refractorholism and refractoritis.

[Ian Robinson]

Be honest.

[AlexN]

Apertureholism has a hold of me... and it seems there is no cure...

Although, due to this ailment I think recently I've become susceptible to refractorholism.... Treatment I understand is horribly expensive :)

[cookie8]

I have mild refractorholism but severe IceinSpaceforumitis:doh:

[gmbfilter]

Buying them is nearly as much fun as using them.

[JimmyH155]

Definitely a refractorholic when the object you are looking at is at a reasonable elevation. When it is overhead (like Joops right now) I come down with Lightbridgeitis.

[Starkler]

Heres one you could buy to satisfy your refractoritis :)

http://www.iceinspace.com.au/forum/showthread.php?t=33716

[Tandum]

They make for great guide scopes, does that count ?

[Dave47tuc]

A cure for this is a good Newtonian, beats them every time.:D

[hector]

Apertureholism has no cure and no matter how bad the case is you have there is always another symptom around the corner. Its the steps you have to make that get longer and harder the worse it gets. It now seems I must get 9 feet of the ground to get relief.

[mick pinner]

:lol:never miss an opportunity Geoff

[jjjnettie]

I've never had enough spare money to fully develop either condition.

[AlexN]

JJJ.... its not about what we have.. its about what we WANT! :)

With infinite funds, I'd have a 100m reflector with a malim-cam (with servo-cat obviously.. I aint pushing that thing ANYWHERE)

Hence. Apertureholism.

[Babalyon 5]

Apertureholism is my suffering too! :eyepop:

Quote "Tandum They make for great guide scopes, does that count ?"
Yeah, they do and I thought that's all they were!!:P

[ausastronomer]

Geoff,

That scope would make a lovely finder for a serious telescope :)

Why don't you just piggyback it on the SDM :thumbsup::thumbsup:

I am with Andrew. I need my feet off the ground to satisfy the photon urge.

Cheers,
John B

[CoombellKid]

Isn't that like hitting your toe with a hammer to stop you thinking about
the ache in your shoulder...etc...etc...

Nah that just changes the disease lol and increases the likely hood of
aperture fever... which is kinda like malaria for me these days it comes
and goes, but I do have the next script about filled :D

regards,CS

[AstroJunk]

Yes, they make cute finder scopes. Trouble is though, you don't get to see many stars through them due to lack of, er, aperture!

[Ian Robinson]

Oh my .... what have I started now ?:help:

[AstroJunk]

It's your own fault - you mentioned the 'A' word in the poll ;)

[toyos]

A big dob is like a keg of beer, a fine Apo is like a glass of 1979 Penfolds Grange Hermitage Magnum. Go for the one that makes you 'happier' :lol:

[AstroJunk]

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 :D)

[Starkler]

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.

[Ian Robinson]

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.

[AlexN]

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...

[ausastronomer]

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 :)

Cheers,
John B

[ausastronomer]

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

[AlexN]

I've put ep's into my imaging scopes, for star alignments and for aligning guide scope with imaging scope :)

[mick pinner]

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.

[Starkler]

Thats a whole 'nother can of worms Mick :whistle:

[Ian Robinson]

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 ?

[toyos]

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 :)

[ausastronomer]

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

[Miaplacidus]

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.)

[Dave47tuc]

Those damn Newtonians, time to fight back:whistle:

What about Astro-Physics Apo's of 130mm and 180mm That will fix em:poke:

Hey who's that Tak MT160 over there;) How much, wow thats a lot cheaper and just as good:D

I really like this debate, its fun:lol:

[PhilW]

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.

[Ian Robinson]

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.

[Ian Robinson]

Now that's what I call terminal refractorholism !!!:D

[AstralTraveller]

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.

cheers,
Dave

[Ian Robinson]

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.)

[PhilW]

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:

http://www.cruxis.com/scope/mirrorcooling.htm

He has a discussion on the front radiation coefficient in a truss vs tube, and his model lets you compare the two scenarios.

[ausastronomer]

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.

Cheers,
John B

[toyos]

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.

[Satchmo]

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.

[AstroJunk]

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.

[TrevorW]

Sorry not big enough 10" TMB maybe

:thumbsup:

[AlexN]

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...

[ausastronomer]

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

[Stephen65]

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.

[AstroJunk]

Hey John, I think you misread what Alex was saying ...

[toyos]

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.

[CoombellKid]

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.

regards,CS

[ausastronomer]

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.

Cheers,
John B

[ausastronomer]

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

[ausastronomer]

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.

Cheers,
John B

[toyos]

For those who are interested in D & G refractors, here is their website:
http://www.dgoptical.com/index.htm

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.

[AstroJunk]

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!

[toyos]

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.

[ausastronomer]

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

http://www.buytelescopes.com/product.asp?t=1&pid=9847&m=

an 8" Takahashi APO costs $200K mounted

http://www.buytelescopes.com/product.asp?t=1&pid=1727&m=

Cheers,
John B

[ausastronomer]

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.

In this thread

http://www.iceinspace.com.au/forum/showthread.php?t=31267

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.

Cheers,
John B

[toyos]

Hi John

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. :D

[Satchmo]

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.

[Stephen65]

I have a 5" APO, a 10" DK and a 20" dob under construction. All of them have their advantages and disadvantages. The views are not comparable but there is something uniquely pleasurable about the high contrast widefield views you get through a good quality APO. When I have it set up at star parties I always get people saying "oooo the stars are so sharp!".

Obviously if I was hunting some specific DSO or trying to make out festoons on Jupiter the 5" is not my choice of instrument, but for appreciating the aesthetic beauty of starfields and larger open clusters its hard to beat it (especially when used with an Ethos).

[NQLD_Newby]

Arpeturitus for me I'm afraid. Must be very contageous, as just last year I was fine.

[Stephen65]

This isn't right. A 10" mirror gathers 2.37 times as much light as a 6.5" lens and even allowing for lower transmission off mirrors and the secondary obstruction the 10" is still going to be considerably superior in light-gathering.

As others have said a lot of comparisons people make refractor v reflector is expensive APO v generic Chinese reflector. If you get an equivalent quality reflector it's going to be a lot fairer comparison.

To give an illustration, my 10" Mewlon has two mirrors and is a pretty decent quality folded reflector. Once it is properly collimated and cooled, and provided the seeing is decent, it will absolutely crush my 5" APO on DSO's and planets. The difference on something like Eta Carina is like night and day, the Mewlon can deliver larger image scale and a much brighter image.

[Starkler]

Actually John to be fair, your challenge should be comparing your 10" newt against an 8" apo. You do however have the right to nominate the place for this showdown.

I cant imagine too many people willing to drag their 8" apo out to the middle of the field at a star party :D

[Wavytone]

A case of mild refractoritis.

I'm aiming to have three catadioptrics and two refractors on the same mount mid next year..

[Ian Robinson]

Don't set it up near any airports , I don't think the cops would be impressed.