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Is aperture still king even during the day?

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#1 Jobryant

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Posted 10 November 2013 - 03:04 PM

Just wondering if there ever comes a point for daytime viewing where there is no improvement in the view by increasing aperture. Example if two scopes are of the same optical quality is there a difference in the view between a 100mm scope with a 120mm scope during the day?

#2 KennyJ

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Posted 10 November 2013 - 04:58 PM

Great question!

My honest answer simply has to be "I'm afraid I don't really know", having never had the pleasure of looking through anything larger than 102mm in the daytime.

It may well be that if viewing something only a short distance away, e.g a bird 50 feet away(when mirage caused by rising heat etc. doesn't play a significant role)a view of that bird with a magnification of around 100x, through a 4mm exit-pupil, provided by a 400mm APO(if such a thing exists!) would blow away all else available.

I can easily imagine there will be some locations, at certain times of day and year, when temperatures and atmospheric conditions are most favourable, where a similar set-up would undoubtedly yield similarly superlative views above and beyond anything currently available.

However, MOST of the time, in MOST locations, for MOST people, "optimum" magnification of objects or scenery from several miles distance is invariably restricted to around 40x, because of the "mirage" or atmospheric "haze".

So, if we stick to a desired exit-pupil of 4mm(my personal favourite), then I would guestimate that a 160mm objective size would probably provide very close to "as good as it is going to get".

Purely speculative, of course, but as I said, this is a very interesting question, which around nine years ago, on this very forum, would almost certainly have attracted a more duly deserving array of responses than it almost certainly, and most sadly, will now.

Kenny

#3 Mark9473

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Posted 10 November 2013 - 06:05 PM

I agree with your reasoning, Kenny, but 40x is perhaps a low estimate on many days. There has to be a good reason why many spotting scopes come with zoom eyepieces going up to 60x magnification.

My exit pupil tolerance is about 2 mm. Below that and I find that colours and contrast go out the window fast.
So I guess that means I could probably be happier with a 120 mm scope (or the 107 I already have) than with the 80 mm I'm typically using in the daytime.

Either way I think on most days there won't be any practical difference between a 100mm and 120 mm scope - that's a small difference in aperture anyway. But I have found myself using 138x in my 80mm in daytime and really wishing I had an 8" scope to put the colour back onto the deer I was watching on the next mountain.

The remaining question is, do you want to have the aperture available for those few days when the higher magnification works fine, but knowing that on many days your lugging around a heavier instrument for nothing.

To the OP: since you have a 114mm scope, please use it several times in daytime with and without a 95mm aperture mask, and then please come and tell us what you found. ;)

#4 mooreorless

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Posted 10 November 2013 - 08:55 PM

This sort of reminds of another thread. I would have to say yes.
http://www.cloudynig...ays/Number/6...

#5 Jobryant

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Posted 11 November 2013 - 01:15 AM

I will make a couple aperture masks and test it out. One of the main reason I asked this question is I just purchased an ED120 to compare it to my ED114. Since my ED114 is such a fast scope I wanted to see if a slower one of similar aperture would be a noticeable improvement in sharpness and it was. I don't know if the reason the ED120 was sharper is because it's a slower scope or if it's something else. I did notice that I prefered the color tone better in my ED114 though. The 120 was too cool for my liking but I think the improvent in overall sharpness is worth it and I should get used to the cooler tone with time. My biggest dilemma is the ED120 is just a tad bit too big for me. As much as I like it I just don't think I'll be able to handle it. So I'm looking at getting a Tak fs-102NSV in hopes that it will give me the same results as ED120 just in a shorter package. I just don't know how going from 120mm to 102mm is going to effect the overall view. There's a few more things I'd like to add but I'm typing this on my phone and its a pain in the butt so I'm going to call it a night for now.

#6 Mark9473

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Posted 11 November 2013 - 06:20 AM

Interesting situation. These scopes are hardly ever discussed in the context of daytime viewing.
I don't think the difference you noticed in sharpness in going from 114 to 120 is related to the aperture. It's either the inherent optical quality of the scopes, or the suitability for fast f/ ratios of the eyepieces you're using.

The Tak 102NSV is only going to be about 2" shorter than the ED120, right?

#7 Jobryant

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Posted 11 November 2013 - 12:14 PM

I have the non-retractable ED120 and the shortest I can get it without taking the dew shield off is 36 inches. I believe the shortest the 102NSV can get is right around 26 inches. The retractable dew shield version of the ED120 goes down to 32 inches(I think) but it weighs 3 pounds more then the non-retractable version that I have.

After using the ED120 for a few months now I'd say my max size and weight for a day time scope would be 12 pounds(bare ota) and about 30 inches long. Anything longer then 30 inches doesn't fit in my current backpack and an ota over 12 pounds would probably require my to get a new mount.

Back to the topic at hand. It sounds like one needs to consider more then just aperture but in general the more the aperture the better even when used during the day. At least this is what seemed to be implied from mooreorless's post even though I couldn't get the link to work.

So far I have a hard time seeing a difference at the eye piece between the ED114 and ED120. Besides the increased magnification of the ED120 the only other major difference is the color tone. I have yet to use either of the scopes at dusk or dawn so there might be a bigger difference during these times of day. Videoscoping is where I start to see the differences in the sharpness and clarity between the two but as stated earlier this might have nothing to do with aperture. I'll try and post some pics or videos of the differences I'm getting between the two scopes.

As far as masking the ED114 I actually have an iris in that scope and the more close the iris the sharper the scope gets to a certain point at least but at the cost of color vibrancy and detail. The only problem with this is that I currently have no way to tell how much I am masking the scope with the iris so I don't know at what aperture that I'm getting my best results at. I'll have to do some more experimenting when I get the time. Hopefully I can get a hold of a Tak fs-102nsv soon enough to compare with the other two scope as well.

#8 KennyJ

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Posted 11 November 2013 - 12:25 PM

Mark,

I simply based my 40x "optimum" magnification figure on impressions and results after using the 20-60x zoom lens on my Zeiss 85mm Diascope for around 6 years.

Perhaps the "slightly above 2mm exit-pupil" had as much to do with that as anything else, but there were definitely many days when I felt there was absolutely nothing gained and a little lost by tweaking the zoom up to 50x.

Kenny

#9 snorkler

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Posted 11 November 2013 - 12:58 PM

I don't have any scientific data or theory to back the birding community opinions, but the consensus there seems to be that aperture only matters in dark conditions.

In full daylight, an excellent 60 mm. scope gives just as good views as an excellent 100 mm. scope. In daylight, atmospheric heat waves often limit top magnification to 40X or even 20-25X. 60x is considered to be the top useful magnification for diurnal terrestrial use.

Well-heeled forest birders, owlers, and crepuscular birders often haul around 80-100 mm. apochromatic spotting scopes, but I see a lot more 60-80 mm. achromatic scopes used by birders in the field, because of budgetary restraints. It's also likely sheer size and weight limit birders' ability or willingness to haul around a 100 mm. scope and tripod.

#10 KennyJ

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Posted 11 November 2013 - 01:31 PM

If it's of any significance, at all magnifications, my TeleVue 76 provides much sharper/ more detailed/ more pleasing views in the daytime than my 102mm Chinese short tube.

#11 curiosidad

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Posted 12 November 2013 - 10:23 AM

Hello,
Many birders prefer 30X for most situations .... which does not mean that in those "perfect days" can be reached with good resolution at 50X-60X or some more ....As for the size of the telescope I think there is now a trend towards smaller diameters ..

#12 star drop

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Posted 17 November 2013 - 02:14 PM

I would say that aperture helps. When looking at a electric transmission tower from a distance of one eighth mile one day I noticed a tiny black dot at the top. Using 10x70 binoculars revealed that dot to be a bird. Using a 635mm (25") reflector at 98.5x magnification I identified the bird as being a crow. There was a fly flying around pestering it and I could see the crow blink its eyes.

#13 KennyJ

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Posted 17 November 2013 - 02:23 PM

What about the fly's eyes though, Ted?

#14 star drop

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Posted 17 November 2013 - 03:17 PM

The fly's eyes were easily visible. Whenever the fly rested on the tower I could see it in enough detail to be certain that it was a common housefly. I was surprised that the crow tolerated the flies persistence.

#15 KennyJ

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Posted 17 November 2013 - 06:30 PM

What about the fly's persistence?

Did it tolerate that? :-)

#16 Hikari

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Posted 20 November 2013 - 11:35 PM

I doubt you would see a difference between 114mm and 120mm apertures. The quality of the optic would be more defining.

#17 Jobryant

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Posted 21 November 2013 - 10:45 AM

I think your right Hikari. I think the differences I'm seeing between the 120mm and 114mm have more to do with their focal ratio then aperture.

I've been doing a few test with aperture masks on my 114mm scope but I wasn't satisfied with the results. Mainly because masking a scope slows it down and I'd like to compare different apertures with similar focal ratios. So I just bought a Sky 90 to compare with the 114mm scope it should be here next Monday. Both are very fast scopes with the 114 at f/5.3 and the 90 at f/5.6. Comparing these two scopes should help cure my curiosity.

#18 Mark9473

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Posted 21 November 2013 - 01:35 PM

Now that's the spirit! Want to compare? buy a scope! I'm looking forward to that. My money (figuratively) is on the Vixen.

#19 mooreorless

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Posted 22 November 2013 - 05:53 PM

I don't see that "much" difference between my Nikon 50ED and Nikon 60ED Fieldscopes. ;) I can see more with a Celestron 80ED and even more with an Orion 100ED, higher powers etc. etc. etc. This is daytime use and I beg to differ on the 60mmED[excellent} giving just as good views as a 100ED scope during the daytime. But that is just me having views as far as 2.8 miles & [5 miles] away and looking quite often, well the side of the mountain looking for deer etc.



#20 Jobryant

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Posted 28 November 2013 - 09:50 AM

Got the Sky90 a few days ago and so far I like it a lot. Only bad thing is that I can't reach infinity with my modified diagonal with iris and mag switch. So unless I can figure something out to get it to reach focus with my diagonal setup the little Sky90 will probably not be replacing the 114mm which is a real bummer because I'm really digging the Tak.

I've only been able to use the Sky90 during the middle of the day and during that time I find I like the Sky90 just a bit better then my Vixen 114. Contrast and sharpness are just a hair better with the Tak and the overall view is just more pleasing to me. So far the only EP used has been a 30mm leitz and a 12mm Ortho so I have yet to push the tak to see how it performs at higher powers. The main test for me and my uses will be to see how the Tak does during dusk and dawn timeframes and how well the Sky90 will do with videoscoping.

#21 Mark9473

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Posted 29 November 2013 - 06:02 AM

Please keep us posted on the Sky90. Will you be testing it at night too?

#22 Jobryant

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Posted 29 November 2013 - 11:27 PM

Probably not going to compare the scopes at night. Planning on doing a side by side test during the day with the Sky 90, Vixen ED114SS, Sky-Watcher Pro ED120 and my little custom Nikon ED50. Out of the 3 big scopes I'll be just keeping the one I like best and selling the other 2.

#23 Jon Isaacs

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Posted 30 November 2013 - 04:37 AM

Only bad thing is that I can't reach infinity with my modified diagonal with iris and mag switch.



I am quite sure that if the iris is in the diagonal then it is close to the focal plane and would not act as an aperture mask.

As a birder, I think aperture choices are made on the basis of portability rather than maximum possible performance. Some situations allow for high magnifications but hauling around a 120mm refractor and tripod is quite a task..

As far as the typical 60x limit on spotting scopes, there are probably a lot of reasons for that, the need for a more stable mount at higher mags, various optical compromises induced by the more complex light paths and optics as well as the generally poor seeing..

Jon

#24 KennyJ

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Posted 30 November 2013 - 06:50 AM

Yes, Jon, I agree.

Another restriction would be an exit pupil below around 1mm being undesirable for the majority of people and daytime situations.

#25 Jobryant

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Posted 30 November 2013 - 11:24 AM

Technically I'm not a 100% sure if iris I installed is working as it is supposed to. All I know for sure is that it is doing what I want it to. To be clear the iris is not located directly in the diagonal but attached to the outside of the diagonal between the diagonal and the scope. I don't know the exact science behind it. I just tinkered with it until I got it to work the way I wanted it to. Being that the Vixen ed114ss is such a fast scope I find I use the iris quite a bit especially during bright conditions were I don't need as much aperture. By closing the iris I can effectively clean up the image quite noticeably without losing very much light.






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