The "Erecting Prism & Plössl" Experiment (1 Viewer)

Well after a bunch of reading I decided I could improve my digiscoping a bit if I changed out a few of the components. My big goal was to squeeze a bit more light and sharpness out of the scope for the overcast mornings I'm constantly subjected to on my early digiscoping morning forays.

I use a Celestron C90 as my scope. I bought it as-sold, which is as a spotting scope with a big aperture. As it turns out what it really is is a compact Maksutov-Cassegrain telescope outfitted for terestrial viewing. Go figure. At least I bought a well-regarded unit in this class of compact telescopes.

In my puttering around online I decided there were some possible gains in brightness, sharpness, and color accuracy by switching out the erecting prism and the plössl (stock Celestron) for TeleVue components. I figured this is a good investment because I'd like to also use it for stargazing, and the prism and Plössl can be moved from telescope to telescope if I upgrade.

And so begins the big experiment. And the orders from Amazon.com.

TeleVue 45° Erecting Prism:
I received the erecting prism last night, a simple 45° prism, just like the stock Celestron. I did some side-by-side viewing with and without the Celestron 32mm plössl, with just me eyes and in the house (the weather is rainy and overcast the last few days).

RESULT: I can't really tell through the plössl at such short ranges, but viewing straight through the erecting prism the difference is quite apparent: the color is more accurate on the TeleVue. The Celestron has a noticeable yellow or yellow-green cast. The TeleVue is more neutral on greys, more vivid on whites, and does indeed seem to be just a small amount brighter (luminance, not color). This can only be a good thing for my photos.

TeleVue 32mm Plössl:
Haven't received this yet, will give feedback once it comes in.

Photos:
This may take longer...darned weather looks to be drizzly, rainy, or at least miserable through the weekend. And I'll have to have opportunities for good back-to-back comparisons.

TeleVue 32mm Plössl:
Received my TeleVue plössl last night. Sure is quite pretty and hefty from an engineering standpoint. The key difference is the TeleVue has a much bigger diameter across the rubber eyepiece. Instead of the viewing lens being a tad bit smaller than the tube/bezel (like the Celestron), the TeleVue has a larger metal rim (perhaps 1/16") around the lens, making for a much larger rubber cup on the eyepiece. The rubber cup also has much softer rubber than the Celestron. Overall, it makes the use of the plössl against my eye much easier and more comfortable.

RESULT: This turned out to be difficult to compare side-by-side because each eyepiece requires a re-focusing of the telescope, they appear to have slightly different focal points or magnification.

The Televue seems to have better color overall. A bit flatter (less vibrant) than the Celestron, but more accurate and more detailed. For example, green ink on a brown paper bag at 4.5m: the Celestron shows the green a bit more saturated, but the TeleVue shows more nuances in the shades of green, which brings out more details in the paper fiber.

Seems like the TeleVue is at once a slightly larger field of view and slightly larger magnification. This could be subjective, though it might have something to do with the TeleVue also seeming to have a much better eye relief (was much easier to keep my eye in a position to see trough the eyepiece).

I can say the TeleVue does have very slightly, but discernible, better depth of field. Focusing on a given object shows just a slight bit more detail in the objects just behind and just in front. This may amount to less than 2mm at 4.5m distance. Put another way, it was much easier to bring fibers and hairs into focus at 4.5m with the TeleVue than on the Celestron. Whether this translates from 4.5m in the house to 200m outdoors photographing a white-tailed kite remains to be seen.

Photos:
This may prove to be tricky. Given that the scope has to be re-focused each time I switch from the Celestron to the TeleVue plössl, and birds aren't famous for sitting still that long, and switching the camera adpater between plössls takes a couple minutes, this may be incredibly subjective.

In any case, I need to wait for good digiscoping weather to even make any sort of comparisons at all.

Looks like...overall the Televue was a bit better.
Seems to have less contrast than the Celestron, but if it has a wider field that's an unavoidable trade-off.

The extra sharpness you noticed several different ways commonly does give the illusion
of higher power. There might be better depth of field, but it might also be an indirect consequence
of sharper resolution....because you can find the reference focus so precisely.

I'd be tempted keep both on hand. Having less resolution but better contrast could be handy
for some targets.

OPTIC_NUT said:

Looks like...overall the Televue was a bit better.
Seems to have less contrast than the Celestron, but if it has a wider field that's an unavoidable trade-off.

The extra sharpness you noticed several different ways commonly does give the illusion
of higher power. There might be better depth of field, but it might also be an indirect consequence
of sharper resolution....because you can find the reference focus so precisely.

I'd be tempted keep both on hand. Having less resolution but better contrast could be handy
for some targets.

Click to expand...

I would agree with your conclusions. For my own digiscoping, any gain in DoF and sharpness is a plus, since both are extremely touchy right now (razor-thin DoF, for example). I understand this is a problem for all digiscoping, but particularly at really long distances and using astroscopes. So any gain is a good thing.

(I knew there would be trade offs, I'm just trying to do the best I can with what I have.)

The color on the TeleVue is the hands-down winner. Never realized the yellow/green cast on the Celestron erecting prism. I'm a graphic designer at heart, and a medium-level pixel-pusher, so I dislike artificial color casts.

I had read the Celestron prism/plössl are "not bad" for the price range they sell the C90 in. So far, I'd have to concur. I managed good photos, certainly acceptable, from the supplied Celestron components. Reviews give the scope itself high marks as a package (price, size, weight, abberration, color cast, etc.). This experiment is to push the bounds of the equipment I have, in preparation for a new camera, hopefully before the end of the year.

I'd rather have a little less contrast but more actual image detail. My current camera doesn't shoot in RAW, and I have to post-process all my photos anyway, so the more data in the histogram, the better.

I hope others find my experience useful. If I had to sum it up in a nutshell: The TeleVue components ARE better than Celestron's, at least for the C90 scope. The improvements are not HUGE, but a good step increment, and important ones.

Followup—Photos:
Well, I decided that doing side-by-side comparisons with the old Plössl-and-prism wasn't really practical, but subjectively...

I was lucky enough to get my usual overcast/party-sunny conditions and my usual White-Tailed Kites as subjects.

The verdict: the depth-of-field made getting a good focus in the tiny little low-res viewscreen on my Coolpix 4500 alot easier, and the resulting photos showed more of the bird in-focus. I consider this a huge win.

The overall light available seemed greater as well.

In the interest of fairness, I was also fiddling with the settings on my camera this time out as well, so brightness could be attributed to that. The depth of field could have been affected as well (by the new camera settings), but not to the extent I'm seeing in the photos and most especially in the viewscreen.

On the contrast: It looks like the 4500 has 4Megapixels. That's still over the 1-2MP typically
viewed on a PC monitor. In post-processing, you can scale down to 60-70%, bump contrast and saturation up
a little, sharpen optionally, and improve your contrast. I often take pics from 16MP to 2MP to improve quality.
More megapixels typically adds more noise than detail, but the excess can be thrown into improving the picture.

Bumping the contrast on non-reduced pics increases the noise...

OPTIC_NUT said:

On the contrast: It looks like the 4500 has 4Megapixels. That's still over the 1-2MP typically
viewed on a PC monitor. In post-processing, you can scale down to 60-70%, bump contrast and saturation up
a little, sharpen optionally, and improve your contrast. I often take pics from 16MP to 2MP to improve quality.
More megapixels typically adds more noise than detail, but the excess can be thrown into improving the picture.

Bumping the contrast on non-reduced pics increases the noise...

Click to expand...

I've been using that trick for years, actually. People always asked "How'd you get that gorgeous shot" when the reality was I shot at high-res to capture detail, then down-res'd and sharpened.

But while that worked well when the typical web page image was maybe 640x480, and home printers were "meh" for quality (and even high-end color printers were "meh" outside MatchPrints), it doesn't really work when the common monitor size is exceeding 21" widescreen and my own monitors are 1920x1200. I can't even make a decent desktop for myself anymore with the Coolpix's images (regular vacation and casual photos are still okay, but digiscoping images, not so much).

The trap I've gotten into is knowing enough to know why the 4500 has been failing me this last year or so. Up the shutter speed to get rid of blur on long shots, it suffers in noise. Turn on noise reduction, it re-blurs. Allow Aperture Priority, the blur comes back. Down the ISO to get rid of noise, the images are too dark.

Now, I know a reasonable amount about Photoshop (graphic designer by trade, pixel pusher for over a decade), but 4MP + Coolpix's limited dynamic range + tiny little sensor are just not enough to "rescue" shots with the aforementioned problems. Just not enough data.

The irony is "full auto" on the Coolpix has probably been taking only a little less worse shots than when I use M, S, or A modes, a credit to Nikon really.

However: despite planning to upgrade my camera soon (with luck), I decided to take my first try at astrophotography during the lunar eclipse yesterday. And on images like the moon, I got some really nice shots for my first try. So there may yet be a place in the toolkit for the 4500, even after I get an E10 for digiscoping or an SX50 (and just skip the scope). ;-p