does bigger aperture mean more magnification

[faulksy]

funny realy stu with what your saying about stopping down your scope, i read lots of articles on colimation with compound scopes and one article said you need to use max magnification if you can and then defocus to get your defraction rings, if you cant use much power due to the seeing being poor make a ring to go around your corrector to reduce it by 50% i did this and my airy disk was perfectly stable so i could colimate, i have tried this disk a few times on jupiter when seeing was poor and it realy improved the views but wasnt as bright.thanks all for the input. mike

[Baffa]

I found my larger scope gives the better views, if I use the same magnification. More aperture allows me to see fainter objects. I guess it could be a diffrence when observing the moon but I'm usually not out observing when the moon's up.

[Moonshane]

I think it really depends on what you are observing, where and what you want to see. e.g. I often use ridiculous magnifications (500x) on double stars at the zenith even with my 6" f11 dob. this is not possible I hear you say (500x is 83x per inch of aperture - surely impossible?) yet I use it relatively often when tight doubles are high in the sky. I also use 300x or more regularly when the moon is high in the sky and seeing decent.

quite often the for planets like Jupiter, and for my site, the seeing does restrict magnifications to 150-200x though for any scope. this changes rapidly during the night too.

I see a big difference in the performances of my scopes on different nights and generally aperture does not greatly affect magnification available. it does though affect the brightness of the object at a given magnification so Jupiter is a lot brighter in my 12" dob than it is in my 6" dob at say 200x. when the seeing pings into sharpness aperture does as mentioned above increase detail visible. that said, the detail is generally visible if you take the time to observe correctly and for an adequate amount of time.

in general terms I would always choose more aperture over less aperture as I do mask off frequently. the best views I have ever had of planets have been through my 16" masked to 170mm. take off the mask and you then get stunning DSOs, or more Saturnian moons, or more obvious crepe ring.........as always it's horses for courses.

we all know that seeing and sky conditions are often better away from housing estates but we have to put up with them in the main; such effects have more of an impact than any change in aperture. despite this, I'd still choose aperture every time.

[ronin]

As the mirror or lens gets bigger then the aberrations get more. A lens will give spherical and chromatic aberration a mirror will give whatever is the equivalent to spherical. A parabolic mirror does not give a perfect image - a parabolic profile does not bring all light to a single point. The bigger the mirror then the more the aberration is from the edge region. Simply it get progressively more difficult to achieve a good profile.

The mirrors in Chile, Hawaii etc all change shape and profile to give the best result. The space telescope mirrors are not parabolic.

In general terms you may well get more magnification the bigger the mirror but a 300mm f/3 mirror will almost 100% sure give less magnification then a 120mm f7 APO refractor.

[RobertI]

I'm not pretending to understand all of the attached article, but there is definitely documented information which explain the fact that larger apertures are more affected by poor seeing than smaller.

http://www.telescope-optics.net/seeing_and_aperture.htm

Wow that's a detailed article - Not sure I fully understand the detail but i get the point about the speckling being worse in larger apertures. Obviously more to this than meets the eye (excuse the pun).

[John]

Another factor in all this is the quality of the optics. My 12" Orion Optics dob has really good optics compared to the Meade Lightbridge 12" I used to own. The OO primary has 2 year old Hilux coatings is around 1/9th wave PV and has a strehl ratio of .987. I did not have test info for my Meade Lightbridge 12 but I'd guess it was around 1/4 wave PV and diffraction limited but not much better than that. The Meade coatings were older and I suspect less effective than the Orion Optics ones as well. 

My OO dob can handle noticeably higher magnifications than the Meade LB 12 could regardless of the conditions. I never found 250x or more much use with the Meade but the OO seems very comfortable at 250x - 300x and even 400x under good conditions. Shanes scopes have OO optics in them as well, as it happens !

I found the same when I owned some chinese 6" F/8 achromat refractors. In their native form (ie: no correction) they worked well enough up to around 200x-220x, after which things started to get a little "mushy", but when I fitted a Chromacor corrector, which corrected much of the spherical aberration ( which many of these scopes possess) and 80% of the chromatic aberration, and the ability of the scope to handle high magnification was noticeably improved with 250x-350x becoming much sharper. 

Accurate collimation is another factor in a scopes ability to perform well at high magnifications as well.

So many factors which can degrade the view - it's amazing we can see anything at all really ! 

[mert]

so in truth a large ota around 16" upwards is only better than a small ota on good nights, so a ota around 12" - 14" is probably better most of the time

I went from a 12" sct to a 22" dobsonian, under the same dark skies the dobsonian gives incredible views over the 12" scope, so much more detail at the same mag.

[ollypenrice]

I often use high power in the TEC140 and rarely use it in the 20 inch, which I prefer to use for what it does best - faint objects.

I do think a narrower incident beam can be an advatnage in poor seeing but its's a trade off, as ever, with a point of best compromise.

Going to very high powers is one thing but seeing more detail as a result is quite another.

Olly

[faulksy]

very true olly, the reason for the original post was i was trying to find out what members prefered large or small aperature. i originally liked/wanted the 11" version of what i have but a member on here that owns a 11" said im not going to gain much at all over mine, next up was the lx90 meade 12" ot"a is very good but 50/50 split with the mount some say its fine some say no good with the 12", then im left with dobs. i emailed 2 suppliers yesterday, first one was orion optics, asked about availability and upgrade options, told them i was thinking 14"vx with 1/10 optics upgrade and moonlite focuser they said i could have it before astro fest mid jan, but there new in house focuser beats the moonlite in all aspects hands down.

next up was david lukehurst, there 16" deluxe with 1/10 optics as standard and moonlite as standard approx 4 months delivery, but the eyepiece height at the zenith was 2 meteres and yes thats with the 16, so i dont think that will suit. so i think the 14 00 looks good.not to slate 00 as there glass is amongst the best, but i have heard things on the net about there tubes being poor so im not sure if there focuseres are as good as they claim, but they said i could collect and check it out before hand and have a tour of there plant, there new web site shows the focuser which looks the same as the badder steel tracks very strange. need time now to digest and work things out

[faulksy]

any thoughts on the above guys ?

[Mike73]

I'd think long and hard about what you plan to use it for and concentrate on that rather than trying to get one scope which will be good at everything.

If you are looking at a big dob for DSO's at dark sites then you want the most aperture thats practical to you, a 16" GSO will still have more light grasp than a 14" Ultra Grade OO and its that light gathering ability that counts for DSO's. When you are looking at distant galaxies you are looking for slight changes in contrast not detail.

On Saturday night I was observing the Blinking planetary nebula, at x236 it looked like a green pea with a central star, upping the mag to near x400 and x500 didn't improve the view to show detail but I could see inside it and could see a disk shape which was brighter at the bottom, a 1/10pv primary at those mags probably won't have shown much of an improvement.

My last object I viewed that night was Jupiter and at x236 with full aperture it really was stunning, at that moment in time I would have loved a Ultra High grade primary but it was the end of the night and I was tired so after 10 minutes I packed up and came home.

Couldn't you source your own primary and get David Lukehurst to make a scope around that? A 16" f/4.5 will leave your feet on the ground and an f/4 even better! I think you will be better of getting a tested mirror showing high strehl and HiLux coating for DSO's and a separate scope for home observing with more modest aperture for planets etc, thats just my opinion of course.

[jetstream]

Well,I'm going to relay some thoughts,anybody feel free to comment,criticise.....First on the aperture vs coatings (system efficiency).With standard mirror coatings of 88% reflectivity(GSO),the system efficiency would be-16" x .88 x.88=12.4" effective aperture

With 97% reflectivity-         14" x .97 x .97=13.1" effective aperture

So GSO 16"=12.4"

   OOUK 14"=13.1" effective aperture,if the 9.25 SCT was calculated like this it may surprise people....adding the corrector in...and the eyepieces (to them all)

My OOUK focuser is fantastic, very smooth,no drawtube "twist",little focuser shift when locked and holds my heavy eyepieces no problem.No need for me to upgrade....and I called Starlight for adapter and FeatherTouch in case this one was lousy.But the OOUK focuser is still on the scope as it performs great for me.....

My 10" 1/8 p-v OOUK offers great planetary detail and nice,bright DSO images from dark skies(aperture considered) with its FOV,but I wouldn't want to get so big of a scope that the TFOV gets too small,but this is just me

[Mike73]

GSO advertise there mirrors at approximately 93% not 88%, mine was tested by Huygens and came back at 92% at 1/6Pv. 

I just don't think that there is such a thing as one scope which is good at everything, I'd rather have a large aperture scope for DSO's with a smooth mirror and something with a high Pv for home.

Your perfectly right Gerry in that the TFoV gets smaller with bigger scopes but it certainly doesn't retract from the observing experience.

[Moonshane]

Personally, I'd not buy a scope that I have to use a ladder for. it's a royal pain.

I have two bigger dobs, a 12" f4 and 16" f4 plus a 6" f11 and a 4.5" f4.3. I use the 16" almost only at star parties. it's becoming a 3-4 times a year scope since I bought the 12" which I use almost exclusively now. will I sell the 16"? never as it's such a wonderful instrument. is paid for and will pretty much last forever. for me it's worth keeping all of them as all four stand me at <£2k as I bought them used.

only you can really decide on what you want to do I'd say but I really do like the variety of the four scopes I have.

[Daniel-K]

for me its aperture every time even the low power views beat a 10inch with a 8mm EP.  being able to resolve globs at x90 rather than x150 is just awesome therefore beating the seeing dont get me wrong i do like using high power but only on planets and the moon. big scope with the widefield views in the low power EP'S are just majestic. i rarely  go above x150 i just love seeing the DSO with some dark sky and bright stars in the FOV

[jetstream]

Great to here that the GSO mirror tests so well,this dispells some questions floating aroung SGL about GSO mirror coatings,durability and p-v #'s etc.I may build a largeish dob,down the road-or buy the Skywatcher if it reviews well,so it is comforting to know these manufacturers are putiing out good products.

The right tool (scope) for the job is important thats for sure,a nice 5"-6" refractor may be a nice addition,but the big dob will be first.I will compare the two(10" & 18") and then find someone with a nice 5" (or so ) refractor to compare them all at the same time.I'm sure the TFOV choice is a personal one,it would be nice to have the aperture to support the narrower,but more detailed view.Spending this kind of money on a scope,one just has to be sure what he wants/expects out of the instrument.

Off topic Mike,what is the best SQM reading you have taken and what was the NELM at the time?

[faulksy]

nelm is around 6-7 and never done a sqm reading only using a phone app which states mag 5

[AlexB67]

Well,I'm going to relay some thoughts,anybody feel free to comment,criticise.....First on the aperture vs coatings (system efficiency).With standard mirror coatings of 88% reflectivity(GSO),the system efficiency would be-16" x .88 x.88=12.4" effective aperture

With 97% reflectivity-         14" x .97 x .97=13.1" effective aperture

So GSO 16"=12.4"

   OOUK 14"=13.1" effective aperture,if the 9.25 SCT was calculated like this it may surprise people....adding the corrector in...and the eyepieces (to them all)

My OOUK focuser is fantastic, very smooth,no drawtube "twist",little focuser shift when locked and holds my heavy eyepieces no problem.No need for me to upgrade....and I called Starlight for adapter and FeatherTouch in case this one was lousy.But the OOUK focuser is still on the scope as it performs great for me.....

My 10" 1/8 p-v OOUK offers great planetary detail and nice,bright DSO images from dark skies(aperture considered) with its FOV,but I wouldn't want to get so big of a scope that the TFOV gets too small,but this is just me

The SCT will have a fairly hefty obstruction size too, cutting out even more, of course they all do, including Dobs where perhaps 20 - 25% obstruction is more typical, but it is not negligible in any case for all of them.  Another interesting way to look a it is to compute the brightness factor including the eyepiece and coating attached to the scope, if your exit pupil matched the eyepupil and you have a theoretical brightness of one say, by the time you take all these factors into account  rough working value of 0.7 is not out of the question as more representative, these are sort of hand waving figures a little bit I know, but as you say , there is  a lot less left by the time the stuff reaches the eye. 

I guess nothing I wrote will change the fact that for DSOs aperture still rules,  in fact, such figures only tell me you need even more of it to make up for the losses. If you can minimise the transmission loss and have high PV to go with it even better if you can afford it.  

Now, for a  bit of a conundrum which I cannot answer myself through lack of experience, not having worked with such high grade equipment, for DSO would a 12 inch 1/6PV be more useful for hunting down  some of the fainter stuff like galaxies compared to say a 1/10 PV 10 inch, assuming coatings are the same for a minute ? My gut would tell me yes, perhaps, though my stool I am sitting on wobbles as I write this, that tells you I don't feel very confident about saying that  . On the other hand, when it comes to revealing a cleaner image such as planetary details or general features where object contrast is also important, particularly at higher mag, a higher PV would come into its own too ( assuming the skies will let you) . 

Different courses for different horses I suppose. In an ideal world I want all the benefits :0)

[Mike73]

Great to here that the GSO mirror tests so well,this dispells some questions floating aroung SGL about GSO mirror coatings,durability and p-v #'s etc.I may build a largeish dob,down the road-or buy the Skywatcher if it reviews well,so it is comforting to know these manufacturers are putiing out good products.

The right tool (scope) for the job is important thats for sure,a nice 5"-6" refractor may be a nice addition,but the big dob will be first.I will compare the two(10" & 18") and then find someone with a nice 5" (or so ) refractor to compare them all at the same time.I'm sure the TFOV choice is a personal one,it would be nice to have the aperture to support the narrower,but more detailed view.Spending this kind of money on a scope,one just has to be sure what he wants/expects out of the instrument.

Off topic Mike,what is the best SQM reading you have taken and what was the NELM at the time?

When I first received my Sumerian I quickly found out that the mirror was very rough, the views were really dim and not good at all. Obviously the mirror went back to TS in Germany where it was tested and confirmed that it was bad. Wolfi at TS was so bothered by this that he sent all (4 of them) his in stock 16" mirrors to Huygens for testing and all of them came back at being above the advertised 1/4thPv. So this is pretty reassuring but if I was ever going to buy a large aperture scope again I'd definitely buy a tested mirror like a 1/6th OO.

Yeah I definitely think its a case of right tool for the job, I just don't really enjoy observing from home hence my current and only scope. 

Best SQM was 21.79 but NELM was only 6.3! 

[YKSE]

A very interesting discussion has been carried on here, I'd like to add fuel about the more degraded performance of big scopes against small ones.

Considering the two extreme cases, i.e. wall to wall cloud and the 100% good seeing condition, a scope's performance could look like this:

scope_perf_1.bmp

Kind of diminishing return graf towrads both ends.

If we put graphs for a big and a small scope together, it may look like this:

The middle section between A and B are our usual observing conditions, it is possible that the graphs here look like this: 

Where the performance of big scope drops below the small one, this would explain some observed worse performance of big scope.

Even if the big scope doesnot actually drop under the small one, the much bigger deterioration in performance comparing to in better conditions might give perception of worser performance in close calls.

Just my two cents.

[jetstream]

Even if the big scope doesnot actually drop under the small one, the much bigger deterioration in performance comparing to in better conditions might give perception of worser performance in close calls.

This is a great explanation,IMHO

[jetstream]

When I first received my Sumerian I quickly found out that the mirror was very rough, the views were really dim and not good at all. Obviously the mirror went back to TS in Germany where it was tested and confirmed that it was bad. Wolfi at TS was so bothered by this that he sent all (4 of them) his in stock 16" mirrors to Huygens for testing and all of them came back at being above the advertised 1/4thPv. So this is pretty reassuring but if I was ever going to buy a large aperture scope again I'd definitely buy a tested mirror like a 1/6th OO.

Yeah I definitely think its a case of right tool for the job, I just don't really enjoy observing from home hence my current and only scope. 

Best SQM was 21.79 but NELM was only 6.3! 

Glad to here the mirror got sorted out and judging from your sketches it is excellent... I love travelling to a dark site to view and do it a lot.I noticed the same thing with the SQM readings the difference being extinguishment from whatever source I think-my best reading is 21.3.This thing is a good tool to hunt for darkness ....so when the sky is transparent,awesome views.Aperture vs magnification,great topic

[Mike73]

haha yeah I can see your logic but your putting a big scope in its worse possible scenario. Your 'usual' conditions maybe very different to Faulksy's dark skies in North Wales so to even it up you could add another graph comparing the two scopes in mag 7 skies?

Large apertures really shine under dark transparent skies, seeing conditions mean next to nothing for DSO observing its all about dark skies and sky transparency.

[scarp15]

very true olly, the reason for the original post was i was trying to find out what members prefered large or small aperature. i originally liked/wanted the 11" version of what i have but a member on here that owns a 11" said im not going to gain much at all over mine, next up was the lx90 meade 12" ot"a is very good but 50/50 split with the mount some say its fine some say no good with the 12", then im left with dobs. i emailed 2 suppliers yesterday, first one was orion optics, asked about availability and upgrade options, told them i was thinking 14"vx with 1/10 optics upgrade and moonlite focuser they said i could have it before astro fest mid jan, but there new in house focuser beats the moonlite in all aspects hands down.

next up was david lukehurst, there 16" deluxe with 1/10 optics as standard and moonlite as standard approx 4 months delivery, but the eyepiece height at the zenith was 2 meteres and yes thats with the 16, so i dont think that will suit. so i think the 14 00 looks good.not to slate 00 as there glass is amongst the best, but i have heard things on the net about there tubes being poor so im not sure if there focuseres are as good as they claim, but they said i could collect and check it out before hand and have a tour of there plant, there new web site shows the focuser which looks the same as the badder steel tracks very strange. need time now to digest and work things out

Good to see that your process is involving making numerous enquires that includes through David Lukehurst and OOUK, with an option to visit the OOUK factory.

Concerning the 14" VX, the proportions and weight are quite manageable by one person, if moving it only short distances. I can fit mine length ways into my small hatch back and so can travel to dark sites. It was a fairly recent purchase and I haven't had the opportunitie's so far to use it extensively as yet. It has 1/10pv mirrors and a baader steeltrack focuser, which is nice and smooth. At F4.5, positioned on the OOUK dob base it is fine for seated observing (I am approx 5' 11") and is border-line in terms of preference concerning whether you prefer a paracorr or not (I dont use one). Not unlike yourself, I gleaned a lot of useful additional info from forum member, OOUK dob users. I feel that it complements my 8"SCT really well and both will be eagerly used when the opportunities present themselves.

[faulksy]

Good to see that your process is involving making numerous enquires that includes through David Lukehurst and OOUK, with an option to visit the OOUK factory.

Concerning the 14" VX, the proportions and weight are quite manageable by one person, if moving it only short distances. I can fit mine length ways into my small hatch back and so can travel to dark sites. It was a fairly recent purchase and I haven't had the opportunitie's so far to use it extensively as yet. It has 1/10pv mirrors and a baader steeltrack focuser, which is nice and smooth. At F4.5, positioned on the OOUK dob base it is fine for seated observing (I am approx 5' 11") and is border-line in terms of preference concerning whether you prefer a paracorr or not (I dont use one). Not unlike yourself, I gleaned a lot of useful additional info from forum member, OOUK dob users. I feel that it complements my 8"SCT really well and both will be eagerly used when the opportunities present themselves.

im a bit shorter than you im 5' 10 but 5' 11 in my heels , so your enjoying your vx14 then, did you have any problems with it ?