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FlorinAndrei
scholastic sledgehammer


Reged: 09/28/10

Loc: California
Re: What point is a 2" focuser on 6" dob? new [Re: galexand]
      #5541495 - 11/27/12 03:53 AM

Quote:

there is a point to buying a 2" EP for any scope that will take it: the wider field of view will help you in a "finder eyepiece".




6" f/8 newtonian tube with a 2" focuser and a 1.83" secondary (30% the size of primary). It gives me over 2 deg TFoV in a 30 mm ES82 eyepiece, pretty decent illumination at the edge, actually. The Pleiades fill the whole field and are visible pretty much entirely at once.

It's nice that I can use good 2" eyepieces that would otherwise be unusable in a classic 6" reflector with a small focuser.

I can also do prime focus photography with a Micro Four Thirds camera, using a low profile T-minus adapter, which wastes much less back focus than a classic T-adapter/T-ring and allows the back focus to stay small (also helped by the small flange-focus distance of a MFT camera). The sensor gets decent illumination all the way to the edge - not 100% perfect, but good enough for Moon and solar shots.

Incidentally, the entire lunar or solar discs fit on the sensor entirely in one shot, with a little extra room on the edge. That would not be doable in a small focuser at this focal length. The results are fairly pleasing:

http://imgur.com/a/TGqlG#0

The price paid for all that is the large 30% secondary. But in 9 cases out of 10, atmospheric turbulence blurs details much more than the effects of the large secondary. When seeing is good, at high magnification it resolves the galilean moons as tiny discs, no problem (but without any surface details, of course). So the large secondary ain't a big issue, if the rest is in good shape.

This is the scope I use to do demos for my kids, their friends, my friends, neighbors, random passersby on the street, etc. Being small it's perfect for ad-hoc sessions.

So yeah, it's totally worth it having a 2" focuser on a 6" reflector for a general purpose, flexible instrument. This is not a single-purpose "apo killer" with a tiny secondary.

Edited by FlorinAndrei (11/27/12 04:01 AM)


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: What point is a 2" focuser on 6" dob? [Re: galexand]
      #5541513 - 11/27/12 04:26 AM

Quote:

Wow! Quite a few interesting posts here. I don't know that anyone answered your practical question though.

If you're like me and you don't use a finder scope, then there is a point to buying a 2" EP for any scope that will take it: the wider field of view will help you in a "finder eyepiece". Poorly-illuminated, ridiculous giant exit pupil, no matter. Having a 2 or 3 degree true field of view to look through makes star hopping *much* easier. To say nothing of the aesthetic delight of such a rich field.






I think the original question was whether a 6 inch F/8 Dob with a 1.33 inch secondary could take advantage of the larger field of view offered by a 2 inch eyepiece and the short answer is yes...

Jon


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markgf
super member


Reged: 10/31/12

Loc: Gold Coast, Qld, Australia
Re: What point is a 2" focuser on 6" dob? new [Re: Jon Isaacs]
      #5541669 - 11/27/12 08:34 AM

OK then! Guess I'd better start saving for a 2" wide field eyepiece and a 2" barlow/telextender.

A pity the Orion 6" dob only has a 1.25" focuser, but the Saxon/Skywatcher 6" dobs don't have intelliscope. Oh well.

Thanks All...Mark


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howard929
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Reged: 01/02/11

Loc: Low End of High Ground
Re: What point is a 2" focuser on 6" dob? new [Re: markgf]
      #5541794 - 11/27/12 10:22 AM

Depending on how much time and money you'd want to invest into that telescope, this focuser might be a worth while upgrade if you can see yourself opening the hole in the OTA a bit larger.

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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: What point is a 2" focuser on 6" dob? new [Re: markgf]
      #5541990 - 11/27/12 12:09 PM

Quote:

OK then! Guess I'd better start saving for a 2" wide field eyepiece and a 2" barlow/telextender.

A pity the Orion 6" dob only has a 1.25" focuser, but the Saxon/Skywatcher 6" dobs don't have intelliscope. Oh well.

Thanks All...Mark




That's the catch 22. And if you spend the money to add the 2 inch focuser to the 6 inch Orion, you might as well just buy the 8 inch Orion.

There was a short time when Optics Planet was selling the 6 inch Synta Dob under the Bushnell name with the 2 inch focuser for $220 shipped to your door.

That was a deal.

Jon


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BDS316
Carpal Tunnel


Reged: 09/16/09

Loc: Sol 3
Re: What point is a 2" focuser on 6" dob? new [Re: Jon Isaacs]
      #5542270 - 11/27/12 02:49 PM

" There was a short time when Optics Planet was selling the 6 inch Synta Dob under the Bushnell name with the 2 inch focuser for $220 shipped to your door.

That was a deal. "

You got that right... after they went on sale, Buschnell realized that it was too good a deal and they were yanked, never to be seen again. My club bought one of these to test out for our Young Astronomer's Program which awards 5 or 6 6"f/8 dobs to deserving 4th to 8th graders annually.(we are a 501C3) It tested great. They wouldn't let us order any more so we got XT6's instead.


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markgf
super member


Reged: 10/31/12

Loc: Gold Coast, Qld, Australia
Re: What point is a 2" focuser on 6" dob? new [Re: galexand]
      #5544657 - 11/28/12 10:03 PM

Greg
Yes, I'm hoping a telrad or similar plus a wide field eyepiece will see me set for finding, and with a nice exit pupil in an F8 scope.

Jon and Florin.
Ummm...OK, so the real FOV is identical to the angular width of the field stop if viewed from the centre of the objective lens. (Or from the centre of the primary mirror in a reflector, assuming the secondary mirror's angular width isn't less than the field stop's).

but:

1. Why is it assumed that the rays from the distant object (the top, middle and bottom of a distant tree, say) all pass through the centre of the objective lens? http://www.novac.com/nl/91/Fov.pdf . Surely they spray all over the objective. Is it that the spray from a particular bit of the tree is either focused to the same point as the ray through the middle, or else refracted/reflected completely away from the field stop? (And I thought I was finally getting it. At least I'm zeroing in on it.)

2. Just confirming: Is the focal plane located at what I used to think of as the focal point? (ie one focal length from the objective/primary?)

Cheers...Mark


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FlorinAndrei
scholastic sledgehammer


Reged: 09/28/10

Loc: California
Re: What point is a 2" focuser on 6" dob? new [Re: markgf]
      #5544904 - 11/29/12 01:09 AM

Quote:

the real FOV is identical to the angular width of the field stop if viewed from the centre of the objective lens.




Exactly. That's the angular size of the chunk of sky visible in the scope. That's the keyhole. Commonly called, not "real", but "true" field of view, TFoV. As opposed to AFoV, apparent field of view, in the eyepiece.

Quote:

1. Why is it assumed that the rays from the distant object (the top, middle and bottom of a distant tree, say) all pass through the centre of the objective lens?




No, they don't.

Quote:

Surely they spray all over the objective.




Yes, they do.

Quote:

Is it that the spray from a particular bit of the tree is either focused to the same point as the ray through the middle




Correct. You're doing very well, you just need a bit more practice.

I used the central ray simply because it illustrates my point more easily. But indeed light from the object takes advantage of the entire aperture, not just the center.

Rays emanating from the same point in the object, pass through the objective, then all converge in the same point in the focal plane. The image of a point is a point.

In reality, they don't converge quite in the same point, just close enough, and the difference from the ideal perfect point is called "aberration". When you make a telescope, you try to minimize aberrations, you try to coax the rays to converge as close to each other as possible.

In some cases, the perfect point convergence is actually achieved. E.g., a high quality parabolic mirror has zero aberrations on-axis; the image of a point-like object, situated at infinity, on the axis, is a perfect point when reflected in a parabola.

Now move the object slightly off-axis and even a perfect parabola starts to show a little bit of aberrations. But in practice it's quite close to perfection if it's just slightly off-axis.

Quote:

2. Just confirming: Is the focal plane located at what I used to think of as the focal point? (ie one focal length from the objective/primary?)




Indeed.

The "focal point" is merely the image of a point-like object, on axis, at an infinite distance from the objective.

Now, we call it focal "plane" but in reality it's not exactly flat. For the vast majority of objectives, it's actually a shallow curved surface - but close enough to flat if you're not doing ultra-wide angle stuff.

Also, due to aberrations, the rays may not converge all in the exact same place, so the definition of the focal "plane" can be a little bit arbitrary for non-ideal objectives. It's just the locus where the rays emanating from the same point in the object come closest to each other.

Edited by FlorinAndrei (11/29/12 01:22 AM)


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markgf
super member


Reged: 10/31/12

Loc: Gold Coast, Qld, Australia
Re: What point is a 2" focuser on 6" dob? new [Re: FlorinAndrei]
      #5544982 - 11/29/12 05:13 AM

Woo hoo! I've got it!!

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