Aperture vs. Size
The first difficulty with very large scopes is portability. Since I have to travel to do my best observing, my telescope has to be reasonably portable. It can't be too big to fit into a wide range of cars, as I don't own a car and am never sure what will be available when I rent. Ideally, the largest component, probably the mirror or rocker box, would fit in a trunk. Failing that, it has to be able to fit in a back seat.
The telescope also can't be too heavy for me to handle easily myself, as I sometimes observe alone. I figure the heaviest component I'd really want to carry would be somewhere around 50 lbs. This is a bit arbitrary, but moving something too heavy around has risks, especially in the darkness after a long night of observing!
The final consideration in size is the eyepiece height. I want to be standing flat footed for all of my observing. I'm about 5'6", so I'm hoping for an eyepiece height, near the zenith, of around 5'. In general, the larger the aperture, the longer the focal length and the higher the eyepiece when you point the scope up.
The exact length depends on the focal ratio (focal length/aperture) of the mirror I choose. You can get very fast (low f-ratio) mirrors that will reduce the length of a given aperture, but they tend to be more expensive and have other undesirable optical characteristics; namely coma. Realistically, for reasons I'll cover in a specific post about my mirror options, I'm probably going to settle on an f-ration between 4 and 5... They are generally available, don't necessarily require a coma corrector, and won't be too long.
A spreadsheet seemed like the best way to juggle all of these variables and come up with some concrete numbers. Luckily, before I actually finished one, I ran across a nice Telescope height brainstorming spreadsheet a user on Cloudy Nights worked up! It will also do rough calculations of mirror weight. Awesome.
A spreadsheet seemed like the best way to juggle all of these variables and come up with some concrete numbers. Luckily, before I actually finished one, I ran across a nice Telescope height brainstorming spreadsheet a user on Cloudy Nights worked up! It will also do rough calculations of mirror weight. Awesome.
Playing with the numbers for a while, it seems that my most stringent requirement is probably eyepiece height. Any scope with a mirror or rocker box large enough, or heavy enough that it violates my restrictions has already exceeded the eyepiece height. So simplifying a bit I came up with this table which gives an overall sort of range:
| Diameter | f-ratio | 8" mult | Weight | Eyepiece Height |
|---|---|---|---|---|
| 14" | f5 | 3.1 | 23 | 6' 1" |
| 15" | f4.5 | 3.5 | 26 | 5' 10" |
| 16" | f4.5 | 4 | 30 | 6' 2" |
| 16" | f4 | 4 | 30 | 5' 6" |
| 17" | f4 | 4.5 | 34 | 5' 10" |
| 18" | f4 | 5 | 38 | 6' 1" |
From left to right I've listed the mirror size, f-ratio, how much more light than my current 8" scope it will gather, the weight of the mirror only, and the eyepiece height pointed straight up.
I'm a bit suspect of the eyepiece height calculation. The math seems sound, but in spot checking some listed eyepiece heights for various scopes on the web it seems high. I'll have to do some more specific calculations when I'm closer to choosing a design to build. In any case, I'm probably not going to use the scope straight up because it's somewhat uncomfortable with a dobsonian, or any alt-az mount for that matter.
So I think any of these will meet my three requirements, although the 18" would probably be pushing it on the mirror box size, and I've never really seen a 17" mirror around. However I want to keep an open mind, and 5 times the light gathering of my current 8" scope would be wonderful.
Aperture Vs. Cost
This is a trickier trade off to optimize. The cost of the mirror is the largest, but not only, cost in a telescope and not all the components scale in price the same way. Some things, like a good focuser, are going to be the same price regardless of aperture. Others, like the secondary mirror, or support structure, do get more expensive with increasing aperture, but at rates different from each other, and different than the mirror.
I'm going to crib off of the work of Matt Wedel again and copy a section of a chart he posted about What Aperture Costs, and expanded it a bit to include a couple of the more common larger scopes:
Not all these scopes are equally equipped; some have goto, some do not, and they vary in the quality and types of accessories. However, one thing is pretty clear, larger aperture costs more money!
I've not fully worked out my budget yet, but I think I'm aiming to come in under the $4000 mark. Based on what I have saved so far, this seems like the absolute max I could really do in a year. True, the year is somewhat arbitrary, but that is my goal and I'd like to stick to it.
The Verdict
Based on all the factors so far, I'm going to build my design around a 15, or 16" f4 or f4.5 mirror. I've not done all the budgeting yet on every scope, but it seems like I really can't afford anything larger than 16". Depending on the design, 16" might even stretch my budget and portability requirements, so I'm going to design and budget a couple of options right around that size.
The difference between 15" and 16" may seem small, but the aperture increases with the square of the diameter, so the 16" has 15% more aperture. However, it's eyepiece height is also 4" higher, and I bet that extra inch of diameter means a noticeably larger mirror box as well.
I'm going to have to see what is available in commercial scopes or parts, figure out the specific dimensions for a given implementation and decide exactly what size a bit later. Until then, Clear Skies!
I'm a bit suspect of the eyepiece height calculation. The math seems sound, but in spot checking some listed eyepiece heights for various scopes on the web it seems high. I'll have to do some more specific calculations when I'm closer to choosing a design to build. In any case, I'm probably not going to use the scope straight up because it's somewhat uncomfortable with a dobsonian, or any alt-az mount for that matter.
So I think any of these will meet my three requirements, although the 18" would probably be pushing it on the mirror box size, and I've never really seen a 17" mirror around. However I want to keep an open mind, and 5 times the light gathering of my current 8" scope would be wonderful.
Aperture Vs. Cost
This is a trickier trade off to optimize. The cost of the mirror is the largest, but not only, cost in a telescope and not all the components scale in price the same way. Some things, like a good focuser, are going to be the same price regardless of aperture. Others, like the secondary mirror, or support structure, do get more expensive with increasing aperture, but at rates different from each other, and different than the mirror.
I'm going to crib off of the work of Matt Wedel again and copy a section of a chart he posted about What Aperture Costs, and expanded it a bit to include a couple of the more common larger scopes:
From left to right he's listed the brand/size, aperture (in MM), square inches of mirror real-estate, scope cost, and most interestingly, price per square inch.Meade 10″ Lightbridge – 250mm – 78.5 in^2 – $700 – $8.92/in^2
Meade 12″ Lightbridge – 300mm – 113 in^2 – $1000 – $8.85/in^2
Orion XT12i – 300mm – 113 in^2 – $1100 – $9.73/in^2
Orion XX14i – 350mm – 154 in^2 – $1700 – $11.04/in^2
Meade 16″ Lightbridge – 400mm – 200 in^2 – $2000 – $10.00/in^2
Hubble Optics 16" - 400mm - 200 in^2 - $2300 - $11.50/in^2
Obsession 15" classic - 381mm - 177 in^2 - $4500 - $25.42/in^2
Obsession 18" classic - 457mm - 254 in^2 - $7000 - $27.55/in^2
Not all these scopes are equally equipped; some have goto, some do not, and they vary in the quality and types of accessories. However, one thing is pretty clear, larger aperture costs more money!
I've not fully worked out my budget yet, but I think I'm aiming to come in under the $4000 mark. Based on what I have saved so far, this seems like the absolute max I could really do in a year. True, the year is somewhat arbitrary, but that is my goal and I'd like to stick to it.
The Verdict
Based on all the factors so far, I'm going to build my design around a 15, or 16" f4 or f4.5 mirror. I've not done all the budgeting yet on every scope, but it seems like I really can't afford anything larger than 16". Depending on the design, 16" might even stretch my budget and portability requirements, so I'm going to design and budget a couple of options right around that size.
The difference between 15" and 16" may seem small, but the aperture increases with the square of the diameter, so the 16" has 15% more aperture. However, it's eyepiece height is also 4" higher, and I bet that extra inch of diameter means a noticeably larger mirror box as well.
I'm going to have to see what is available in commercial scopes or parts, figure out the specific dimensions for a given implementation and decide exactly what size a bit later. Until then, Clear Skies!
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