I confess that I am increasingly confused. Based on the advices following up my last post, I started to delve into the world of telescopes to replace my ordinary zoom lens.
Let's start with figures: my Sigma zoom 150-600mm, which I always use at the maximum available aperture ranging from f/5 to f/6.3, has a diameter of 95mm with maximum aperture f/6.3 at 600mm focal length. This is my benchmark.
I assume that when comparing a telescope vs. a photo lens in terms of diameter, aperture and focal length, I am comparing analogous quantities with the same units of measurement in the same reference system. Am I wrong? And, in this case, why?
Let's say that currenlty my ideal telescope has a focal length between 500mm and 750mm, since most of the DSOs I'm used to shoot at fall within a FOV that now my Sigma lens, mounted on my Canon 90D, covers from 400 to 600mm, plus 1.4x or 2.0x teleconverters when needed, which I could replace with a couple of barlows, in case.
In other words, this means that today my worst scenario is to shoot at 1200mm (600mm + 2.0x) at f/13 with a 95mm diameter. I assume this is the real benchmark to look at, whose results I wish to be improved.
If comparison has some meaning, I don't understand how, on a like-for-like basis, I would gain a truly appreciable quality shooting with a telescope in place of my zoom (apart from spending a lot of money, that's maybe the answer).
- So far, I'm discarding Maksutov, Cassegrain et similar, since they have much longer focal lengths that, given this scenario, keep them out of scope.
- Considering refractors, it seems that I cannot find any APO under 1800€ (which is double the Sigma cost) with an aperture/focal lenght/brightness ratio comparable to my Sigma.
At 600mm most APOs seem to have apertures f/6.5 - f/7 and below, and diameters smaller than mine. Either moving up with diameters, or down with aperture, we end up with sidereal costs over 2000 to 4000€, plus new mount (and accessories, etc)… with all that money I could almost buy a used Canon L 600mm with a a greater aperture and diameter, without the need to change my mount, and also being able to shoot at f/8 at 1200mm…
The issue of price could be circumvented by moving to achromatic rrefractors which, on the other hand, seem to have serious issues with chromatic aberration, a problem that my Sigma, at 600mm, has quite limited. However, compared to Sigma, all the other limitations affecting APO lenses as well remain.
If I stick to a refractor < 500mm I don't see advantages: both diameters and focal lengths are always smaller than my Sigma's figures.
Everything, not to mention that with a large refractor I would still definitely have to change my mount, meaning other significant costs.
- The only meaningful solutions that would really seem to improve my life are Newtons: much lower costs, perfect parameters on paper to be compared with my Sigma.
For example, the SW 150/600: large diameter, f/4. Compared to my zoom a significative leap forward, but, if I go by what I read, less contrast and above all star spikes that cannot be definitely eliminated (I read that the star spikes are a constant with Newtons).
Also, maintenance issues, collimation, etc. Ok, I could deal with all of those annoiying sides, but not with star spikes, that's really artifacts I don't like. Tell me I can avoid star spikes at all and maybe this could be my path.
Jokes apart, it seems to me that unless you have a few thousand euros to invest, let's say at least >3000€ for refractors + suitable mount, there is no way out to make a leap ahead in quality while staying in the same target DSOs range (or accepting spikes and more equipment care needed).
What's wrong with my reasoning?
I now mostly shoot at nebulae, as galaxies are generally pretty impossible to catch with my gear. Maybe, truth is that if I'd like to get a real improvement, spending by spending, it would then be preferrable for me really considering something like a Maksutov with much longer focal length and larger diameter than mine, to catch galaxies, planets and small nebula that now I cannot target at all.
That's unless there is a fundamental flaw in my reasoning or a parameter that I am not taking into account.
Clear sky to everyone, of course!
Let's start with figures: my Sigma zoom 150-600mm, which I always use at the maximum available aperture ranging from f/5 to f/6.3, has a diameter of 95mm with maximum aperture f/6.3 at 600mm focal length. This is my benchmark.
I assume that when comparing a telescope vs. a photo lens in terms of diameter, aperture and focal length, I am comparing analogous quantities with the same units of measurement in the same reference system. Am I wrong? And, in this case, why?
Let's say that currenlty my ideal telescope has a focal length between 500mm and 750mm, since most of the DSOs I'm used to shoot at fall within a FOV that now my Sigma lens, mounted on my Canon 90D, covers from 400 to 600mm, plus 1.4x or 2.0x teleconverters when needed, which I could replace with a couple of barlows, in case.
In other words, this means that today my worst scenario is to shoot at 1200mm (600mm + 2.0x) at f/13 with a 95mm diameter. I assume this is the real benchmark to look at, whose results I wish to be improved.
If comparison has some meaning, I don't understand how, on a like-for-like basis, I would gain a truly appreciable quality shooting with a telescope in place of my zoom (apart from spending a lot of money, that's maybe the answer).
- So far, I'm discarding Maksutov, Cassegrain et similar, since they have much longer focal lengths that, given this scenario, keep them out of scope.
- Considering refractors, it seems that I cannot find any APO under 1800€ (which is double the Sigma cost) with an aperture/focal lenght/brightness ratio comparable to my Sigma.
At 600mm most APOs seem to have apertures f/6.5 - f/7 and below, and diameters smaller than mine. Either moving up with diameters, or down with aperture, we end up with sidereal costs over 2000 to 4000€, plus new mount (and accessories, etc)… with all that money I could almost buy a used Canon L 600mm with a a greater aperture and diameter, without the need to change my mount, and also being able to shoot at f/8 at 1200mm…
The issue of price could be circumvented by moving to achromatic rrefractors which, on the other hand, seem to have serious issues with chromatic aberration, a problem that my Sigma, at 600mm, has quite limited. However, compared to Sigma, all the other limitations affecting APO lenses as well remain.
If I stick to a refractor < 500mm I don't see advantages: both diameters and focal lengths are always smaller than my Sigma's figures.
Everything, not to mention that with a large refractor I would still definitely have to change my mount, meaning other significant costs.
- The only meaningful solutions that would really seem to improve my life are Newtons: much lower costs, perfect parameters on paper to be compared with my Sigma.
For example, the SW 150/600: large diameter, f/4. Compared to my zoom a significative leap forward, but, if I go by what I read, less contrast and above all star spikes that cannot be definitely eliminated (I read that the star spikes are a constant with Newtons).
Also, maintenance issues, collimation, etc. Ok, I could deal with all of those annoiying sides, but not with star spikes, that's really artifacts I don't like. Tell me I can avoid star spikes at all and maybe this could be my path.
Jokes apart, it seems to me that unless you have a few thousand euros to invest, let's say at least >3000€ for refractors + suitable mount, there is no way out to make a leap ahead in quality while staying in the same target DSOs range (or accepting spikes and more equipment care needed).
What's wrong with my reasoning?
I now mostly shoot at nebulae, as galaxies are generally pretty impossible to catch with my gear. Maybe, truth is that if I'd like to get a real improvement, spending by spending, it would then be preferrable for me really considering something like a Maksutov with much longer focal length and larger diameter than mine, to catch galaxies, planets and small nebula that now I cannot target at all.
That's unless there is a fundamental flaw in my reasoning or a parameter that I am not taking into account.
Clear sky to everyone, of course!
