After all, I spent so much on my smart telescopes, why don’t they really have great planetary imaging capability? Well, actually we can observe the planets, but we’ll probably be disappointed with most of the smart telescopes. (Vaonis’ Hyperia does pretty well, but it’s way beyond my budget or capability of being sited at my back alley observing location.) The best I’ve done is with my eVscope is on Jupiter (see first image below), but it’s really not as good at all as just looking through my 4 inch Celestron. So, really, why should we not be able to get good planet images with the smart scopes?
The answer is really not too difficult to understand. It all has to do with focal length and the sizes of the imaging chip pixels. Knowing both numbers we can calculate the angle of the sky subtended by an imaging pixel with our scopes. Since the planets are so small, it really doesn’t matter how many pixels are in the sensor. Even Jupiter barely gets to 50 arc seconds, which isn’t even an arc minute. My eVscope has the smallest field of view and it subtends a whopping 27 arc minutes, which could hold over 30 Jupiters placed side to side.
What matters is what angle each pixel sees. And that can easily be calculated if you know the pixel size of the sensor and the focal length of the objective mirror or lens. Since the pixel sizes don’t very greatly, it’s all about focal length. These scope most of us have sport focal lengths ranging from: 100 mm (DWARF II) to 450 mm (eVscope or eQuinox). The image pixel sizes range from 3.75 micrometers (eVscope 1) down to 1.45 micrometers (DWARF II). If you want to do the math, then the angular resolution of any one pixel is =
2arctan[(Pixel Size)/(2FocalLength)].
When putting in numbers for a few scopes, here’s what I get for their angular pixel size in arc seconds: eVscope 1 = 1.7; Vespera = 2.99; DWARF II = 2.99. Of course these are theoretical and we have to worry about the actual seeing while observing, but we’ll deal with seeing at a later blog post.
The traditional planet sizes range from Jupiter at up to 50 arc seconds to Mars, which can be as low as 4 arc seconds. (N.B. Mars is almost always disappointing for all but the most sophisticated planetary imagers.) Even my eVscope can barely get twenty five pixels on Jupiter. I’ve attached my best image below. Note that even with it, Unistellar has modified their software to just take a short image, since Jupiter is so bright. Stacking with these scopes really doesn’t happen for this type of image.
Hyperia should do better because it’s effective pixel size is 0.7 arc second. (I’ve attached an image of Saturn, which was processed a bit, through a Hyperia below too, courtesy of Vaonis.) But, really, Hyperia is only just about two times smaller than the eVscope when it comes to pixel angle. The software might improve things a bit in the future, but we’re really limited primarily by the focal length of these scopes.
So, is there any way to make our focal lengths greater? That’s the appropriate question if you been paying attention here. You really can’t insert a Barlow or something like that in this type of system. Yes there is a way to make one of these designs with a longer FL, but it would take a different mirror and tube in the case of an eVscope and the designs of the affordable others really couldn’t support going much longer. These scopes are compact for a couple reasons. One is ease of use and transport, but the other is that the mounting mechanics would have to be much more robust for longer focal length scopes. (The only exception might be the Hyperia by Vaonis, which boasts a focal length of 1,050, but that’s not even the 1,325 focal length of my 4 inch Maksutov.)
And then there is the software, which in stacking mode is not really suitable for planets, which are so bright. Planet imagers with larger scopes (11 inch SCTs) take movies, then sort through the frames and then stack them. In principle I suppose the software could be improved to do this, but we still wouldn’t have then the resolution of the larger diameter telescopes.
Well, where does that leave us? For now, you simply need to get a separate telescope for the planets. (I consult on education observatories and my ideal set up is a smart telescope combined with a large SCT and a Hydrogen alpha scope for the Sun.) For me, a simple C4 works wonderfully on the Moon and planets. I don’t take images with it, however.
With a smart telescope it’s important to understand this point about the planets ASAP. That’s why it’s an early blog topic. Like it or not, with these scopes we’re being hurled into interstellar and intergalactic space… as Buzz Lightyear once said,
“To Infinity and Beyond!”
Hmmm, well we’ll get to the infinity issue at some point in later entry ;-)
You didn’t mention the Dawes Limit, which is the limiting resolution based on the diameter of the objective. For the 114mm eVscope it is 1.0 arcsec. Even for the $50K, 150mm Hyperia it is 0.77 arc seconds, so their 0.7 arcsec pixel doesn‘t buy you much! With traditional scopes, we simply pop in a shorter length eyepiece to get “more power”, i.e. A longer effective focal length. Even there we are limited to about 50X per inch of aperture for reflectors and maybe 100X/inch for refractors, and then there are atmospheric conditions to consider.
Good explanation! I think Vaonis has done a pretty good job letting prospective buyers know that they can’t image planets. I knew that going in, but for me it is no problem, as even in my Bortle 7 sky I can find and look at planets and the moon easily with my other scopes. The smart scope let’s me see deep sky objects that I could never see without imaging.
Thanks for that clear simple explanation!
Hi Jim
I tried to illustrate some comments here with a photo. I got the message “file was not uploaded.” Any idea why? This site will be limited if visitors can’t add images.
Daniel