Coming back again to amateur Astronomy after 15 years on & off, and just purchased a GSO 12" Dobsonian and an imported 152mmx1200 refractor.
I owned several reflectors before but never had a large refractor. I always knew that refractors would provide clearer images with much better contrast, but I didn't expect that the 6" refractor would significantly outperform the 12" Dob (including many deep-sky objects) after comparing them side by side under the moderately light-polluted sky. Omega Centauri stands out crisply against a much darker sky background showing better detail.
I'm starting to wonder if there's probably something wrong with my 12" Dob, but everything seems fine and properly set up.
There is clearly something not right with the 12" reflector.
If both scopes are properly set up under the same conditions the 12" reflector will clearly outperform that 6" achromat on every single target in the sky; and by quite a margin.
I am guessing the 12" reflector is not properly collimated and also you may have some stray light getting in the tube if you are observing from metropolitan Sydney.
I suggest you check the collimation on your 12" reflector and also get it to a dark sky sight in order for it to do its best.
Coming back again to amateur Astronomy after 15 years on & off, and just purchased a GSO 12" Dobsonian and an imported 152mmx1200 refractor.
I owned several reflectors before but never had a large refractor. I always knew that refractors would provide clearer images with much better contrast, but I didn't expect that the 6" refractor would significantly outperform the 12" Dob (including many deep-sky objects) after comparing them side by side under the moderately light-polluted sky. Omega Centauri stands out crisply against a much darker sky background showing better detail.
I'm starting to wonder if there's probably something wrong with my 12" Dob, but everything seems fine and properly set up.
Any inputs?
Thanks
Well toyos, there is no doubt that theoretically the 12" dob should easily outperform the 6" achro, though it is interesting to read asimov's review of his 6" job wherein he makes comparisons to his 12" dob in the IIS reviews section: I doubt whether asimov would take too kindly to certain individuals questioning his expertise and judgement, from my observations of him.....
What I can tell you is this: I own a 10" GSO dob and a 6" achromat, and first of all there are 2 things I can tell you about my dob that are unequivocal.
(1) it appears, from the small sampling I have been able to do with other GSO dobs, that its optics are par for this particular instrument (measured against 3 other newish GSO 10 inchers.)
(2) it is kept collimated to a very high degree: my own capabilities here are more than competent - I alllways perform star-tests to ensure the optics are kept properly aligned, and achieve excellent diffraction rings, uniformly concentric, before considering the scope fit for obbing.
However, on a wide range of objects, there is very little to separate these 2 scopes: I do use one of the Baader "semi-apo" filters and a quartz dielectric diagonal in the achro train; but with 13mm Televue T6 Nagler or 17mm Vixen LVW the above appraisal is unequivocally true; also when employing a Televue 2x barlow in each scope's optical train! These 2 scopes are often set up side-by-side on my ob deck, and although I don't have the luxury of 2 sets of these types of eps, I have frequently swapped the same ep between the 2 to compare views.
With focal lengths of 1200mm and 1250mm respectively for the achro and dob, the difference in magnifications are so close as to be insignificant. (the aforesaid eps give approximate magnifications of 70x, 90x, 140x and 180x)
What is also unequivocal is the superior planetary detail revealed with the achro; specifically on Saturn and Jupiter. Other than the fact that my vision is excellent and I have been obbing on and off for over 50 years and don't need any johnny-come-latelies to lecture me on collimation, observing techniques et al, I have nothing more to add except that the clarity, contrast and definition of this (relatively) small achro refractor constantly amazes me with its ability to stand up against a (my own) significantly larger apertured, well maintained dob newt!
ps toyos: I do have very good dark skies down here, except for the sky up to about 25 degrees above the horizon between north and nor-north-east; where Adelaide's sky glow impinges - sigma Octans is an easily seen star from my place.
I agree with John - it's likely your collimation is off.
What eyepieces are you using in both the 12" and the refractor?
Can you describe some of the differences you see between them both?
We're having a new-moon meet up at Mangrove Mountain on Saturday night - you're more than welcome to come up and join us. Bring your 12" dob and I'll happily check/adjust the collimation for you if it's out. It's also significantly darker there than the 'burbs, so you'll be able to see a lot more too. Check the star parties forum for a map and other details.
The collimation of the 12" dob is fine. The big difference in image contrast between the two is what surprised me the most. Everything is brighter including the background through the 12" dob, whilst the 6" refractor shows a slightly dimmer but much clearer Omega Centauri with a much darker background allowing me to see details better. I used the same set of Plossl eyepieces 6, 9, 15, 18, 20, 25, 28 & 32 on both telescopes.
I should probably take the dob to some place darker since most of my observation time was more conveniently done in my backyard.
Mike, thanks heaps for the invitation. I'll be there if I can manage to first find a bigger car than my 2-seater.
The collimation of the 12" dob is fine. The big difference in image contrast between the two is what surprised me the most. Everything is brighter including the background through the 12" dob, whilst the 6" refractor shows a slightly dimmer but much clearer Omega Centauri with a much darker background allowing me to see details better. I used the same set of Plossl eyepieces 6, 9, 15, 18, 20, 25, 28 & 32 on both telescopes.
I should probably take the dob to some place darker since most of my observation time was more conveniently done in my backyard.
Mike, thanks heaps for the invitation. I'll be there if I can manage to first find a bigger car than my 2-seater.
Hi,
The lack of contrast you describe predominantly stems from the fact that the GSO dobs are very poorly baffled when they come from the factory. While the flat black paint used on the inside of the tube in theory should be "non reflective", However, an enormous amount of stray light bounces around the inside of the tubes on these scopes. You can rectify this yourself pretty inexpensively. You can use "flocking paper" or any one of a dozen other methods to properly baffle the telescope tube itself. Flat Black epoxy paint with crushed walnet shells mixed into the paint is another option.
Some suggested mods which will improve the contrast of your dob by up to 40% :-
1) Baffle the tube. Flock the entire tube or as a minimum flock a large section opposite the focuser. In addition you can add a tube extension which helps a lot.
2) Blacken the edges of your secondary mirror with a black texta pen.
3) Baffle the inside of the focuser. You can do this by using some 1/4" self adhesive foam weather stripping.
4) You can go a step further and install annular baffles (ring knife edge baffles) into the tube at the top on either side of the focuser and at the bottom just above the primary mirror to prevent stray light entering the tube.
I have done all of the above mods to my 10" newtonian and several more; and it gives "refractor like" views in terms of contrast. It will beat the daylights out of any 6" refractor on the planet. There are plenty of IIS members who have looked through it to attest to this. Whilst a refactor gives its maximum performance straight out of the box, unfortunately these GSO dobs while having the makings of a very good scope, need quite a bit of tweaking to deliver of their best.
I have some photographs on my work computer which I will post tomorrow from work, showing exactly the modifications I have done. If you go to the Pony Club on Saturday night I will be happy to explain them all to you at the scope. I can't bring the scope because that would mean leaving the 18" behind.
Well toyos, you certainly got two-bob's worth from a couple of us: obviously from your reply you know how to/are confident of your scope's collimation - all the other points made are certainly worth a try: except the one about epoxy paints!
Flat black epoxies certainly have a degree of durability, but not when I require a matt surface: I use them extensively for certain applications; but they are one of the least matt or "flat" blacks around. Krylon flat black has been a favourite for many. I personally would advise using a pure acrylic binder black paint (not the same as "acrylic" paints that are actually PVA's) that you mix a powerfull flattening medium into yourself - you can get this in archival paints at your local fine arts supplier.
This way you can make it as flat as Hades (if Hades is flat!) and be assured of the quality/longevity of the surface as well as its "flatness."
The unknowing confuse "flatness" or the degree of matt in black paints with chroma, the degree of colour intensity or saturation; doubly confusing when one remembers that "black" is essentially "no colour!"
If you mix walnutshells/whatever into the above, you have the extra comfort of knowing these paints/binder bases are expressly formulated to "glue/hold" the brew together.
Pardon the waffle, but this is my profession and especial expertise!!!
The requirement here when mixing crushed walnut shells with the paint is to get a paint with the highest level of durability, not necessarily the most non reflective black surface. Krylon certainly works well and I have used it before. I have also used White Night flat black epoxy paint and it works equally as well as Krylon and is very durable. The last thing you want in this situation is pieces of paint or walnut shell flaking off onto the primary mirror.
The object of the exercise is to have a non reflective paint combined with another medium which breaks up all available angles of incidence for the light to reflect off. The crushed walnut shells having no defined shape or form do this very effectively as they essentially point at all different angles all over the place and block all available light paths. The crushed walnut shells are doing the majority of the baffling work, not the paint. Of course having a very non reflective paint in this situation can't hurt. For the same reason, felt or flocking paper IMO is the best choice because it is made up with minute fibres pointing all over the place which break up the angles of incidence for the light to reflect off. Plain and simple blue felt, green felt or any colour felt, would do an equally good job of baffling the scope as would black felt. It is the medium that is non reflective not the colour. We use black felt as a matter of choice because of the mental association of black being non reflective. Plus it looks the part.
If you are not going to mix walnut shells into the paint, then your advice to use Krylon is correct. It is the most non reflective black paint available. Krylon on its own without the walnut shells will improve the non reflectivity over the standard scope, but it doesn't do nearly as good a job as felt, flocking paper or paint mixed with walnut shells and isn't what I would be doing. When you mix the walnut shells into the paint you need the walnut shells so thick they touch each other when you paint the surface. One piece of walnut shell here and there doesn't do squat.
My first choice to properly baffle the scope is the self adhesive craft flocking paper available from Spotlight Stores. It costs about $40 for enough to do an entire 12" scope. Protostar in the USA aslo sell it but it will cost a lot more than buying it from spotlight. It comes in pieces the size of an A4 piece of paper and costs about $2-00 a sheet from Spotlight. It has a very pale blue backing sheet onto the self adhesive. Tip: The glue on this stuff sticks like anything and once one piece touches the other or a piece doubles over and touches itself they are stuck. Hence cut the sheets in half before peeling off the backing paper making them easier to handle and minimising the risk of doubling it onto itself and ruining the sheet. Overlap each sheet when applying by about 4mm. This flocking paper will do an infinitely better job than krylon without walnut shells.
The unknowing think its the colour and non reflectivity of the paint that properly baffles the scope; when in fact its the non reflectivity of the surface medium itself and colour matters not near as much, if the job is done properly
The unknowing think its the colour and non reflectivity of the paint that properly baffles the scope; when in fact its the non reflectivity of the surface medium itself and colour matters not near as much, if the job is done properly
Cheers,
John B
High John - I've deleted most of your post, because in essence I have no real objections to the general gist of your comments; however, the above bit of your quote I've deliberately left to comment on: this is done as a sincere (and also humorous) attempt to enlighten you re the indisputable facts: part of our mutual contradictionary relationship I'd like to think we've entertained IIS members with over time. Absolutely nothing personal, but in the spirit of friendly contention, and in a purely objective and accurate appraisal, the above is sheer nonsense!
By admitting that "its the non reflectivity of the surface medium itself" you are acknowledging that it is the paint (which is, after all, the surface of whatever is underneath, be that walnut shells or ground up yaks' teeth!)
Or perhaps you are suggesting that these troublesome, scattered/stray light rays in the tube are something akin to superman's x-ray vision; wherein (cartoon-wise) there is some hidden ability for them to detect the actual substance of "walnut shells" beneath the paint surface!
As I've inferred John, it could be walnut, almond, pistachio or the aforementioned ground yaks' teeth: the material that effects/causes the broken-up nature of the surface is important, but only as a substrate*: but of utter paramountcy is the non-reflectivity of that substrate's surface: ie, the paint! THE SURFACE MEDIUM IS THE PAINT!
* "substrate" noun: something which underlies, or serves as a basis or foundation.
It's akin to you saying you could chrome-plate the walnut shells (which I've actually done with an artwork) and the substance of the shells themselves would still prevail as the prime factor!!!
And, colour does actually matter very much: anyone who works with colours knows that colours have differing tonal values dependant on their hue! This directly affects their ability to reflect light, regardless of whether they have a flat or broken surface!
Well John, I know that I've got you dead and centre on that one: I like to think that it's something we each of us are capable of doing (the going OTT) as we strive to outmanoeuvre each other over some of these posts with our pedantry - no doubt you'll nail me somewhere down the track; but be warned, some of my postings are bait that should be approached circumspectly!
All the best; and if I'm ever up your way I might be darn cheeky enough to ask for a squizz through your scope - but not the GSO dob!
As a result of your posts in this thread, at least I have been able to work out your problem.
When you made this comment:-
Quote:
What I can tell you is this: I own a 10" GSO dob and a 6" achromat, and first of all there are 2 things I can tell you about my dob that are unequivocal................on a wide range of objects, there is very little to separate these 2 scopes
That comment immediately told me one of three things were going on and these are listed below in the likely order of probability:-
1) There was something wrong with your 10" reflector.
2) There was something wrong with your eyes.
3) The 6"/F8 Synta refractor was on steroids.
Having used all of the scopes you mention I can tell you that the difference between the two is not marginal, particularly on DSO's. However, to get the newtonian to deliver of its best performance takes quite a lot of tweaking. The refractor will do its best straight out of the box.
You made the following comment regarding my advice about the use of flat black epoxy paint as a substrate to use to apply a "crushed baffling medium".
Quote:
- all the other points made are certainly worth a try: except the one about epoxy paints!
Whilst I don't dispute the fact that your recommendation of Krylon is an excellent choice to use as the base paint, I dispute the fact that epoxy paint is totally unsuitable. I have used the White Night Flat Black epoxy paint successfully on two occasions. Your comment that it was "bad advice" made something immediately apparent to me. That was that you know a lot more about paint, than you do about properly baffling a newtonian telescope. Therein lies the answer to your telescope problem. "It isn't properly baffled"
You should possibly read this review by Ed Ting where he compares a 5.25" APO refractor to a 7" Newtonian to a 6" Mak Newt. You should also note that the jump from a 6" refractor to a 10" newtonian is a lot larger in terms of resolution and light grasp than the jump from a 5.25" refactor to a 7" newtonian.
Some suggested mods which will improve the contrast of your dob by up to 40% :-
1) Baffle the tube. Flock the entire tube or as a minimum flock a large section opposite the focuser. In addition you can add a tube extension which helps a lot.
2) Blacken the edges of your secondary mirror with a black texta pen.
3) Baffle the inside of the focuser. You can do this by using some 1/4" self adhesive foam weather stripping.
4) You can go a step further and install annular baffles (ring knife edge baffles) into the tube at the top on either side of the focuser and at the bottom just above the primary mirror to prevent stray light entering the tube.
I have some photographs on my work computer which I will post tomorrow from work, showing exactly the modifications I have done. If you go to the Pony Club on Saturday night I will be happy to explain them all to you at the scope. I can't bring the scope because that would mean leaving the 18" behind.
Cheers,
John B
Attached are the photographs.
In the first photograph you should observe the following
1) tube is completely flocked and PROPERLY BAFFLED
2) annular knife edge baffles (plywood rings) inside the tube on either side of the focuser drawtube.
3) baffles inside the focuser. you can see the first baffle right at the edge and the second baffle deeper in the focuser drawtube.
In addition, although you cannot see it from the photograph, the edge of the secondary mirror is blackened with a texta.
In the second photograph you will notice a row of screw heads about 4 inches above the machine screws holding the primary mirror cell. These screws hold a plywood ring baffle identical to the two at the top end of the scope, just above the primary mirror stopping stray light entering from the bottom end of the scope. Believe me if you observe in a light polluted environoment and observe the moon, planets and double stars, a lot of light gets up the bottom of the tube. Later design mirror cells may have rectified this problem, but in my scope it was a major source of stray light.
Hi John - blow me down me hearty - those piccies certainly demonstrate a passionate desire to determine the effects of baffling upon your newt: obviously you feel that they provide some tangible benefits, though it's interesting for readers to note that many experienced users question the benefits of baffling such as your own attempts, in said optical system.
No problems though John, each to their own as they say; and of course you're right, I do actually know a helluva lot about colour, colour theory and the effects of light; which is of course all about optical effects! That, and my own personal knowledge of the properties and use of the Barloworld Coatings* formulae (now isn't their name an interesting pun for us AA's) lets me say unequivocally that to improve your GSO's reduction of scattered light interference:
(1) overpaint that matt sheen White Knight "Flat Black" epoxy (1part) enamel
(2) apply the Krylon Ultra..... or much better still my recommendations from the previous post, in sufficient layers to cover up your previous mistakes.
This should result in a marked improvement, though I'm unable to quantify a specific percentage improvement, apropos the 40% figure you managed to concoct!
I'm sure others will benefit from our lively exchange in determining their own attitudes to this and other vexed questions; satisfied in their own views, after reading (each) our own infallible opinions....!
btw John - I've got to confess to a rather cheeky thing last night: I was watching you pouring over my last responses to this thread for a considerable length of time whilst I was compiling some reports etc - I had a full night's paperwork and waited and waited (only relatively speaking, that is) for the reply I felt sure you were machinating over - you must've decided you needed more time to think!
*perhaps that implies exponentially extra sheen to their "matt" enamel White Knight paints!
btw John - I've got to confess to a rather cheeky thing last night: I was watching you pouring over my last responses to this thread for a considerable length of time whilst I was compiling some reports etc - I had a full night's paperwork and waited and waited (only relatively speaking, that is) for the reply I felt sure you were machinating over - you must've decided you needed more time to think!
Cheers and best regards, Darryl.
Well that just shows you Darryl that not everything may be as it appears.
Did you consider the possibility that I left the page open on the computer to make it easier to come back to that page later in the night, when I had time to reply and that I may have gone and done something else for 2 hours like watch a movie on TV
Only one problem here, Darryl. I doubt the yak you extracted the teeth from would take too kindly to you (or anyone else for that matter) using them to baffle a scope
I'm afraid the reason as to why the yak would destroy your scope and trample you into the ground wouldn't be so baffling (pun intended)
And, in the case of a dead yak, it's relatives would do the same
Only one problem here, Darryl. I doubt the yak you extracted the teeth from would take too kindly to you (or anyone else for that matter) using them to baffle a scope
I'm afraid the reason as to why the yak would destroy your scope and trample you into the ground wouldn't be so baffling (pun intended)
And, in the case of a dead yak, it's relatives would do the same
Ok, ok r/n, skip the yak's teeth suggestion; llamas are much more amenable to teeth extraction if paid in the proper currency!
That, or crushed almond shells; but just in case John queries this suggestion, I'm specifically referring to Giant Californian Papershells with the outer fibrous husks of the shells removed: these I have on good authority are even better than those redoubtable wallnuts when processed thus!
Glad to hear about the movies last night John; as I said, my "spying" was only a relative thing: I'm actually preparing a paper (up most of the night) on my use of field flattening substrates and overlays to create optical effects, as per my most recent exhibition entitled "Ceremony of Meaning."
Images from this exhibition are not available on my website (too busy to upload them yet) but you should be able to access some of them in the pages of google search guff on me: "darrylpfitznermilika"
I've just realized John, apropos to my last post below, the avatar image you see with my postings is one of the images from the aforementioned exhibition: only the actual image is approx 1.5 x 1.3 metres.
The relevance to this thread is in the optically flat field and treatment of all surfaces, be they polymer coated panel or metal (as well as the compositional manipulation.)
The background of the top third of the composition would create the most marvellous anti-reflective surface for scopes: however I'm afraid I must decline to elaborate, from my selfish desire to maintain the "edge" to my reputation as one of Oz's most innovative contemporary visual artists! (oh well, sometimes modesty needs to be dispensed with to establish basic awareness of one's expertise....!)
I am afraid contemporary art is a subject that has never floated my boat. I am continually amazed by some of the pieces of such art that bring thousands of $$$$ yet they look like my 11yr old painted them, in the middle of a brain freeze. Never forget, ones mans' pleasure is another mans' pain.
That having been said, I can appreciate quality pieces of landscape and nature scene art which I do enjoy.
Cheers,
John B
Interesting comments those John, and they actually carry far more weight to someone like myself than you would likely realize: but I'm afraid that neither you nor your 11 year old son would ever be able to emulate any of my work.
Forgetting about the decades of training and knowledge accumulation that sees me working with architects, engineers and other professionals on issues as diverse as large scale engineering and load structuring etc through to basics such as occupational health and safety; the undertaking of durability and maintenance factors of 40 plus years etc etc: there are the managerial capacities et al now known under the "human resources" etc banners that have been an essential, integral element to my professional career. Some of these "artistic" projects span several years from planning/design through to fruition.
Besides my own private practice involving the abovementioned, I have also been the head of a governmental statuory organization, responsible for a large number of staff and liasing with other organizations; dealing with large numbers of the general public as well as governmental and NGO's of all levels.
All this of course is seperate from my academic involvments; which sees me speaking/presenting to anyone from primary school children through to post graduates and other professionals.
So you see John, we don't all stand there with a dripping paint brush attired in a beanie et al in some disney-esque prose, marking canvasses with paint slops or waiting for inspiration - whatever that may be!
Glad your last post gave me the opportunity to enlighten you on my own professional background John; I'd also be interested to know a bit about your own background that perhaps may be found on public record, and some of your experiences: one of the good things about the IIS forum is the benefit of letting people get to know each other quite a bit more than many do.
Good one John - now why did I suspect you could possibly be an accountant....?
Anyway, it was nice to hear your story, but I think we should really let this thread either get back on track, or let it fade away.
Btw, you'll remember Quark and a couple of post exchanges you had with him not long back - you might wish to read a little about his story that I've posted in the "Astronomy Science" section John: he's a real astronomer, not a play one like you or me.
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