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Should I focus more on Aperture or Focal Length for a telescope?


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Let's assume there are three types of telescopes with the following specifications:

    Telescope                  Aperture              Focal Length

    --------------------------------------------------------------

    Example 1                     70 mm                    400 mm

    Example 2                     60 mm                    700 mm

    Example 3                     60 mm                    900 mm

As a beginner, should I look for a higher value in the aperture or the focal length?

P.s. this question was asked by a user of astronomy.stackexchange. Since there aren't any good answers there, and I would really like to get a professional answer for this question, I posted it here to get a fresh perspective from seasoned stargazers. 

 

 

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Simple question but complicated answer.

It all depends on what you want to look at. Generally the longer the focal length the narrower the field of view. So longer focal length can be better for the moon and planets but for DSOs then a shorter focal length is better. Bigger aperture  is “better” but in refractors can get expensive.

Longer focal length with cheaper refractors means less CA or false colour.

However the quality of the optics is perhaps the biggest factor..

 

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Depends :D

Can you provide use case scenario and telescope type?

General rule is to go for as much aperture as you can manage. This means use / mount / transport / store.

Aperture rules both light gathering capability and resolving power and there is no substitute for that. It lets you see fainter things, and lets you see finer detail (all other things equal).

What is focal length good for? Some telescope designs call for long focal lengths, some can be made in both short and long focal lengths. Effective magnification of eyepiece is directly tied to focal length - you divide focal length of telescope with focal length of eyepiece to get magnification of particular eyepiece.

Other important aspect of focal length is actually tied in to aperture - and it is their ratio, called F/speed or F/ratio. This is very telling number in many ways. We can sort of divide range of F/ratios in 3 types: short, medium, long. Anything below F/6 would be considered short. From F/6 to let's say F/9 is medium, everything above is long.

Benefits of Long F/ratio - easier on the eyepieces due to shape of light cone entering eyepiece. Generally less aberrations of any type, or easier to manufacture to higher standard. In achromats - less chromatic aberration. In parabolic newtonians - less coma. In spherical newtonians - less spherical aberration. Downsides? - not suitable for wide field observing due to long focal length (higher range of magnifications).

People don't prefer such scopes for astro photography - shorter F/ratio scopes provide bigger light grasp at lower resolutions.

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vlaiv in my scenario it is a question of 102mm refractor with 600mm focal length vs a 90mm refractor with 900mm focal length! I feel that the gain in aperture is minimal (only 12mm) but the loss of focal length in this case is huge (300mm), so does it still make sense to go for more aperture even if it means foregoing so much focal length?

I'm torn between these 2 refractors and cannot make an informed decision

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That really depends on what would be your primary interest with scope.

Many amateur astronomers end up with couple of telescopes precisely because each telescope design and format is better at some things and worse at other things.

If we are talking about high end refractors in 4" department - meaning 4" or 102 mm APO (apohromatic telescope - uses special types of glass that minimize chromatic aberration and is of triplet design in F/6 variant) - then I would say go for 4" scope! But that sort of telescope costs about 10 times as telescopes I presume we are talking about.

If you prefer to have wide field views and are interested in deep sky objects - like galaxies, nebulae and star clusters, then I would say go for 102mm. But that scope will be really poor performer on planets, moon and double stars. On the other hand 90/900 refractor (achromat again) will be much better all around instrument. Yes you will be giving up some aperture, but due to fact that it is F/10 scope, chromatic aberration (that is often seen as purple/blue halo around stars and planets) will be much smaller and better managed. While you will be giving up some in field width department (not that much 900mm is really not that long focal length, and with decent 32mm Plossl you will still be having around 1.5 degrees TFOV view) and some aperture, difference is small - less than 20%, you will have much more versatile instrument in 90/900 (planets, Moon, double stars, galaxies, nebulae, clusters - you will be able to observe them all).

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I'd say in that case the 70mm Aperture is better. You can always make up the shorter focal length with eyepieces/barlow but the light gathering power is always more beneficial... as slight it is in this case.

 

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5 hours ago, cassuto said:

102mm refractor with 600mm focal length vs a 90mm refractor with 900mm focal length

I'm imagining a star field in say Cygnus. The 90/900 is gonna be dull by comparison with the 102/600.

The best bet is to find a club and go and see for yourself. Between them the members will be able to show you examples of both but if you can't, and as they're both available economically, I'd say get both, otherwise your curiosity will almost certainly get the better of you;)

HTH

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Also consider the mount. Many of the smaller telescopes are sold with the cheapest (and often flimsiest) mount that the manufacturer can get away with to get below a certain price point.

My Celestron Travelscope 70 is Example 1. With my 32mm eyepiece, it gives me the widest FOV of all my setups. However, the tripod mount, supplied as part of the Travelscope, is almost impossible to use at higher magnifications (it moves as you lock it onto a target). The good news is that the OTA has a "Vixen standard" dovetail plate, and so will fit on most of my (much more friendly) other mounts.

In my opinion (others may well disagree) the best (aperture) / (price) * (convenience) beginners setup is the Skywatcher Heritage 130P.

Geoff

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As a beginner it has taken me a while, but finally I have decided that the apparent magnification of an object with a single EP is determined by focal length.  Thus if you want something with the most apparent magnification you really need the most focal length possible.   However, there is a limit to how much magnification you can effectively use at a single focal length and that is governed by apperture, so if you have a 2m long focal length you will need a mirror of some size to make effective use of it.  Within a certain focal length the only thing a bigger mirror will get you with that same EP is more light, so faint grey fuzzies are more visible, but the bigger mirror will allow the use of an EP with more apparent magnification (lower mm value) and so you get more magnification.  Therefore, as mentioned above it depends on what you want to see, if you want to chase FGFs you want the largest mirror possible and added focal length will help apparent magification which is useful with some of them.  Otherwise if you want to look at things like planets and double stars, extra focal length will be useful for apparent magnification at a single EP length, but unless you have enough apperture you won't be able make use of it.

The above is as I currently understand things having learned from SGL, but I am only a beginner myself.  

The answer to the best starter telescope question is often a SW 200P

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I think aperture is the first thing to consider, you can use different eyepieces to achieve the same magnification from different focal length scopes but the aperture of a scope can't be increased other than by buying a bigger scope.

Size and ease if handling is another consideration. I have scopes similar to example 1 and example 2, and in practice the scope similar to example 1 gets more use as it is less bulky (by which I mean the tube is shorter), even though the quality if the views is slightly poorer.

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I have a 90/900 achromat, and mounted upon an EQ-2...

kit.jpg.96c69a1b1877936b414599549bfc0e98.jpg

The achromat is rather under-mounted there, as the images do shake and wiggle a bit before settling, but not as badly as that might imply.

At 45x, the Trapezium and its environs within Orion is nothing short of glorious.  A 90/900 would be more versatile than a 102mm 600(or 660).  Also, the longer the achromat, the less false-colour to be seen when viewing brighter objects.

But there's one thing we know nothing about: the amount of light pollution where you'll be observing.  If you observe beneath darker skies, a 90/900 will not disappoint.

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7 hours ago, cassuto said:

    Telescope                  Aperture              Focal Length

    --------------------------------------------------------------

    Example 1                     70 mm                    400 mm

    Example 2                     60 mm                    700 mm

    Example 3                     60 mm                    900 mm

As a beginner, should I look for a higher value in the aperture or the focal length?

Example 1: more aperture means better resolution and brightness. However the approx f/6 ratio in an achromat means more aberration that will negate some of the theoretical resolution gain. I'm assuming you're dealing with achromats here, but if you switch to an apochromat, aberration becomes negligible and aperture wins, no contest. Another caveat is if you use the 70mm achromat at low magnification only, the aberrations won't be magnified, and the 70 wins the comparo as a rich-field scope exclusively.

Example 2: smaller diameter means a little less resolving power but the approx f/12 beam gives very little aberration, the image will be clearer. Similar eyepieces will yield higher magnifications but also show a smaller piece of sky. 60mm tubes are in disfavor since larger scopes have become affordable.

Example 3: f/15 beam will make aberrations almost nil (I had a 60mm f/13.3 with barely visible aberration) but field of view is narrowest.

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7 hours ago, cassuto said:

vlaiv in my scenario it is a question of 102mm refractor with 600mm focal length vs a 90mm refractor with 900mm focal length! I feel that the gain in aperture is minimal (only 12mm) but the loss of focal length in this case is huge (300mm), so does it still make sense to go for more aperture even if it means foregoing so much focal length?

I'm torn between these 2 refractors and cannot make an informed decision

This seems like an artificial choice. If you want a 102mm f10 refractor, then get one.  I have a 102mm f5 refractor and I rarely use it, as it is mainly suited for looking at large star clusters.  If you are limited by budget, Newtonian reflectors are generally cheaper, even in small sizes.

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Thanks all for your advice, greatly appreciated. I think I will get the 90/900 since it would be more versatile as most answers suggest here, and I am more interested in viewing planets and moon than star clusters, so it seems more logical. The only downside with the 90/900 is that it is mounted on a EQ, and from my research an EQ could be pretty challenging for beginners. So this is one downside I need to consider.  

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1 hour ago, cassuto said:

Thanks all for your advice, greatly appreciated. I think I will get the 90/900 since it would be more versatile as most answers suggest here, and I am more interested in viewing planets and moon than star clusters, so it seems more logical. The only downside with the 90/900 is that it is mounted on a EQ, and from my research an EQ could be pretty challenging for beginners. So this is one downside I need to consider.  

You can find 90/900 models that come on both AZ and EQ mounts. For example this:

https://www.firstlightoptics.com/evostar/sky-watcher-evostar-90-az-pronto.html

Or if you decide to go for EQ mount after all, here is something more substantial:

https://www.firstlightoptics.com/evostar/skywatcher-evostar-90-eq3-2.html

I would indeed suggest to skip both AZ3 and EQ2 class mounts - I had them both, and really you want something more serious from the start (no such thing as beginner mount - only good mount and less than good one ...).

I don't know how good AZ pronto is, but I have AZ4 and such scope would "sing" on it. I've just purchased my self a 102mm F/10 achromat (just got it in the mail, hence clouds here for next 5 days :( ) and its going on AZ4 for those casual Moon and Planet (and even DSO stuff) "looksies" from the balcony.

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Interesting. I had the same dilemma. I started off with a 70/700 and wanted to upgrade with another refractor. I was on the verge of getting a 102/660 while thinking about a 90/900. I just thought the extra 20 percent more light of the 102 over the 90mm (pi r squared) would swing it for me.The shorter focal length will also show more sky,which helps star-hopping. In the end I broke the bank to get an ED 100/900 because I didn't want to buy another telescope ever. How foolish I was. 

I would definitely recommend some sort of AZ mount over an EQ. Apart from that, don't stress too much. They'll both show you a pretty decent Orion nebula and, depending on conditions, some very very faint but pretty amazing fuzzy stuff. Beware that if you enjoy it, you'll probably want to upgrade some time between two months and two years.

Welcome to the hobby.

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The AZ-4 is a good AZ mount.  I recommend avoiding the flimsy lightweight mounts unless you really need something portable.  For my vintage 70mm brass refractor, the AZ-4 and the EQ-5 are the minimum AZ or EQ mounts.

Avoid EQ mounts unless you are willing to incur the bother of aligning it each time you use it and the headache of figuring out how it is supposed to work.

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On 18/10/2018 at 08:39, cassuto said:

Thanks all for your advice, greatly appreciated. I think I will get the 90/900 since it would be more versatile as most answers suggest here, and I am more interested in viewing planets and moon than star clusters, so it seems more logical. The only downside with the 90/900 is that it is mounted on a EQ, and from my research an EQ could be pretty challenging for beginners. So this is one downside I need to consider.  

In the getting of the kit, you're after the telescope primarily.  Suitable alt-azimuth mounts are more expensive, and less commonplace; my 90/900 on an older alt-azimuth...

100818.jpg.4a09df816f9291ac1a815d35f59efde0.jpg

That doesn't make much sense, given that those first starting out overwhelmingly prefer an alt-azimuth.  But it does make "sense" when you factor in marketing, and profit.  You're going to get a good telescope, else word would get round and they wouldn't sell at all; but the mount, eyepieces, and accessories, are not necessarily going to match the telescope, ideally, not at those price-points.

The EQ-1 and EQ-2, are the smallest of equatorials...

2012375255_EQ-1EQ-2comparison3.jpg.9e27130d11bc3ffdfb28a78a58af096a.jpg

Both can be placed into an alt-azimuth mode, and by simply throwing the RA-axis all the way back to the 90° position...

987778395_alt-azmode7.jpg.d3ab064d0e4ab97f4cc6d3fcd73f2d40.jpg

All the controls and components that operate in equatorial mode will also function in alt-azimuth, including the counterweight.

The EQ-1 and EQ-2 have been around for quite a long time, since the early 1960s at least...

https://astromart.com/images/classifieds/16327/326641-1.jpg

No sense in throwing away perfectly good molds.

In the end, you would find that you would need to do what the rest of us do, and in the customisation of a kit.  For example, this 70/900 achromat is woefully under-mounted upon an EQ-1, and as it had arrived...

kit4.jpg.4607801efdce88b26d5026fd34d43d95.jpg

Instead, and improved...

845268487_mountingmodes.jpg.897eba5f2c15be1a8343368f41afbd0d.jpg

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7 hours ago, Alan64 said:

Both can be placed into an alt-azimuth mode, and by simply throwing the RA-axis all the way back to the 90° position...

Beautiful, never thought of it, but it much such a sense (as does turning Alt/AZ in EQ via wedge).

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With my 70/700mm, I went for the mount, not the OTA. I already had the 127mm Mak. (Skymax with Synscan), 90/910mm refractor (Cosmos 90GT WiFi), on similar mounts; but the Skyprodigy 70 (with built-in Starsense) was, a little while ago, on sale for under £200. Celestron produce the Skyprodigy range, essentially the same mount, with different OTAs, and different price points. The "70" being the cheapest, and the "6" the most expensive. The only difference in the mounts was the plastic cowl surrounding the dovetail clamp; each one being a snug fit on the external diameter of the OTA's tube. The 70's was the smallest diameter, and therefore had the most plastic.

After about a quarter of an hour with tin snips and a file, I had a Starsense mount (now with a lightweight dovetail clamp cowl), for a whole range of OTAs ( the Skymax 127 & Cosmos 90 - above, and adding the Heritage 130P Newtonian, ST120 refractor, Virtuoso 90 Mak., Travelscope 70 refractor, and Astromaster 130 Newtonian). With a bit of bodging, (probably using the "L" bracket from the Virtuoso) I hope to add my Strathspey 15x70 bins.

Geoff

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