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Most aperture for 4° TFOV?


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It turns out that there are other "more affordable" solutions out there.

Siebert Optics does custom made 3" eyepieces - there is 42mm 75° eyepiece. That can be paired with ES 3" diagonal and TS152 F/5.9 achromat scope (this part is affordable, first two - not so much).

I can't find data on field stop of 42mm 75° eyepiece, but it comes also in 2.7" flavor - which means it is less than 68mm. According to AFOV/Fields top math - FS should be around 55mm. That is 3.5 degrees of sky at 152mm

And here we go again - just a bit more aperture with just a little narrower TFOV - now you see why I wanted firm 4° limit.

Bresser 127mm F/5 achromat with Aero ED 40 - and yes, this eyepiece is still available as Lancerta ED eyepiece:

https://tavcso.hu/en/product/LA40ed

That will keep things nice and low cost and give quite a bit of aperture. Only thing I can't really get past - I find Bresser refractors really ugly - white paint and that oversized dew shield and that plastic finder base and all of that :D.

Oh nooo - it is 8mm exit pupil. See the problem?

By the way - say we go for 6" newtonian. It would need to have at least 40% secondary to get illumination over whole 46mm field stop (I just came up with this figure - did not do the math, so it can be a bit more or a bit less - but I figure it's about right).

Now if we take 94% per mirror, 4 element coma corrector which is x8 air glass at 99.5% we get - 126.6mm of clear aperture - or about same light gathering of 127mm scope (which itself should be corrected for diagonal and at least 4 air glass surfaces so it ends up being 125mm).

F/4 newtonian would need 28mm eyepiece, so I really need to go with at least F/4.5, but then FL is 675mm and we can't get 4° out of that in 2" format.

 

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My best contender for this is my Televue Genesis f5. With a 31mm Nagler it gives 4.8 degrees using the field stop calculation (5.1 using the afov/mag method), with a 6.2mm exit pupil. Lovely flat field too 👍👍.

With a 41mm Panoptic it would give 5.27 degrees but that would have an 8.2mm exit pupil.

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28 minutes ago, Stu said:

My best contender for this is my Televue Genesis f5.

I don't think you get the game :D

We want to maximize aperture while still giving 4° or more TFOV.

There are many options for 100mm giving 4° or more of TFOV.

But I do appreciate the idea - TeleVue NP127is with 660mm of FL would provide 127mm (of excellent aperture) and paired Pentax XW 40mm (46.5mm field stop?) would give 4° TFOV.

I think that is by far the most expensive realization of large aperture idea though :D - and I'm not even sure Pentax XW 40mm is in production any more.

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5 minutes ago, vlaiv said:

I don't think you get the game :D

We want to maximize aperture while still giving 4° or more TFOV.

There are many options for 100mm giving 4° or more of TFOV.

But I do appreciate the idea - TeleVue NP127is with 660mm of FL would provide 127mm (of excellent aperture) and paired Pentax XW 40mm (46.5mm field stop?) would give 4° TFOV.

I think that is by far the most expensive realization of large aperture idea though :D - and I'm not even sure Pentax XW 40mm is in production any more.

I get the game, it’s just that was my best shot at it. I thought you had already reached conclusion with the 152mm being biggest?

40mm XW is still in production, and on offer at FLO currently, if they can get stock.

https://www.firstlightoptics.com/pentax/pentax-xw-40-r-2-eyepiece.html

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The William Optics 110mm Megrez ED doublet had a focal ratio of F/5.95 so an Aero ED 40mm eyepiece will give a true field of 4.27 degrees at 16x and an exit pupil of 6.72mm.

These scopes are out of production now but when they do come up are not terribly expensive - maybe £600-£700 ?.

 

 

Edited by John
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15 minutes ago, vlaiv said:

I wonder how wide field eyepieces perform at say F/2.8 to F/3.2?

Because if they perform ok - we have a winner then :D

https://www.teleskop-express.de/shop/product_info.php/info/p11772_TS-Optics-200-mm-f-3-2-Parabolic-Newtonian-Astrograph---Carbon-Fibre-Tube.html

 

I’d come to a similar conclusion, however to achieve the 7mm exit pupil constraint you’d be looking for a 22mm fl eyepiece with a field stop of 45mm- not sure I can find one of those

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15 minutes ago, vlaiv said:

I wonder how wide field eyepieces perform at say F/2.8 to F/3.2?

Because if they perform ok - we have a winner then :D

https://www.teleskop-express.de/shop/product_info.php/info/p11772_TS-Optics-200-mm-f-3-2-Parabolic-Newtonian-Astrograph---Carbon-Fibre-Tube.html

 

To keep the exit pupil below 7mm you max focal length eyepiece will be 22.4mm. To get a 4+ degree true field in that scope the eyepiece will need a 120 degree AFoV I think.

 

 

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My best shot might be a Tak Epsilon 180ED with an Ethos 21- gets 4.1 degree FoV with an effective aperture of approx 140mm (80mm secondary and light losses at each reflection or air/glass surface). 
 

I’m not sure if vignetting will kill it though- specs say 44mm image circle - 60% not vignetted. If that’s linear dimension it’s only 26mm so probably going to get more than 10% drop off at the 36mm field stop of the ethos…

Turns out this is a trickier problem of optimisation than it first seemed…

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The ES coma corrector (that doesn’t waste your precious fast speed) and the 21mm Ethos… people are using <f3 reflectors and not reporting crummy views… some people really don’t like ladders at all!

interesting to see a commercial scope <f4, let’s hope more appear:

Peter

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