Stopping down...a question

[Qualia]

I've been viewing Jupiter recently in both the 4" f/10 and the 10" f/5 and after this morning's session I've been thinking about 'stopping down' the aperture to see what happens. I've had a look around the web but can't really find anything to help.

The question is, is it really that simple?

You take the 10" f/5, make off-set cardboard circles to avoid the secondary and vains with a diameter of say 8", 6" and 4" place it over the OTA and you've converted the telescope into an 8" f/6, 6" f/8 and a 4" f12.5. Is it that straight forward? If I then use a Barlow would that mean the 8" becomes an f/12, the 6" an f/16 and the 4" an f/25 ? In theory, one gets decent aperture with unobstructed, CA-free views making the 10" a planetary buster with apo-like quality ?

The downside would be lower resolution ( a darker image) but a gain in contrast. Is that how it works?

A confused observer with a little DIY free time on his hands.

[Naemeth]

Yes it is that simple. The reason you get a larger (or slower) focal ratio is that the focal length remains fixed, but the aperture is stopped down.

Think of a camera lens, it is the same principle (fixed focal length, varying aperture diameter)

[John]

I think thats broadly how it works. With a 10" F/5 though the max aperture stop that will fit between the secondary vanes and avoiding the secondary would be limited to around 4.5" I would have thought. So you get a 4.5" f/10.4 with slightly lower light throughput than a refractor of the same spec.

[Moonshane]

hi Rob, yes that's it but you need to make the holes so they fit within the vanes and avoid the secondary. http://stargazerslounge.com/topic/143780-aperture-mask-for-16-f4-dobsonian/

you only use one hole at a time and in your case, one hole only would probably be Ok. as John said maybe 100mm would do it so you'd have a 100mm f12.5 or maybe a 90mm f14. the only downside to doing this is that I fear your TAL may be in the firing line.

[Qualia]

Thank you all so much for your kind replies. Okay, so I'm going to give this a shot. It'll be interesting to see what happens. This afternoon I'll head out, buy some cardboard and see if I can get something together for 4am when Jupiter does his kingly thing. I'll make a little report and let you guys know how this goes.

The only downside to doing this is that I fear your TAL may be in the firing line.

Ay, Shane, you warned me of this all those months back and you're right. The Tal gets a look in each morning with the Sun and that's about it. But I don't think I'd ever sell the Tal. I mean, I'd only get about €150, €180 tops. That's about the price of a decent secondhand EP and it just doesn't seem worth it. It's paid itself back more than enough times, makes a lovely white light observer and within reason, sticks up for itself well.

And of the Moonshane. Well, that's a completely different story.

I've had the gorgeous Moonshane now for what? 6 months, or so? It's been up mountains, across deserts, into rolling green fields and dry, barren scrub-lands. It's been (protected) out in the rain, the fog, the snow and the blazing sun. It's already travelled way over 7,000km, climbed tortuous heights of over 1,000 metres and back down to relax near golden sands and mellow blue seas. The Moonshance has been to Italy, Portugal, France, and the furthest reaches of Spain. It's an international, cosmopolitan cosmic-space-cruiser and if I were a poet, I'd write an ode to the Moonshane.

[bingevader]

I'd be interested in the results of your little experiment and the comparison with the 4" Tal.

Cheers

[Moonshane]

Thank you all so much for your kind replies. Okay, so I'm going to give this a shot. It'll be interesting to see what happens. This afternoon I'll head out, buy some cardboard and see if I can get something together for 4am when Jupiter does his kingly thing. I'll make a little report and let you guys know how this goes.

Ay, Shane, you warned me of this all those months back and you're right. The Tal gets a look in each morning with the Sun and that's about it. But I don't think I'd ever sell the Tal. I mean, I'd only get about €150, €180 tops. That's about the price of a decent secondhand EP and it just doesn't seem worth it. It's paid itself back more than enough times, makes a lovely white light observer and within reason, sticks up for itself well.

And of the Moonshane. Well, that's a completely different story.

I've had the gorgeous Moonshane now for what? 6 months, or so? It's been up mountains, across deserts, into rolling green fields and dry, barren scrub-lands. It's been (protected) out in the rain, the fog, the snow and the blazing sun. It's already travelled way over 7,000km, climbed tortuous heights of over 1,000 metres and back down to relax near golden sands and mellow blue seas. The Moonshance has been to Italy, Portugal, France, and the furthest reaches of Spain. It's an international, cosmopolitan cosmic-space-cruiser and if I were a poet, I'd write an ode to the Moonshane.

What a fantastically nice thing to say and what a story. From my house (surely my first light in England to check focus etc counts as another country for the list? ) via UPS and then to you and all those travels. I agree actually about the TAL, different scope for a different purpose - perfect.

[JamesF]

I've struggled to think my way through this on more than one occasion. I can see what you lose by stopping a large aperture scope down, but what do you actually gain? The focal length remains the same, but you've reduced the amount of light you're receiving. So where's the up-side?

James

[Moonshane]

hi James, it is somewhat counter-intuitive but no harder than a de-rotation device on an alt az mount

think about it this way, people often refer to e.g. long focal length fracs and SCTs as 'planet and double killers' and this is certainly the case. effectively stopping down recreates this design and removes the diffraction effects of the vanes/secondary.

also, it reduces the exit pupil size markedly and this creates a darker background and more contrast. another effect is that it produces tighter stars and therefore with the reduced diffraction it also creates easier splits with doubles.

furthermore, the seeing often means that larger apertures produce less stable images. smaller apertures do seem to 'cut through' the seeing and although full aperture is wonderful and produces much more detail and brightness when available, this is not always the case and to my eyes, the view of e.g. Jupiter is better with my 16" masked to 170mm than it is with full aperture unless the seeing is top notch. it may just be an effect/illusion but views seem to be more contrasty and sharp when masked other than those fleeting moments of clarity with full aperture.

[JamesF]

Yes, getting rid of the diffraction effects and reducing the exit pupil size might well be desirable. Those effects hadn't occurred to me. And of course people do talk about the C9.25 being better than the C11 and C14 unless the seeing conditions are near-perfect, so the aperture reduction makes sense there too.

Thanks.

James

[swamp thing]

Be interested to read how you get on Rob. I've found stepping down my 10" has provided very mixed results, and TBH offers very little gains.

On nights where seeing is bad enough to warrant stepping down I just observe something else rather than planets. I find the loss of resolution off sets the gains.

On larger scopes like Shane's 16" stepping down can prove very effective, because the stepped down scope is still of large enough aperture to offer some real resolution still. A stepped down 10" just isn't big enough IMO.

I'd personally only bother stepping down scopes 12" and over.

[Qualia]

Okay, here's what I did which might be of some use to future readers:

Early evening I went out for a stroll with my girlfriend and as luck would have it came across a child's snare drum left out for rubbish. It looked like a broken version of this:

I carried it home, took it apart and ended up with this:

Knowing that by using the frame it wouldn't be necessary to measure the inside diameter of the telescope, I covered it with a sheet of stiff, thin black card and went on to measure the diameter of the primary and secondary. With a compass I drew two concentric circles of those sizes on the card. In my own case the radius of the primary was at 125mm and the secondary at about 31mm.

With a ruler I drew a line from the edge of the small secondary circle to the edge of the primary circle. This line was about 100mm long. I marked its mid-point, divided the length of the line by 2, set the compass radius to 50mm and scored another circle with the compass point on the marked mid-point of the line.

Making sure that this new circle neither touched the inner circle (the secondary's edge) nor the middle circle (the primary's mirror edge), I ended up scoring a circle with a radius of about 45mm. This would avoid slop, dodgy scissor cutting and misalignment. Finally I cut this circle out and ended up with this:

The seeing and skies this week have been exceptional. I didn't start observing until around 2.30am and even in the city at this hour badly positioned faint nebulae like M 1 could easily be seen through the LP.

So how did the mask work? Well, not bad actually. It just depends what you were looking at.

I found that the stopped down image was a disaster on doubles. The image in the new 90mm masked Moonshane is a lot dimmer than its usual 250mm, the sky background significantly darkens (no bad thing when viewing in the city), but what are bright, snappy, cleanly split beauties blazing forth under the 250mm aperture were fainter, harder to split/see and lost a lot of their inherent colour.

By contrast, when Jupiter started to rise and was 'sufficiently' high enough to observe, I found the image was sharp, with good contrast and no false colour or other types of aberrations. Indeed, with Jupiter being pretty low and never reaching the zenith, the new Moonshane cut through that thick atmosphere and offered up a very stable, crisp and clean image.

What was most surprising was the attention to detail which could be tweaked with the stopped down Moonshane. Without the aperture mask, Jupiter was very bright which often made it tricky to see the polar regions or fainter temporate belts. I usually use a light blue filter under such conditions but with the mask these features came through clearly and I even saw what can only be described as a very dark, distinct blob on one of the equatorial belts which wasn't nearly so apparent when using the Moonshane's full aperture.

Visually, then, this given experiment turned out to be successful with extremely pleasing results. I'm not sure whether the images thrown up this morning would be like those offered by a top-class apochromatic refractor - if they were no one would buy such an instrument - but I sense we get a feel of what is going on.

My own sentiment on this is that anyone who has a Newtonian with 10" or more ought to experiment with an aperture mask. It costs almost nothing, takes no time to make and you may enjoy these dimmer but frac-like images. The only way you can decide if it is worth your time is with a side-by-side comparison. I found that doubles were a great disappointment but the view of Jupiter threw up really nice surprises. Later on this year - at a more reasonable hour - I'll try the experiement again and see whether I come to the same conclusions. Nevertheless, at this point, I think one will still walk away feeling that the advantages of greater aperture significantly outweigh any disadvantages.

[foundaplanet]

Interesting Rob. I made an aperture mask for my newt, from memory I think it was 86mm, I tried it once. I imagine that swampthing is right about 12" and upwards as there is just not enough aperture between the veins on a ten to out do a 4" frac. In comparing the two I do find the frac a far better instrument to observe the moon with over the newt as I feel I can make out more shades of colour and I get good magnification with less boiling up, I imagine that is down to brightness as I struggle with the moon through the newt due to brightness.. Plus using a frac on the moon is proper cricket..

[Moonshane]

nice report Rob. what you have to bear in mind of course is that the exit pupil of your f12 dobfractor is a lot smaller than your f5 newt scope. at 150x your exit pupil is 0.69mm in the dobfractor and 1.66mm in the dob. that said, I like the dimmer images of doubles as they provide an easier split I think and round stars with nice airy rings.

[foundaplanet]

nice report Rob. what you have to bear in mind of course is that the exit pupil of your f12 dobfractor is a lot smaller than your f5 newt scope. at 150x your exit pupil is 0.69mm in the dobfractor and 1.66mm in the dob. that said, I like the dimmer images of doubles as they provide an easier split I think and round stars with nice airy rings.

You are right Shane, sounds to me that what you need is a nice long focal length 4" refractor..

[Moonshane]

why?? when I have a (two) dobfractor(s)

[John]

While tight doubles may seem easier to split, probably because of the reduced diffraction, the resolution ability of the stopped down scope does actually decrease because the airy disk of the stars gets larger as the aperture diminishes. So despite the diffraction effects, a larger aperture should always be able to resolve tighter binary stars than a smaller one.

My 12" F/5.3 OO dob has curved secondary vanes and a 21% central obstruction both of which help to reduce diffraction effects. The star images it produces have no diffraction spikes and are much "tidier" than any other newtonian I've used, though not quite as tight as my refractors produce. While I love the views through the refractors, the 12" will beat them every time even on really tight binary stars. I've yet to split (convincingly !) a sub-arc second double with my 4.7" ED refractor but I've managed that a number of times now with the 12". It's also the only scope I've owned that has spilt Sirius for me.

[Qualia]

Thanks for the replies.

When I can, Foundaplanet I will try viewing the Moon with the aperture mask (maybe tonight) and see what happens. Generally speaking, though, I don't have a preference with viewing the Moon, through the 10" or 4" the views are nice.

Ay, that is true, Moonshane. I feel this has a lot to do with it. By reducing the exit pupil I effectively 'cleaned' the image. I can see now why folk would go for local focal lengths and by this have learnt something new, 'hands-on' An aperture of 10" or more and a focal length of about 4 meters would be sweet for planetary viewing

Great information, John and thank you. I've found exactly the same thing with resolving binaries, the 10" dances circles around the 4". Brigher, cleaner split and more colourful than the Tal.

[AlexB67]

You are right Shane, sounds to me that what you need is a nice long focal length 4" refractor..

Okay. I'll stir the pot a bit What Moonshane knows is that you can get a good bang for buck dobfractor. Now turn it around and lets see the 4 inch refractor people make a fractordob by growing aperture for a few quid, much more difficult. The Newt always wins in versatility and allround performance for the money, okay slap me, I deserve it

Interesting thread and good read btw. It sort of got me thinking to make one with two holes for the fun of it on the heritage ( seeing it has only one strut for the secondary ) and what would happen to the diffraction patterns, ooh the fun that can be had a telescope version of Young's slits, let's call it the double hole experiment.

[foundaplanet]

Okay. I'll stir the pot a bit What Moonshane knows is that you can get a good bang for buck dobfractor. Now turn it around and lets see the 4 inch refractor people make a fractordob by growing aperture for a few quid, much more difficult. The Newt always wins in versatility and allround performance for the money, okay slap me, I deserve it

Interesting thread and good read btw. It sort of got me thinking to make one with two holes for the fun of it on the heritage ( seeing it has only one strut for the secondary ) and what would happen to the diffraction patterns, ooh the fun that can be had a telescope version of Young's slits, let's call it the double hole experiment.

T'was merely jesting and of course Shane is absolutely right. However, now it is my turn to ''stir the pot'' as you say.. I have yet to see a solid tubed Newtonian of which I could truly describe as an object of beauty.. Fracs do look like proper telescopes whereas solid tubed Newt's will always look like a bucket. I will though concede that there are types of truss tubed Newt's that have considerable contraption appeal...

[AlexB67]

I think a newt is a nice looking scope personally, especially the flextube designs as you say , I prefer that more over a long tube refractor to look at, but in the end of the day I only care what I see through the eyepiece.

On a more serious note however, not joking around, I am not a one or other scope club type scope person. I find it interesting to learn and understand about all the designs, what they bring to the table and how they can potentially be modded to do extra things. I am quite convinced however that when on a budget I find it hard to see any other alternative but to go a Dob/Newt route to astronomy for allround performance for DSO and planetary visual observing.

[Astro Imp]

This has been one of the most enlightening threads on SGL recently. An intelligent question well answered by posters who know what they are talking about and a bit of humour thrown in as well.

[Scott]

Having only ever owned Newts......and this hurts to say......I find fracs to be more pleasing on the eye .

Now, back to the thread, I was just wondering, when you stop down a scope, does this affect the collimation at all being that the centre of the opening has shifted slightly? sorry if i've missed the point.

just realised, I may not get a reply after my opening gambit

[foundaplanet]

Having only ever owned Newts......and this hurts to say......I find fracs to be more pleasing on the eye .

Now, back to the thread, I was just wondering, when you stop down a scope, does this affect the collimation at all being that the centre of the opening has shifted slightly? sorry if i've missed the point.

just realised, I may not get a reply after my opening gambit

I can't give you the physics but I can give the answer, no.

[Moonshane]

T'was merely jesting and of course Shane is absolutely right. However, now it is my turn to ''stir the pot'' as you say.. I have yet to see a solid tubed Newtonian of which I could truly describe as an object of beauty.. Fracs do look like proper telescopes whereas solid tubed Newt's will always look like a bucket. I will though concede that there are types of truss tubed Newt's that have considerable contraption appeal...

really ?