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Old 19-10-2018, 07:37 PM
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silv (Annette)
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102/500 & binoviewer in the city: which filters and eypiece?

My questions are mostly regarding visual, not imaging.
I'd like to sweep the milky way band, enjoying star fields, hover over a cluster if I find one.
If possible with this scope, maybe shooting MOV of a planet and the sun. Not really interested to "see" grey-ish smudges of DSO, although saying Hello to the Andromeda galaxy via averted seeing would be a nice moment.

I'm awaiting my SW 102/500 simple refractor with different focuser and 2" to 1,25" zenith prism, and the Arcturus binoviewer with 30mm eyepieces and 1.7 and 2.5 correctors.

1. Should I get 2 simple moon filters or rather one (or 2) which hides 25% transmission? I thought maybe looking at the sun (through ND 5 solar foil of course. Got that here from a DIY hood for my 500mm lens) the 25%-filter could be beneficial on the sun, as well?


I'm not sure at all how the binoviewer on the fast refractor will impact the light path and focal ratio. It surely must make the scope slower? Some say 2 f-stops, even. Making the scope a planetary one? That'd be grand. And in spring/summer here on 54th latitude, the sun will be all I can see, astronomy wise. It'll be too bright for stars during the "night" at comfortable hours, I assume. For both, planets and the sun, the 25%-filter seems a better investment than a cheap moon filter, isn't it?

2. And what about light pollution, filter, and fast-ish refractor with binoviewer? I'll be using the system directly in a big city, although looking out on Western skies which get a little less direct light from below than my own location. I can't tell the Mag-number for light pollution...(how can I tell?) - population is 210,000. Looking through a 7x50 at the milky way band IS a real pleasure. But I know I "should" be seeing many, may more stars, really, if it weren't for the light pollution. Would a "moon and skyglow" filter improve the star count?

3. Eyepiece. Is it a good choice to get 2x15mm? I understand the binoviewer plus possible light path corrector act as ~2x barlow. So in this imitated 2xbarlow on 102/500 (or 102/600 as the bino adds 100mm to the light path), 15mm eyepieces would be enough sensible magnification?

Thanks heaps for your input!
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Old 20-10-2018, 07:09 AM
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Atmos (Colin)
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I've found that while using binoviewers I've never wanted to dim the image with a moon filter. Because it splits the light between both eyes it effectively already works as a 50% decrease in brightness per eye but you don't notice it because you're using both eyes and your brain is amazing
So, I wouldn't bother about a moon filter at all personally.

The binoviewer itself doesn't increase the telescope focal length; ie. your telescope has a 500mm focal length whether you use a diagonal, binoviewer or straight through.
The 1.7 and 2.5 GPC do work as 1.7x and 2.5x barlows though and you'll likely have to use at least the 1.7 for the binoviewer to come into focus.
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Old 20-10-2018, 07:12 AM
astro744
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WARNING. To view the Sun safely you must use an approved solar filter at the front of the telescope before any light passes through the optics. Not having an approved filter at the front and even having an ND5 filter at the eyepiece or diagonal end will cause excessive heat build up at the filter, eyepiece, eye!

Approved filters typically have a metal alloy, i.e. Nickel Chromium, Aluminised Mylar to block harmful IR and UV.

As for Moon filters and especially for binoviewing, you wont need one as the binoviewer splits the beam. In fact even for mono viewing of the Moon you will find no filters fine.

Note your binoviewer doesn't add to your focal length so your 500mm telescope will still have a 500mm focal length whether the binoviewer is used or not (without Barlow attachment). The binoviewer does chew up approx. 100mm (depending on model) of focal length so you will have to rack inward an extra 100mm to achieve focus with the binoviewer attached than without. Depending on you telescope you may or may not have that much inward travel available in which case you must have a Barlow attachment (say 2x) or one of your 1.7 or 2.5 correctors which are in essence a Barlow attachment. This will push out the focal plane and allow you to achieve focus within the focal travel of your focuser. It will give you higher powers, 1.7x or 2.5x depending on which corrector you use.

With your telescope you get the following:

102mm aperture
500mm focal length
f4.9 focal ratio (It will likely be labelled f5).

Magnification = telescope focal length/eyepiece focal length
exit pupil = aperture/magnification
exit pupil = eyepiece focal length/focal ratio
true field of view =apparent field of view/magnification.
(There is another method to calculate TFOV but requires field stop diameter of eyepiece)

No binoviewer or with binoviewer but no corrector, 30mm Plossl/50 deg.
Mag=500/30=16.7x
TFOV=50/16.7=3deg. (nice and wide especially for Pleiades)
Exit pupil=102/16.7=6.1mm
Exit pupil=30/4.9=6.1mm

With binoviewer and 1.7x corrector and using results above:
Mag=16.7x1.7=28.3x (Note Mag. is 1.7x greater)
TVOV=3/1.7=1.8deg. (Note TFOV is 1.7x smaller)
Exit pupil=6.1/1.7=3.6mm (Note exit pupil is 1.7x smaller)

Note the image with a 3.6mm exit pupil will be dimmer but there will be more contrast. In fact at 6.1mm in the city will give you a more grey sky background less than say a 7x50 binocular gives. (50/7=7.1mm).

With binoviewer and 2.5x corrector and using results above:
Mag=16.7x2.5=41.2x (Note Mag. is 2.5x greater)
TVOV=3/2.5=1.2deg. (Note TFOV is 2.5x smaller)
Exit pupil=6.1/2.5=2.4mm (Note exit pupil is 2.5x smaller)

With a 15mm Superview eyepiece (68deg apparent field) magnification for above is doubled in all cases and exit pupil is halfed. However as apparent field is 68 and not 50 the true field is 68/50=1.36x greater.

e.g. 15mm/68deg eyepiece and 2.5x corrector in binoviewer
Mag=500/15x2.5=83.3x
TFOV=68/83.3=0.82deg. (nice for the Moon)
Exit pupil=102/83.3=1.2mm
Exit pupil=15/4.9/2.5=1.2mm

83x will show you the bands on Jupiter and the rings of Saturn but it is a little on the low side for planetary viewing so you may consider a shorter focal length pair of eyepieces if that is what you like viewing. (Say 7-10mm). However your telescope is a fast (f5) refractor designed for low power wide field viewing will show all objects nicely at low power but does have chromatic aberration (as all achromatic refractors do) and is is particularly severe and shows itself when viewing bright objects (e.g. planets) at high powers. It is not an ideal planetary 'scope but nonetheless can provide pleasing views and can provide that wow factor especially if you've never seen a planet in a telescope before. Adding Barlow to change the effective focal length does not make it a slower focal ratio for the purposes of chromatic aberration effects. It is still f5 for this unfortunately.

As for light pollution filters nothing beats a dark sky but a broad band filter may work to provide a little darker background in your case without taking out too many stars. If you can borrow and try it would be better than buying as there are so many filters out there with so many different uses. I would perhaps try a Lumicon Deep Sky or Astronimik UHC-E on the 30mm at low power viewing and perhaps a Lumicon UHC for mid range power on objects such as Eta Carina and Orion Nebula. The original Tele Vue Bandmate Nebustar type works well on smaller refractors and would be idea for your telescope. Note though if you keep the 1.7x corrector in place your exit pupil will be 3.6mm which is ideal and will produce a nice darker background and you may not need a light pollution filter, (depends on your city lights).

Whatever you choose, enjoy and remember safety first for solar viewing (and don't forget to remove of cover you finder telescope if solar vielwing with the main 'scope (proper filter attached on it).
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Old 20-10-2018, 09:04 AM
Hoges (John)
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If you find the moon a little too bright, pop the lens cap back on minus the smaller 2" lens cap and presto, you've got a 50mm f10. I also use the smaller opening in the lens caps when I'm viewing the sun as I've attached some baader solar film to the inside - that actually works pretty well.


You will need some sort of barlow for the binoviewer to come to focus. I have a 4" F5 that I cut the tube a couple inches shorter and attached a low profile crayford focuser - this allowed the binoviewer to reach focus without having to use a barlow. I probably ended up cutting into the light path/light cone ??? by shortening the system this way with an F5 scope but I can't tell visually and it worked just fine. (see attached pic)



You certainly lose a bit of image brightness with the binoviewer. If you've ever switched from a mirror diagonal to an erecting prism and noticed the image was noticeably duller......well, it's twice as worse with the binoviewer. You can always buy a second 100mm F5....
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Old 22-10-2018, 04:40 PM
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silv (Annette)
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Okay, Atmos, I'll be forgoing the filters for now. Thanks. Since pondering going visual, I read a few observation reports online. But none were done with (my) binoviewer or from city lights. So I'll wait and see how grey washed-out or black contrasty the background will actually be.

@Hodges, Good advice about the dust cap, - and I’ll consider the 2nd 102/500 if someone feels the urge to donate 2 EMS .
Thank you both!


Astro744, man, great teaching, very much appreciated. I'm glad I don't own a printer. So, because I wanted to have your info available on paper, I wrote it down by hand. Which, in the process of writing, edged the info into my brain circuity much deeper than e-reading or typing ever can. Handling pen and paper triggers more brain areas for me, it improves storing stuff in memory. Thank you very much for your time and the info.

And you’re right about the warning re sun observation. I’d have thought about fitting the main scope with ND5-foil - but I guess I would have indeed forgotten about the finder scope. The ones I made for my 7x50 should fit. I’ll remember using them.

Re the Nebula filters you recommend: I might get tempted to look at DSO. But I'm spoiled from all the colour DSO images so I can't imagine I'd be enjoying looking at them in b/w ... with averted viewing even ... as much that I feel like I'd have to spend money on getting a slightly better image.
I really only want to have back what I experienced under New Zealand skies. Feeling myself connected with the vast starfields and beyond, getting drunk on the cocktail of black velvet and diamonds and the emotionally overwhelming promise that there's so much more to know.
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Old 22-10-2018, 04:43 PM
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silv (Annette)
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From what I understand now regarding my fast refractor, it'd be unfair to put 9mm eyepieces into it. But I have this Maksutov-like camera lens here, with 500mm f/6.3 essentially a 79mm reflector without colour falseties. With a T2 15mm spacer and a very short diagonal the binoviewer should get into focus alright. Tested it with a 10mm eyepiece in a 30mm T2-1.25" holder and there was still room. (The 15mm spacer is necessary to clamp on a mounting bracket bc handheld is impossible due to the lens body IS the focuser ring.)
Maybe that weight plus bino is still okay for my nano.tracker, too, so binoviewing planet or moon on it could be relaxed fun.
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