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Planetary features using the GO 100mm f5.3?

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#1 jdown


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Posted 23 November 2012 - 11:15 AM

I've a new Garrett Optical 100mm f5.3 BT. The optics seem good enough to encourage me to use them for planetary observations. My first impression is that I need more magnification than the 23x mag provided by the 23mm eyepieces that came with the BT. So my questions are, first, is it reasonable to use the 100mm's to see the following, and if so, what eyepieces (i.e., what magnification?) would work best for:
1. resolving the rings of Saturn including Cassini's division
2. seeing the Great Red Spot on Jupiter
3. seeing the shadow cast on Jupiter's surface by any of its 4 moons
4. seeing the polar ice cap on Mars when Mars is farthest from the Earth, 10 degrees above the horizon, on a windy night (no, just kidding!!)

Garrett will, of course, gladly sell me more eyepieces. Should I spring for the 10mm Plossls (53x) or perhaps the 12.5mm (~40x?)? Or be satisfied looking at double stars at 23x?

#2 Mark9473



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Posted 23 November 2012 - 12:49 PM

53x is definitely inadequate for any satisfactory view of the planets you mention. I'm willing to bet that 1 and 2 will be impossible, and 3 very unlikely.

Now, if you want to insist on using this instrument, you could consider getting an eyepiece giving 100x or more and using just one side of the binocular. But be aware that such instruments are not built to optical quality specifications anywhere near that of a basic telescope.

#3 Rich V.

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Posted 23 November 2012 - 01:10 PM

Mark's right; you need some serious magnification. I consider my 71x eyepieces minimum for planetary obs. with my Miya f7.5 100mms. 100x would be more like it but the next step up for the Miyas is 150x which is a bit too much mag but still useful.

Just the same, depending on how well figured your binos are, some planetary viewing is possible. I've watched Jovian shadow transits, seen the GRS, the Cassini division rarely, Plato craterlets on Luna, but not much detail on Mars (not enough mag.). I have to admit, though, that my 80mm apo refractor shows more detail on any of these targets than the binos. Just sayin' that it can be done but there's a lot of CA at these higher mags, even in an f7.5 instrument.

If your BTs are the only optics you have out, it's worth a try at planets but it's better just to use the BTs up to maybe 75x on clusters, DSOs, etc. and of course the lower mags where the BTs excel. Serious planetary observation still is best with a telescope!


#4 charen


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Posted 23 November 2012 - 02:53 PM

I have the BT 100/45 Oberwerk version and have gone to 100x with good collimation, however that is taking the BT past it's limits and CA becomes problematic and really I do not 'see more'.
75x is really the maximum, however that is still pushing the envelope. 60x gives me the best image of superficial details of Jupiter and Saturn and that’s on a steady, night, Mars ? - no.
Tracking becomes problem at the higher mags. Yes, you will see some detail on the brighter planets but really I do not think the large BT 's were designed for that purpose. I have a long tube F.15/108mm refractor for planets. Yes, keep trying but at a later date getting, at a minimum, a ED80 mm on solid mount with a RA drive and let you see more. Even better still a ED100 which can be acquired relatively cheaply – they make very nice planetary scopes. Yes, you will be using one eye to observe but most optical instruments are a compromise.


#5 Goodchild


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Posted 23 November 2012 - 06:40 PM

I used to own a pair of Apogee RA-88 binos (wish I still had them) with 23x, 32x and the optional 40x ep's. I almost always used the 40x ep's in this scope. I can't speak to most of the items you wish to view but looking at Saturn through these binos was one of the most pleasing views I've ever had. The rings were clearly defined with empty space between the planet and the rings. I didn't notice whether I could see the Cassini division but I was just mesmerized with the view. Yes, the image was fairly small but the contrast was extraordinary and Saturn seemed to just float in space. Loved it. M31 was also very nice. Hope this helps.

#6 jdown


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Posted 24 November 2012 - 11:51 AM

Thanks Royce et al. The input from everyone has been helpful. Based on what I'm reading here, the best plan seems to be to view at 23x until I'm thoroughly familiar with what my BT can & cannot show me. Then if I want to pursue Saturn's rings & detail on Jupiter, I can try a pair of 7mm or 8mm eyepieces to boost the mag up to 65x - 70x.

#7 Joad



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Posted 24 November 2012 - 03:34 PM

I had my Oberwerk 100mm BT out last night to double check my answer to this question.

I viewed Jupiter at 86X. At that magnification the chromatic aberration blurs detail quite badly, and the Oberwerk is an f/6. An f/5.3 achromat is only going to have worse CA.

Planets need high power, and not only will CA be excessive at high power, but collimation has to be absolutely perfect. You will be able to see the NEB and the SEB on Jupiter at lower power, but the power needed for the GRS will likely not be doable on an achromatic binocular.

Mars is a very poor sight in binoculars. The CA alone makes high power viewing very poor. And it's tough focusing a binocular on Mars.

Saturn: doesn't take much to see the rings, and you might detect the Cassini division at around 80X, but collimation will have to be perfect.

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