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Is aperture actually king?

midge91

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Paul N

Paul N

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When I bought my first scope (Skywatcher 150P) 2 months ago I went in with the 'aperture is king' mantra. For the budget I had set aside for it (£300) I could have got either the 130P on a goto alt-az mount or the 150P on an EQ3-2 mount with no goto. I didn't really like the idea of me just dialling in numbers and some black box taking me there, I wanted to get to know the sky and feel the excitement of having found some obscure target myself. Also, the EQ3-2 had an upgrade path, namely fitting RA and DEC motors (£95) and a polar scope (£35) for alignment with Earth's axis of rotation using Polaris, enabling me to track the object in RA as it 'moves' E-->W across the night sky.

If you want to see DSOs, then it is more about light collection. So yes, larger aperture = more light collection and you will be able to see more with your eyes through a given eyepiece, and get a higher useful magnification. For example with my 150P I can just make out the core of Andromeda (M31) as a fuzzy area and just make out a feint glow around the central trapezium of stars in the Orion Nebula (M42). The real magic happens when you plug a DSLR onto the end of the focuser tube (prime focus) and start COLLECTING light on a CMOS sensor! Even just 5-10 secs at ISO 1600 and you can see the pink cloud and the overall outline of the whole nebula (my first ever photo was 5 secs (was expecting it to fail) and I was jumping up and down like a kid, couldn't believe what I was seeing!). Pushing longer exposures, I noticed the stars started trailing despite having RA tracking. Doing far more careful alignment the next time I was able to get 3min exposures without trails. This brings me onto the topic of mounts....

Up to this point I had been doing this all alone. I then joined my local group and when I got there for the first time I was surprised to see that I had the largest telescope on the smallest mount :). The others had EQ5 and EQ6 PRO mounts with AUTOGUIDING (dedicated little refractor scope with CCD on the end to actively track a reference star), and their main scope was something like an 80mm refractor. They were able to take for example 10min exposures without any star trails! Now if they had my 150P collecting 3.5 times the light per unit time, they could get the same number of photons hitting their camera sensor in 2.8mins. So I can match their performance with my setup only up to this duration. The lesson here is for astrophotography of DSOs you could go for a smaller scope with a more intelligent mount. These mounts also offer the advantage of being heavier and more sturdy and a better platform for photography anyway.

I have no regrets with my choice of scope, it easily fits in my old Nissan Micra, easy to handle, collimation is very stable even after changing temperature from room to near zero and then being transported. Reflectors don't suffer from chromatic abberation (like low-end refractors do) but do have coma abberation. I'd rather have the latter and it can be easily corrected (extra £100 lens). As for the mount, probably should have gone for the EQ5.... but hey, I will push the EQ3-2 to its limits and see what I can get.

To conclude I would say that if you plan to just use your eyes and no photography to view DSOs then go for the largest aperture you can afford (and transport!). If you do think you might try some prime focus photography in the future, then consider getting a beefier mount like a barebones EQ5 and then adding stuff to it when you are ready (this also helps to spread out expenditure, and things usually get cheaper in the future as newer products are brought out). I have not said anything about Dobsonians as I have no experience with them, but they do offer very large apertures!

Aperture enhances light collection. So why not say 'light collection is king'? Well, aperture also enhances your angular resolution, so you can distinctly see more distant points that are close together (Rayleigh criterion for circular aperture is angular resolution=arcsin(1.22*Wavelength/Aperture). For red light at 656nm (H-alpha) and 150mm aperture this is 5.34microradians or 1.1 arcseconds. The atmosphere typically limits seeing to about 1 arcsecond (around here in winter anyway), so apertures above 165mm have no added advantage in resolution for ground-based observations without adaptive optics to correct for atmospheric distortions caused by fluctuations in refractive index.

Therefore: aperture is almighty, but light collection is king for ground based observation through the atmosphere with simple scopes of aperture larger than 165mm.

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midge91

midge91

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When I bought my first scope (Skywatcher 150P) 2 months ago I went in with the 'aperture is king' mantra. For the budget I had set aside for it (£300) I could have got either the 130P on a goto alt-az mount or the 150P on an EQ3-2 mount with no goto. I didn't really like the idea of me just dialling in numbers and some black box taking me there, I wanted to get to know the sky and feel the excitement of having found some obscure target myself. Also, the EQ3-2 had an upgrade path, namely fitting RA and DEC motors (£95) and a polar scope (£35) for alignment with Earth's axis of rotation using Polaris, enabling me to track the object in RA as it 'moves' E-->W across the night sky.

If you want to see DSOs, then it is more about light collection. So yes, larger aperture = more light collection and you will be able to see more with your eyes through a given eyepiece, and get a higher useful magnification. For example with my 150P I can just make out the core of Andromeda (M31) as a fuzzy area and just make out a feint glow around the central trapezium of stars in the Orion Nebula (M42). The real magic happens when you plug a DSLR onto the end of the focuser tube (prime focus) and start COLLECTING light on a CMOS sensor! Even just 5-10 secs at ISO 1600 and you can see the pink cloud and the overall outline of the whole nebula (my first ever photo was 5 secs (was expecting it to fail) and I was jumping up and down like a kid, couldn't believe what I was seeing!). Pushing longer exposures, I noticed the stars started trailing despite having RA tracking. Doing far more careful alignment the next time I was able to get 3min exposures without trails. This brings me onto the topic of mounts....

Up to this point I had been doing this all alone. I then joined my local group and when I got there for the first time I was surprised to see that I had the largest telescope on the smallest mount :). The others had EQ5 and EQ6 PRO mounts with AUTOGUIDING (dedicated little refractor scope with CCD on the end to actively track a reference star), and their main scope was something like an 80mm refractor. They were able to take for example 10min exposures without any star trails! Now if they had my 150P collecting 3.5 times the light per unit time, they could get the same number of photons hitting their camera sensor in 2.8mins. So I can match their performance with my setup only up to this duration. The lesson here is for astrophotography of DSOs you could go for a smaller scope with a more intelligent mount. These mounts also offer the advantage of being heavier and more sturdy and a better platform for photography anyway.

I have no regrets with my choice of scope, it easily fits in my old Nissan Micra, easy to handle, collimation is very stable even after changing temperature from room to near zero and then being transported. Reflectors don't suffer from chromatic abberation (like low-end refractors do) but do have coma abberation. I'd rather have the latter and it can be easily corrected (extra £100 lens). As for the mount, probably should have gone for the EQ5.... but hey, I will push the EQ3-2 to its limits and see what I can get.

To conclude I would say that if you plan to just use your eyes and no photography to view DSOs then go for the largest aperture you can afford (and transport!). If you do think you might try some prime focus photography in the future, then consider getting a beefier mount like a barebones EQ5 and then adding stuff to it when you are ready (this also helps to spread out expenditure, and things usually get cheaper in the future as newer products are brought out). I have not said anything about Dobsonians as I have no experience with them, but they do offer very large apertures!

Aperture enhances light collection. So why not say 'light collection is king'? Well, aperture also enhances your angular resolution, so you can distinctly see more distant points that are close together (Rayleigh criterion for circular aperture is angular resolution=arcsin(1.22*Wavelength/Aperture). For red light at 656nm (H-alpha) and 150mm aperture this is 5.34microradians or 1.1 arcseconds. The atmosphere typically limits seeing to about 1 arcsecond (around here in winter anyway), so apertures above 165mm have no added advantage in resolution for ground-based observations without adaptive optics to correct for atmospheric distortions caused by fluctuations in refractive index.

Therefore: aperture is almighty, but light collection is king for ground based observation through the atmosphere with simple scopes of aperture larger than 165mm.

Thanks Paul,

Sound advice and I am starting to veer away from fancy tech and look for good solid views. The Skywatcher is probably going to be my scope of choice as it suits my situation. Thanks for everyones advice.

Midge

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todd8137

todd8137

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hi from me gotos are not all that ,i have one , and they not that good to be honest,lots buy them thinking thats the problem solved of finding things,but you still need to no a lot of the brighter stars for the goto and it will not point straight at the objects you looking for so my vote is for the dob ,do not be dazzled by the goto system unless the mount is a 6/700 pound one as these single arm ones are not the greatest go for the dob

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