Stopping down...a question

[Moonshane]

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

not at all

the opening just restricts the aperture. the part of the aperture that remains open still hits the mirror in the same place so collimation remains the same.

[swamp thing]

Interesting result on big Jupe Rob.....I found the exact opposite

I found fine detail much harder to define when my 10" was masked down.

Guess we're all different

[foundaplanet]

really ?

Really... That's a drainpipe in a brolly stand..

You know I love my dob.. All dobs for that matter.

[John]

Despite being a big refractor fan, when I look at my 12" F/5.3 dob (on a Moonshane mount I ought to add) it looks great to me because I can work out how much I'd have to spend and how large the refractor would have to be to perform as well as the 12" does. The answers I come up with are £thousands and HUGE

As my wife is still getting over the shock of the 6" F/12 refractor that now stands in a corner of our dining room I don't think I'll try and introduce the topic of an observatory mounted 8"-10" apochromat and perhaps £7-£10K of investment just yet .....

[JamesF]

As my wife is still getting over the shock of the 6" F/12 refractor that now stands in a corner of our dining room I don't think I'll try and introduce the topic of an observatory mounted 8"-10" apochromat and perhaps £7-£10K of investment just yet .....

I believe the appropriate approach there is to wait until she goes away for a weekend or something and then get it built whilst she's away. When queried on her return you just respond "What? That old thing? I've had that ages!"

James

[AlexB67]

really ?

Come oooon, how can anyone not like that, that looks like a piece of modern art to me :0)

My old man is a sculptor, so next week I'll have to convince him, I have some ideas how to make some really cool shapes for dobsonian mount panels. Perhaps there is a business in limited edition mounts for dobs . I can visualise it already how a Dob mount could be made to look like the coolest thing. I don't mind the sky-watcher panel shapes too much, but they could be a bit more curvy

[JamesF]

My old man is a sculptor, so next week I'll have to convince him, I have some ideas how to make some really cool shapes for dobsonian mount panels. Perhaps there is a business in limited edition mounts for dobs . I can visualise it already how a Dob mount could be made to look like the coolest thing. I don't mind the sky-watcher panel shapes too much, but they could be a bit more curvy

I can just see a dob with a nice Rococo ironwork base

James

[foundaplanet]

Oh dear, tangents, sorry Rob completely wrecked your thread.

[Qualia]

Naa, it's a lovely thread and it's nice that we can have a natter like this

Another interesting thing I noticed with the stopping down was the effect of smearing rather than having a sharp image. In the conditions last night although very good there were moments of smearing with the 10" looking at Jupiter but once stopped down this quickly disappeared. In this sense, although the dictum 'bigger aperture suffer more from the effects of bad seeing' may be true when viewing planets, for example, there's a quicker and easier way of getting around it rather than carrying out the mount and 4" frac etc.

I think if you're serious about solar work, want to tweak as much detail from white-light as possible you need a 4" frac and preferably with a Herschel Wedge rather than solar film. I haven't had the chance to look through a top class apochromatic frac within the 5"-6" range but so many experienced observers say the image on planets is crisper, more contrasty and perhaps giving even more information than larger telescopes, then maybe when you get to that kind of price range a bigger newt will get a run for its money. But, then, as John says, you really are going to be paying for that treat.

I was wondering, if, on occassions, Shane's 6" f/11 newt gives better viewing experience on planets than his 16" (hope I got that right) how might that kind of instrument compare with something like a 6" f/12 Skymax Maksutov?

And final question, if I wanted to make the focal length of the Tal longer would all I need is a Barlow? As you can see I'm confused about this. Or again, if that wasn't the case, could I make the Tal into a 4" f/14, say, by unscrewing the lense fitting it on to a pipe of 400mm long and screwing that pipe onto the original Tal's? I don't think I'd ever do that, but there's room for thought.

[Naemeth]

And final question, if I wanted to make the focal length of the Tal longer would all I need is a Barlow? As you can see I'm confused about this. Or again, if that wasn't the case, could I make the Tal into a 4" f/14, say, by unscrewing the lense fitting it on to a pipe of 400mm long and screwing that pipe onto the original Tal's? I don't think I'd ever do that, but there's room for thought.

You could get a 1.4x barlow . But you couldn't just fit 400mm of pipe onto the end without adding an element before which alters the focal length. If you didn't have this element (a barlow for example) you just would never reach focus (unless you had 400mm of in-focus travel ).

Remember, using a barlow increases the focal length and leaves the aperture the same, so you end up with a slower scope. In this case, a 4" F/14 Tal.

[estwing]

I going to watch this closely as this will be my one and only mod to my moonshane ....(honoured member of this wonderful club!)

[John]

....I was wondering, if, on occassions, Shane's 6" f/11 newt gives better viewing experience on planets than his 16" (hope I got that right) how might that kind of instrument compare with something like a 6" f/12 Skymax Maksutov?....

It's a different type of mak but I used to own an Intes 6" F/5.9 maksutov-newtonian. It's central obstruction was just 29mm in diameter so around 19% by primary diameter. It produced images that were just like the refractors produce and it's performance was around the same as a good 5 inch apochromat refractor. It's downsides were that the small secondary caused vignetting when 2" eyepieces with large field stops were used, it was much heavier than a 6" newtonian and it's cool down time was longer than either a refractor or a newtonian due to the relatively think meniscus lens across the front of the scope and the thick tube walls.

From the reports I've read from Shane, I reckon his 6" F/11 OO performs as well as a good mak-newt though, and without the disadvantages that I've listed although the small secondary may still limit wide angle, low power views but thats not what the scope is for really.

[Moonshane]

I think you are quite right John. my secondary is under 25mm although I have not measured it so based on that the obstruction is 16.7% by diameter. it does produce diffraction spikes of course but the contrast is excellent and detail sharp. it's great on doubles and solar system stuff and anything else really within the limits of 6" aperture. I even made an aperture mask for this which gives a 55mm (from memory) f29. as daft as it sounds I feel that in that format I once got close to splitting Sirius.

flocking it (whole tube) was .....a challenge.

inspired by your secondary, I have order a piece of 1.5mm brass and hope to make a curved spider soon to see if I can achieve 'better' results. different really not better.

[John]

...inspired by your secondary, I have order a piece of 1.5mm brass and hope to make a curved spider soon to see if I can achieve 'better' results. different really not better...

My friend Ade Ashford tried this on an OO 6" F/11 newt and reported on it here:

http://www.nightskie...iny/spider.html

I was in two minds about the curved spider approach before I got my scope, which happened to be fitted with them, but I've been very pleased with the scopes performance and I suspect some of that may be due to the spider design. The only way to know for sure would be to re-fit a conventional X shaped spider but thats not too practical really !.

I'll be very interested to hear what your experiences are

[Naemeth]

If it helps at all, I've noticed a small difference between stars between my smaller dobs (with 1 arm needed for the secondary) and my 10" dob (with a conventional 4 spoke? spider). But it's really small, and I only really notice it if I'm out of focus.

[Moonshane]

I've looked at those and might consider one actually. it would be neater and probably more accurate. I'd go for the three vane one though I think as he did and just accept the additional two holes.

[Moonshane]

the main difference is that diffraction spikes are caused generally by straight edges and not by curved ones.

[Stu]

My Mak is in theory an ideal planetary scope, at 8" with a 4000mm focal length. The central obstruction is about 4.8% by area I believe (22% diameter).

I say in theory, as it is not performing to its max at the moment, I recently found a dark spot on the secondary silvering which I think is responsible for the disrupted diffraction patterns I've been seeing. As Shane knows it also needs collimating which is not straightforward. I intend to take it back to OO after PSP for them to get it properly sorted and hopefully performing as it should.

Stu

[Stu]

As an aside to this but relevant to the OP, is airy disk size not related to focal ratio ie it reduces with aperture but increases with focal length? I may be mistaken but have read it somewhere.

Just found it, thought this was interesting

http://www.oldham-optical.co.uk/Airy%20Disk.htm

Stu

[bingevader]

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.

Yep excellent, SGL at its best!

But Qualia, I'd still like to see your comparison of the Dobfractor and the Tal side by side, if you get the chance!

I'd have to agree that solid tube dobs are just big ugly drainpipes I'm afraid. Excellent mind you, but big ugly drainpipes none the less!

And just to finish slightly off-topic, I just spotted Moonshane's signiture and remembered this (amazing what gets stored away and remembered again when triggered by something!):

Just on the border of your waking mind, there lies another time, where darkness and light are one, and as you tread the halls of sanity, you feel so glad to be, unable to go beyond. I have a message from another time.

Cheers

[John]

As an aside to this but relevant to the OP, is airy disk size not related to focal ratio ie it reduces with aperture but increases with focal length? I may be mistaken but have read it somewhere.

Just found it, thought this was interesting

http://www.oldham-op...k/Airy Disk.htm

Stu

That is a very interesting article Stu - thanks for the link

I'm going to need to read it through a few times to fully "get it" though I reckon

[Stu]

That is a very interesting article Stu - thanks for the link

I'm going to need to read it through a few times to fully "get it" though I reckon

Thanks John. Yes, I've really only skim read it at the moment so it will take a few goes to sink in :-)

Stu

[AlexB67]

As an aside to this but relevant to the OP, is airy disk size not related to focal ratio ie it reduces with aperture but increases with focal length? I may be mistaken but have read it somewhere.

Just found it, thought this was interesting

http://www.oldham-optical.co.uk/Airy%20Disk.htm

Stu

I had that in my bookmarks too, but as I pointed out in another thread, can't remember where or bother to search. The airy disk is interesting also from the point of view that the radius shrinks with increasing obstruction size, though I doubt the effect is significant to be noticeable to an observer. The distribution is more important i.e. the central disk reducing in intensity and further rings become more important in their contribution, this therefore plays a part in the the nice images without obstruction that frac people go on about to some degree, but I also think it can be overstated given other factors. I found the following articles to be informative on the topic of obstructing the view.

http://www.hoflink.com/~mkozma/obstruction.html

and

http://www.beugungsbild.de/diffraction/diffraction.html

and

http://www.damianpeach.com/simulation.htm

enjoy

I can appreciate it in theory, not that I ever had the privilege of such instruments.

[AlexB67]

I should have made myself clear, often thinking ahead and not explaining is a bad habit I have ( slapping myself while typing ). When I say obstruction size I am talking about central obstruction caused by the secondary for a given aperture compared to one without.

[foundaplanet]

You have me at it again now Rob, I am making an 80mm disc for the Lyra today to see how it preforms at f14..!