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Achro versus Apo


NGC 1502

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12 hours ago, chiltonstar said:

....ctd. Better than diffraction limited won't change the shape of the diffraction pattern though will it?

Chris

Not the spacing between the diffraction ring and the airy disk perhaps but I think the intensity of the diffraction ring is affected by the figure of the lens :icon_scratch:

The Vixen does resolve Delta Cygni quite nicely though - I've got it out tonight. At 200x the secondary star appears to be just touching the outside of the diffraction ring, rather like a pearl mounted on a ring :smiley:

Also got a nice split of Pi Aquillae at 221x which was pleasing.

 

 

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8 hours ago, John said:

Not the spacing between the diffraction ring and the airy disk perhaps but I think the intensity of the diffraction ring is affected by the figure of the lens :icon_scratch:

The Vixen does resolve Delta Cygni quite nicely though - I've got it out tonight. At 200x the secondary star appears to be just touching the outside of the diffraction ring, rather like a pearl mounted on a ring :smiley:

Also got a nice split of Pi Aquillae at 221x which was pleasing.

 

Never tried Pi Aquillae so will put that on my ‘to do’ list with my 6” or 10” Dob. As expected my Dobs have no trouble at all  with Delta Cyg, but I prefer the way refractors resolve double stars. I lose resolution with my Vixen 80/910, but the doubles it will resolve look delightful. My Pronto also surprises me sometimes with what can be achieved, it may not have the posh glass but it’s a thoroughly enjoyable little scope to use. Was out there with it last night. I suppose a TV 85 would be a great replacement for both my current refractors, from very low wide field to top quality high power but in one neat package.

What a fabulous run of clear nights, but my OCD with being under the stars if it’s clear mean I could do with a break.....but we get loads of long breaks as we know only too well..........

Ed.

Edit - my auto spell checker had a right royal game with “Pi Aquillae”..............?

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8 hours ago, John said:

Not the spacing between the diffraction ring and the airy disk perhaps but I think the intensity of the diffraction ring is affected by the figure of the lens :icon_scratch:

 

If there is spherical aberration, the rings are fattened and the intensity is raised, but where optics are "diffraction limited" and without aberration, the pattern should be the same according to the equation I think :icon_scratch::icon_scratch:?? Never mind, it's what is seen through the eyepiece that matters! Pi Aquilae is a very lovely pair!

Chris

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Pi Aquilae (SkySafari has it with  one l?) is well worth a go Ed. Relatively similar brIghtness and 1.4” split, can be a challenge depending on conditions.

Oh, and the TV85 is a cracker :)

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My friends and I split Pi Aquilae with our club's huge and unwieldy 400mm newtonian fitted with a 150 or 160mm off-axis mask. The image was so quiet it felt completely different from a newtonian. The much blacker background did its part in the alien-looking "huge newtonian" view.

I'm not sure of the numbers but I made several off-axis masks for that scope, they slide over a fixed frame. Their diameter is from 135mm to 155mm in 5mm steps, or was it from 140mm to 160mm? I made several in order to always have a little more than the diameter of a large popular apo, the 130, and not the maximal diameter all the time. Instead we can optimize it according to the seeing.

As the maker of these masks, I asked those would use them the most to tell the result of the comparo but they either forgot to tell me about it, or they forgot to do it.

Kinda off-topic but not completely, we're talking about what makes the best Airy disks, right? A stopped-down large newtonian makes sharp ones, almost like an apo. There, we're back on topic again! ?

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10 hours ago, chiltonstar said:

If there is spherical aberration, the rings are fattened and the intensity is raised, but where optics are "diffraction limited" and without aberration, the pattern should be the same according to the equation I think :icon_scratch::icon_scratch:?? Never mind, it's what is seen through the eyepiece that matters! Pi Aquilae is a very lovely pair!

Chris

The optical theory is a bit over my head Chris - I tend to go with what I see at the eyepiece as you say :smiley:

I was quite pleased to get Pi Aquilae (or Aquillae) with my Vixen ED102SS last night. Later it also showed the whole of the Veil Nebula with my 31mm Nagler and the Lumicon O-III filter in the diagonal. Pretty versatile little scope :biggrin:

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So where do Petzvals fit in?

My F5 TV Genesis & the current Vixen 140 have achromatic long FL front lenses, with the correcting,  FL shortening lenses, giving  some CA reducton, but nice flat fields for space walk simulation etc. 

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I reversed the thread title in a way last night by having my Vixen optic 102mm f13 achro scope on a mount with my ED80 (Apo or semi-Apo), looking at Saturn.

Both views were glorious - for most of the time there were patches of excellent seeing, with heat "wobbles" of course. The ED80, showed (as usual) a very sharp ring outline, nice banding on the planetary disk, very obvious ring-on disk and disk-on-ring shadow lines. Cassini was sharp and visible most of the way around. The Vixen achro was even better. Cassini was thinner and sharper, the shadows better defined and the larger aperture brought out the C ring better. Best of all, there were transient moments where ring detail was visible between Cassini and the C ring - glorious.

The conclusion? Refractors are great!

Chris

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