secondary mirror size than is possible with the tube design. With the tendency to go to faster and faster newtonians, this aids contrast as it is easier to get the secondary obstruction under 20% with the truss design. The MTF curves on a scope with a <20% obstruction are essentially the same as for an unobstructed scope.
Cheers,
John B
Mmmm .... maybe I should consider ditching the PVC tube I was planning on using in my 10" newt V2, and design myself a surrier trust OTA instead. I'll sleep on it.
Another plus for the truss design is that you can use a smaller secondary mirror size than is possible with the tube design.
Sorry if this is obvious but - why?
I imagine the secondary size is determined by the aperture, f-ratio, tube diameter, focusser height and travel and whether you are optimising for visual or imaging.
I imagine the secondary size is determined by the aperture, f-ratio, tube diameter, focusser height and travel and whether you are optimising for visual or imaging.
cheers,
Dave
I can't see why either .... if the secondary is too small taking into account the above , then not all the fov will be properly illuminated as well (may not be important in a dob used visually only, but full fov illumination is desireable if you want to do prime focus imaging and use the full fov).
Think I'll stick with the PVC tube based rebuild - I have already bought my new tube clamps and and a new spider and can't be bothered redesigning the OTA at this stage , maybe V3 will become a truss tube in 3 or 4 years.
My design has a 2.5" diagonal , means 6.5% area based obstuction or 25% diametrical obstruction. Field illumination for my current design is better than 90% out to 25mm from the centre of the fov. A bigger diagonal (a 3" would have given me better field illumination but at the cost of high obstruction - plus I would have had to buy a new diagonal and a new diagonal holder too.)
Last edited by Ian Robinson; 19-08-2008 at 12:35 PM.
One reason I'm aware of is that tubes need to be oversized for thermal reasons, so that currents along the edge of the tube are out of the optical path. This has the unintented consequence of moving the focuser further away from the secondary. For example, a 10" dob needs a 12" tube. With a truss you don't have this thermal problem, so the UTA can be just slightly wider than the mirror (Newt and Kriege & Berry both have sizing calcs to let you calculate how wide it needs to be).
On the general topic of mirror cooling rates in a tube vs truss, Houdart's calculator lets you model it:
One reason I'm aware of is that tubes need to be oversized for thermal reasons, so that currents along the edge of the tube are out of the optical path. This has the unintented consequence of moving the focuser further away from the secondary. For example, a 10" dob needs a 12" tube. With a truss you don't have this thermal problem, so the UTA can be just slightly wider than the mirror
Phil is 100% correct.
Because of the thermal issues the tube needs to be a larger diameter for a given aperture than is needed for the diameter of the UCA for the truss design. This means the focuser is closer to the secondary with the truss design, allowing a smaller secondary to be used.
Using the 10" scope as an example to allow proper air flow and alleviate tube currents the tube needs to be at least 12" in diameter. This is the tube size used on most factory made 10" tube scopes. With a 10" truss dob the UCA can be 10.75" in diameter. This may not sound a lot but it allows you to come down one size in secondary mirror and this becomes important in getting the central obstruction under 20%, which is the cutoff in essentially unaffected views. It becomes more critical as the F-ratio of the scope gets faster, which is the modern trend. Using a 10"/F5 newtonian as the example, it is exceedingly difficult to design the scope and get away with a secondary smaller than 2.14" for a 21.4% obstruction. A 10"/F5 truss scope can easily be designed with a 1.83" secondary for an 18.3% obstruction. This is only an issue for visual observing but it is what is needed to get high quality high contrast views from a newtonian. FWIW the Chinese/Taiwanese 10"/F5 dobs have about a 25% central obstruction, which will reduce contrast compared to a premium truss dob with a secondary obstruction under 20%. For photography it doesn't matter, nor does the thickness of the spider vanes.
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How much, wow thats a lot cheaper and just as good
