Do I understand correctly the limitations of my telescope for prime focus planet imaging? (details in post)

[Cake]

Hi all, I'm very new to astronomy as a whole and need someone with practical experience to tell me whether I actually understand the theory.

I've recently bought on the cheap an old Meade StarNavigator 102 refractor, 102mm/800mm (not NG, as reviewed here). It's physically in good condition and I'm happy with what it is, but I understand it will have limitations and I'm just not sure if I actually know what they really are. Specifically when it comes to imaging the planets.

I've read up a bit about how prime focus photography works and from what I gather this 800mm focal length telescope with no Barlow will give a 0.77 asec/pix image on a 3um size pixel camera (I'm specifically thinking of the SVBONY 105 because it's cheap, I'm on a budget and I care more about learning how it works before I start spending real money).

On the other hand, I believe I understand that the absolute physical limit of resolution for this telescope is its Dawes limit, which (assuming Wikipedia isn't lying to me) would come out to ~1.14 arcsec. In other words, each pixel apparently already catches the telescope's smallest resolvable detail?

Do I undertand correctly that adding a barlow of any power would not have any more effect on the amount of detail than simply cropping the image, i.e. just make it bigger? Altough I imagine that the more pixels per arcsec, the better the signal/noise ratio of what is being captured, but that's a tangential issue.

[CraigT82]

You want the smallest available details to occupy more than one pixel to actually be visible in the image, hence the why its often quoted that you should aim for sampling at around 1/3 the dawes limit, so that those tiny details occupy 3 or more pixels.

The Dawes limit itself is based on separating point sources of light, and in planetary and lunar imaging its common to resolve extended features well below the Dawes limit (the rille in the Alpine Valley, and the Encke gap in Saturn's rings are two examples) because they are linear features rather than point sources. 

You are right in that once you get to a certain point you are actually better off just enlarging the image in post processing rather than adding power by using a barlow, especially when seeing is variable to poor. Perhaps you could experiment with and without a barlow and see what you get. 

[Cosmic Geoff]

Try it in practice, and you will find that the atmosphere is the main limiting factor, though the effect of 'seeing' will be less extreme with a 4" than with, say, an 8" telescope.  Jupiter and Saturn are currently low, so from the UK you will be trying to image through a lot of unstable atmosphere, which also causes some chromatic dispersion.  Often a Barlow, though theoretically advantageous in most cases, gives no improvement in practice. The use of an atmospheric dispersion corrector (ADC) is advised.

It is worth investing in a decent planetary camera rather than the cheapest, as the difference in performance can be significant.

[Owmuchonomy]

Hi, your main issue with that scope is lack of focal length. If it is planetary imaging you wish to do then you will need a lot more focal length. It is also best to use a technique called ‘lucky imaging’. For this you will need a high frame rate planetary camera. For example, I image planets at approximately 4600mm focal length using an ASI174 planetary camera. I achieve this by adding a 2x powermate to my C9.25” SCT to get the desired focal length of 4.6 metres and focal ratio of f/20. You need to aim for a f/ of 4 to 5 times your cameras pixel size. I can also use my ASI290 chip camera at the native focal length of the SCT f/10 because the pixel size is smaller (2.9 micron). Thus f/10 is in the right ballpark for planetary imaging in this setup.

[Tenor Viol]

I'd agree that if your primary aim is lunar and planetary, then you need to look at much longer focal lengths as that affects the image size. 

[geoflewis]

For planetary imaging you should aim for a focal length (FL) of x5 camera pixel size, hence for a 3um px camera, your sweet spot FL will be ~F15, so with your 102/800 (F7.8) scope you should add a x2 barlow. An ADC will help, but you can probably get away without that with such a small scope. Good luck.

[Gfamily]

If you are imaging, you can use software to simulate the effects of an ADC. 

Registax6 and Autostakkert have an RGB Align option that will recentre the three colour images after stacking.

http://www.ianmorison.com/combating-atmospheric-dispersion/

Not tried it myself, but Ian generally knows what he's talking about.

Edited September 4, 2020 by Gfamily

[Cake]

Thank you for your replies

12 hours ago, CraigT82 said:

The Dawes limit itself is based on separating point sources of light, and in planetary and lunar imaging its common to resolve extended features well below the Dawes limit (the rille in the Alpine Valley, and the Encke gap in Saturn's rings are two examples) because they are linear features rather than point sources.

Thanks, that's the kind of stuff that isn't obvious from just theory.

6 hours ago, Cosmic Geoff said:

Try it in practice, and you will find that the atmosphere is the main limiting factor, though the effect of 'seeing' will be less extreme with a 4" than with, say, an 8" telescope.  Jupiter and Saturn are currently low, so from the UK you will be trying to image through a lot of unstable atmosphere, which also causes some chromatic dispersion.  Often a Barlow, though theoretically advantageous in most cases, gives no improvement in practice. The use of an atmospheric dispersion corrector (ADC) is advised.

It is worth investing in a decent planetary camera rather than the cheapest, as the difference in performance can be significant.

I think the seeing and my use of the telescope are the most important factors here. I'm thinking of prime focus as an improvement on EP projection to a smartphone, no more no less. I'm not an avid astrophotographer hunting for the best viewing conditions, so sub-optimal seeing will be the limiting factor. I needed to know the physical limits of the telescope so that I wouldn't exceed them, I don't need to push them. I'm not willing to splash out on even mid-range equipment at this moment. My whole setup (the telescope with it's standard bits plus a cheap 4mm plossl) have cost me only £160, a decent camera and ADC would likely double or triple that. So I'm willing to spend £40 more on a cheap camera and something on a half-decent barlow, but I don't think of imaging planets as priority enough for more.

Alternatively, I might not spend anything more and stick to EP projection if a cheap prime focus camera is just too poor of a tool. I would like to hear your opinion on that.

Is smartphone on a EP (I currently have a 25mm and 9mm 40deg afov whatevers that came with the telescope and a 4mm/48deg afov plossl) better than a cheap prime focus camera, like the SVBONY 105?

EDIT:

I specifically mean the quality of the image, as I did go to https://astronomy.tools/calculators/field_of_view/ to comapare the possible fields of view as in the attached pictures.

5 hours ago, geoflewis said:

For planetary imaging you should aim for a focal length (FL) of x5 camera pixel size, hence for a 3um px camera, your sweet spot FL will be ~F15, so with your 102/800 (F7.8) scope you should add a x2 barlow. An ADC will help, but you can probably get away without that with such a small scope. Good luck.

4 hours ago, Gfamily said:

If you are imaging, you can use software to simulate the effects of an ADC. 

Registax6 and Autostakkert have an RGB Align option that will recentre the three colour images after stacking.

http://www.ianmorison.com/combating-atmospheric-dispersion/

Not tried it myself, but Ian generally knows what he's talking about.

Yeah, I don't mind a barlow, but if I can avoid getting an ADC, that's awesome.

 

Edited September 4, 2020 by Cake
Additional info

[Cosmic Geoff]

2 hours ago, Cake said:

Yeah, I don't mind a barlow, but if I can avoid getting an ADC, that's awesome.

If your whole setup cost only £160 you probably will not be wanting to invest in an ADC (around £120).  Correcting in software does not work as well as using an ADC - there's a technical reason for this, I'm sure.

Planetary imaging is a field where giving it a go may be  more instructive than a lot of theory. I have no idea how well a smartphone would work - I only note that some smartphones have several cameras in them and cost more than a lot of folk's telescopes. 😀

If you look through the Planetary Imaging section of the forum you can see what hardware the more ambitious imagers actually use.

[geoflewis]

35 minutes ago, Cosmic Geoff said:

Correcting in software does not work as well as using an ADC - there's a technical reason for this, I'm sure.

Dispersion occurs across the entire spectrum, so by using software to correct just the Red, Blue and Green channel groups, it will not correct dispersion within each of those broadband groupings. An ADC works right through the spectrum (hence I use one with a mono camera and filters), but for the purpose and size of scope that the OP will be using, software channel alignment in Registax, Autostakkert, Photoshop, etc., will be more than adequate.

[JamesF]

I'll belatedly add another vote for the "focal ratio approximately five times the pixel size in um" rule of thumb.  That's where I've been for some time now.  If you can find it (I can't, for the moment), vlaiv posted a very informative piece on resolution and planetary imaging some time back that I meant to return to at some point before Mars came out to play this year.  As it happens, the weather has made such study largely academic thus far.

James

[Cake]

47 minutes ago, JamesF said:

I'll belatedly add another vote for the "focal ratio approximately five times the pixel size in um" rule of thumb.  That's where I've been for some time now.  If you can find it (I can't, for the moment), vlaiv posted a very informative piece on resolution and planetary imaging some time back that I meant to return to at some point before Mars came out to play this year.  As it happens, the weather has made such study largely academic thus far.

James

Interestingly, when researching this I've seen someone mention another rule of thumb which is "3 pixels per arcsec on the sensor" (which you can calculate based on this http://plato.acadiau.ca/courses/phys/1513/optics.htm), and if you calculate both, they do seem to be more or less equivalent in my particular case. For a 3um pixel and 102mm aperture, that's an ~f/18, so just slightly longer than the five times.

EDIT:

I've checked again, it's 3x pixel per Dawes limit.

 

Edited September 4, 2020 by Cake

[JamesF]

11 minutes ago, Cake said:

Interestingly, when researching this I've seen someone mention another rule of thumb which is "3 pixels per arcsec on the sensor" (which you can calculate based on this http://plato.acadiau.ca/courses/phys/1513/optics.htm), and if you calculate both, they do seem to be more or less equivalent. For a 3um pixel and 102mm aperture, that's an ~f/18, so just slightly longer than the five times.

Yes, I believe both rules of thumb originate from considering the same physical properties of the telescope so it makes sense that they'd be in agreement with each other.

James

[Cake]

5 minutes ago, JamesF said:

Yes, I believe both rules of thumb originate from considering the same physical properties of the telescope so it makes sense that they'd be in agreement with each other.

James

I've checked where I've read that again, it was actually 3x the Dawes limit, which now seems less suprising than it seemed to me.