Heads up: Explore Scientific Ultra Light Dobsonian 305mm

[Space Hopper]

Nice touch with those fans on the side to remove boundary air above the primary. All too often overlooked.

[Moonshane]

not my design - I have distilled a number of designs online to achieve my own results although I really don't have the time even to finish off my own of late! looks a good design to me. I doubt the adjuster would intrude although you might end up with eight diffraction spikes.

[JamesF]

Looks quite nice, but as the OP says, I reckon you would get 8 diffraction spikes off that secondary spider, looks strange and unnecessarily complex.

I was thinking you'd get eight, but I'm not convinced having pondered on it a bit.  I now think that as long as the opposing arms are parallel then you'll only get a pair from each, so four in total.  This may seem counter-intuitive, but you only get six diffraction spikes from a Bahtinov mask -- one pair for each different direction of the slots -- and I think this is effectively the same situation.

Peter Drew would probably be my best guess as someone who'd know for certain.

James

[John]

The specs do seem to suggest it has a mirror, F/5 with 1525mm focal length

Angular Resolution is apparently 0.43 arcseconds

Sounds like somewhere between the Dawes and Rayleigh limit for a 12" aperture. 

[John]

On the secondary supports, I believe you get two opposing spikes from each vane so the 4 symmetrical vane design works because one set of 4 spikes are overlaid on the other 4. Whether this slightly offset 4 vane design would behave like that or whether the spikes would appear stronger because they don't quite overlap I'm not sure. 

[Stu]

On the secondary supports, I believe you get two opposing spikes from each vane so the 4 symmetrical vane design works because one set of 4 spikes are overlaid on the other 4. Whether this slightly offset 4 vane design would behave like that or whether the spikes would appear stronger because they don't quite overlap I'm not sure.

I'd been assuming that you would get two off each vane, and because they are not symmetrical you would see eight in total. They would not each be as prominent as one of the four spikes on standard spiders because they are not reinforcing each other ie you still get eight spikes on a standard 4 vane spider but pairs of them coincide to give stronger spikes.

Stu

[Mike73]

Diffraction spikes are something thats never bothered me at all with newts, intact in a funny way I quite like them.

[JamesF]

I'd been assuming that you would get two off each vane, and because they are not symmetrical you would see eight in total. They would not each be as prominent as one of the four spikes on standard spiders because they are not reinforcing each other ie you still get eight spikes on a standard 4 vane spider but pairs of them coincide to give stronger spikes.

Stu

I think that's perfectly intuitive, but the more I think about it the less sure I am that it's right

I agree that each vane will create two spikes (one either side of the star, effectively).  So actually the design there is optically no different from having two vanes parallel to each other on the same side of the central obstruction and nothing on the other side.  But that is exactly the situation you have with a Bahtinov mask -- lots of parallel vanes creating only one pair of diffraction spikes.

I'm intrigued to know the truth of it now

James

[Stu]

I'm sure one of the clever chaps or chappesses on here knows the answer :-)<br />

<br />

Stu

[JamesF]

In fact, I think it's exactly the same situation has having a single vane in each direction, too, because (I believe) it's the edges of the vane that cause the diffraction effect, not the main physical body of the obstruction.  So, if a pair of edges creates only a single pair of diffraction spikes, perhaps four edges would be no different?

But what do I know about optics?  It's probably just as well that no-one lets me loose with high powered lasers or anything like that.

James

[AlexB67]

Sounds like somewhere between the Dawes and Rayleigh limit for a 12" aperture. 

It depends how you calculate it, Mostly sellers quote the airy disk for an unobstructed mirror, the difference is small. With increasing obstruction the airy disk  actually shrinks slightly. What you see on websites often is a slightly too large airy disk size.   According to my calculator it Is correct for the rayleigh limit given the secondary size for a 12 inch mirror.

[John]

It depends how you calculate it, Mostly sellers quote the airy disk for an unobstructed mirror, the difference is small. With increasing obstruction the airy disk  actually shrinks slightly. What you see on websites often is a slightly too large airy disk size.   According to my calculator it Is correct for the rayleigh limit given the secondary size for a 12 inch mirror.

That explains it then 

[faulksy]

i think it looks great and at that price and your in the market for a dob then this must be better than mass produced surely, if it said tele vue then most would get it

[Mike73]

if it said tele vue then most would get it

hehe aint that the truth! 

[swamp thing]

  I wonder how many diffraction spikes that spider would give?.

Four. My 20" Dob is similar

[JED-E3]

Four. My 20" Dob is similar

That makes sense now, I was thinking that the points where the spider vanes join the tube were equally spaced 90 degrees apart but they can't be.

[John]

i think it looks great and at that price and your in the market for a dob then this must be better than mass produced surely, if it said tele vue then most would get it

What makes you think it's not mass produced ?

I believe it's made, or will be made, by the Jinghua Optics Company who bought Meade last year and also own the Bresser and Explore Scientific brands.

I'm not saying thats a bad thing mind - ES stuff is pretty good as many have found out 

[Mark in Macc]

Spot on John. ES are becoming a real force. I have 3 of their EPs and have been very impressed. Good to have another company shaking things up.

[cathalferris]

Is it just me or do the adjustment knobs for the secondary mirror protrude into the light path? This may cause some interesting flaring.

As for the number of diffraction spikes, you'll see the normal four, as the vanes are co-linear so the spikes from each vane would be superimposed on the spikes coming from the vane opposite.

I'm quite interested in this scope, and if I could combine this scope with a nice lightweight and low-profile EQ table then it may be a realistic option as an upgrade to my current 8".

[Peter Drew]

I've made spider vane assemblies similar to those advertised and can't recall seeing 8 spikes round stars so four would seem to be the answer. The offset arrangement makes for a more rigid arrangement. The method for adjusting the secondary could have merit.        

[bingevader]

Goodness, that's a very keen price. Would be interested to see what the specs on the mirror are.

And if you choose UPS the delivery is still only 9.90 Euros! 

Cheers

[earth titan]

Well I've asked about the mirrors.

If its as good as a Lightbridge (if they are made by the same people) it could be a winner. If they are, I might make a purchase as it is the most compact model I've seen for a reasonable price and lets face it - the biggest drawbacks behind most big scopes are not the optics but:

• Where do you keep them when not being used?
• Can you fit them in the car for holidays?

[michael.h.f.wilkinson]

Sounds very interesting. I am interested in more aperture for galaxy hunting (at 400+ I m pushing the limits of the C8), but my wife would frown on anything that takes significantly more space in the garage

[earth titan]

Just doing a bit or research.

As JOC effectively own Meade, wanna bet these are the same standard mirrors as in the Lightbridge?

Wouldn't make sense to have different production lines for different things, now would it?

[vlebo]

Explore Scientific have a stand at Astrofest this year. I wonder if they will have one on show ? I , for one , will be interested in having a look.