Planetary views 70 - 80 mm aperture short focal length apo ed refractors

[Brave New World]

I've been looking online at a couple of resonabliy priced APO ED refractors for about 300 - 550 Euro. They all have a short focal length of about 400- 600 mm and 70- 80 mm aperture (F6-F7.5). How about planetary views? I was thinking it would be the perfect all round scope for DSO, some imaging and planetary views. What can you expect in terms of planetary detail and size of the planets? And yes, I know they will need an equatorial mount like the CG4 or EQ5. There are so many different roads and so many choices.

[Stu]

Hi

I've used a variety of ED doublets for various kinds of observing, mainly as a grab and go either at home or anytime I head away somewhere.

Scopes? William Optic SD66mm, Televue 76 & 85, Tak FS-60C and 76DC. Not all at once!!

Under dark skies, these scopes can delivery lovely Widefield views of a variety of objects. They also perform surprisingly well on double stars.

Planetary observing is certainly possible, and under the right conditions and with experience you can see detail, although obviously it is easier with 85mm than 66!

As an example, in the 66mm I managed to see the GRS on Jupiter, the main cloud belts plus shadow transits. These things don't jump out at you and need patience and experience to see.

I bought the 76 to give me just that bit more resolution, and it was a definite step up, with better colour correction too.

At each step there is a noticeable increase in resolution and available detail. I would suggest an 80mm if possible.

Imaging I will leave to others to comment, although an 80mm would be ideal for DSO but limited on resolution for planetary

Stu

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[John]

I've owned and used a number of 80mm - 90mm refractors but I concluded that 100mm was the minimum aperture that I found provided interesting images of the moon planets and binary stars. Below that I found the views "nice" but lacking in depth and detail.

With deep sky objects I've found that, with the exception of a few objects, even 100mm is a bit limiting and would strongly prefer 150mm aperture or more.

The above is all relating to visual observing though. I understand that decent 70mm-80mm ED refractors are very effective deep sky imaging instruments. They lack the focal length and therefore image scale for planetary and lunar imaging though.

[Qualia]

Briefly, when looking at objects on Earth, a telescope’s main function is to magnify the object. When stepping from Earth into the Solar System, a telescope helps collect light and also to magnify the distant detail of the object and when peering into deep space, reaching out to objects hundreds, thousands and millions of light years away , the principle function of a telescope isn’t to magnify the object but to collect light from it. When peering into deep space, you are not using a telescope to see objects because they are too small, although in relative terms they might be, but rather because they are so dim, you need light.
 

To get some idea of what this means, imagine we have a 70mm refractor and a 250mm newtonian, the latter has some 1000x more light gathering power and on an average night of seeing is able to observe a galaxy like this:

Evidently, a 70mm scope's resolution and brightness of image would be significantly more compromised. If all other things were equal (which they rarely are) it is for this reason why most astronomers will tell you, aperture rules. In short, the level of perceivable detail rises steeply with each small increment of aperture.  

A quick look at the scope's focal length helps inform us about potential magnification. To work this out, we do scope's focal length / eyepiece's focal length. So, if I have a focal length of 500mm and I have a 10mm eyepiece, I achieve 50x magnification. If I have a 1250mm focal length scope, with the same eyepiece I achieve 125x. This means, that to achieve higher magnifications in the 500mm scope we need shorter focal length eyepieces.

Let's say Jupiter starts to get interesting at around 150x. Here's a sketch of what Jupiter looks like in an eyepiece at about 140x with my 4" f10 on an average night in Spain. Of course, the eye is able to tweak out a lot more information and you will see more, but the size of the image is something akin to this:

Then in the 500mm focal length scope, we will need a 3mm to 4mm eyepiece to achieve this feat. Such eyepieces do exist, but they're not that common, and are often quite expensive and if not, will more than likely suffer from chronic eye-relief. Knowing this makes us realise that perhaps short length scopes are not the most suitable for planetary viewing where we can often go beyond 200x. If we were serious about planets and lunar work, we might want to be looking at longer focal length scopes.

This focal length business also tells us something really interesting about what we may be able to see in the eyepiece itself. If we take the eyepiece's field of view and divide this by our magnification, we end up with a true-field-of-view in degrees, that is, what we see in the eyepiece. So lets go back to our 500mm focal length scope with its magnification of 50x with a 10mm eyepiece, and lets imagine the eyepiece has a field of view like a Plossl, namely, 50º. This means we will see 1º (50º / 50x) of sky, or two full moons. And in our 1250mm focal length scope with the 10mm Plossl, we will see (50º / 125x) 0.4º, or not even a full moon.

With this kind of information, we are now equipped with some useful tools to work out the possibilities of our first potential telescope buy.

Firstly, no telescope is best for every application. To see galaxies and nebulae or deeply resolving globular clusters, a big aperture telescope cannot be beaten. Less aperture means the scope will have less resolution and brightness of image, this means no matter what the object, you will generally see less.

Secondly, more focal length ratio to aperture telescopes provide more magnification with the same eyepiece, making them more suitable for planetary viewing where one is typically using powers between 150x, 200x, 250x and more.

However, small refractors offer levels of portability that an 8", 10" or 16" newt simply cannot. If you're going on holiday by plane, or wish to have a simple grab and go set up, these instruments will be perfect. You're also not fiddling about with collimation, waiting for cool down etc. Due to atmospheric turbulence, they are often able to cut through the bad seeing and might be able to rival larger instruments at that moment and finally, they are perfect for imaging.

They offer the observer a significantly larger true field of view to observe some of the most spectacular deep-sky objects of nature such as the entire Veil Nebula, the full extent of the Andromeda Galaxy, and the Double Cluster. Here's a sketch of the Double Cluster with a 4" f10. I cannot imagine what this would look like in a wider-field setting. The term cosmic comes to mind :

Hope this helped a little

[timwetherell]

I have a WO66 apo and it's a fantastic little telescope but to be totally honest, 66mm of aperture isn't going to give very interesting views of the planets in general. It will show saturn's rings and a couple of cloud belts on Jupiter and do a cracking job on the moon and wider doubles, but I'm inclined to agree that 100mm is about where things begin to get really interesting. I used to have an orion 100mm ED and it showed transit shadows on Jupiter and detail in the cloud belts. A small apo makes a fantastic widefield telescope as well as a portable grab and go but if it were to be my only telescope, I'd be more inclined to spend the money on a different design with more aperture. If you're lucky enough to be able to afford two or more scopes then the little apos are great! I've never got rid of my 66

[Brave New World]

So Jupiter or Saturn would be smaller than the size of a pea at arms length with a 70 mm refractor at clear skies, not pushing the scope beyond its capabilities? Not taking in to account the ever changing different distances between the planets and Earth over the course of a year.

[timwetherell]

That's probably in the ballpark, yes. Though the seeing will limit you to perhaps three times that with any scope really. Everyone will have their own views on this, but generally about 8" of aperture is the point above which the atmosphere will largely negate any additional resolution advantage. Light grasp of course keeps on going up!

Jupiter and saturn do vary a little in apparent diameter throughout the year but Mars is the planet that really exhibits this phenomenon. As big as Jupiter at opposition (closest) to a pin head at arms length at worst!

[John]

So Jupiter or Saturn would be smaller than the size of a pea at arms length with a 70 mm refractor at clear skies, not pushing the scope beyond its capabilities? Not taking in to account the ever changing different distances between the planets and Earth over the course of a year.

The general comment from people new to viewing tha planets is that they look rather small in any scope, 70mm is towards the smallest end of the aperture scale used in astronomy so even more so in such scopes I'm afraid.

To get Saturn or Jupiter to fill, say, half the field of view of an eyepiece you would need to be using a magnfication of several hundred times which is beyond what the atmosphere and most scopes can handle.

Keen planetary observers are more motivated by sharpness and strong contrast than they are by very large but ill defined image scale.

[Stu]

I guess this all comes back to the motivation behind getting a short focal length small apo in the first place.

If the question is, should I buy one on the basis that it will give me excellent planetary views, the answer is no, there are far more suitable scopes out there.

If the question is, I want to but one of these for DSO imaging, or grab and go widefield use, will I also be able to view planets with it, the answer is a qualified yes.

I frequently view planets through small scopes. I do it because it is convenient, because they are easy to take on holiday with me, and because I like a challenge! Can I spot a shadow transit in a 66mm scope? The answer is yes but it takes patience and observing skill to see it.

A large newt, mak/SCT large refractor either long focal length achro or faster apo will give better results but have limitations in terms of convenience and portability. Plus it's a bit too easy to see the detail ;-)

Stu

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[DaveS]

My view is that small, fast apos are really astrographic tools. Yes you *can* see some planetary detail but you'd be better off with a mak-cass for a similar price.

[Stu]

My view is that small, fast apos are really astrographic tools. Yes you *can* see some planetary detail but you'd be better off with a mak-cass for a similar price.

They do also give fabulous widefield views under dark skies

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[Moonshane]

compromise is a big word in astronomy and affects everything. you have to choose the best fit when considering:

cost / budget

aperture

physical size

focal length

focal ratio

type of object

time available for cooldown

imaging or visual

experience

space available

family circumstances

mount

there's no doubt in my mind that aperture matters for most objects and also that there's unlikely to be one scope that does everything well. 

I have five scopes:

16" f4 dob - used almost exclusively when I go to star parties or darker sites on my own

12" f4 dob - my most used scope at home

6" f11 dob - superb on double stars and moon - I use it when the moon is large and bright for moon, doubles and planets - also until recently for solar observing

80mm f11 achromat - bought as a solar scope for use with a Lunt wedge and gives the best white light visual observing I have ever had

102mm f10 achromat - effectively bought for the same reasons as the 80mm but a cheaper option for use at school for me and the astronomy club 

In essence I agree with the above comments that as a main scope a small (aperture) scope is a small scope and will almost certainly lead to wanted more aperture quite quickly, even if it is an ED of decent quality. it can be used for imaging of course with the right mount and for nice wide field views when a compact unit is needed for travel etc (I have plans in the future to buy e.g. a 72/80mm fast ED scope for wide field observing and maybe white light with the wedge when travelling).

the one scope to my eyes that comes close to all things for all 'men' (visually at least) is an 8" dobsonian. my recommendation is to buy a larger visual scope (of whatever type) and a smaller imaging scope which can of course double up as above.

not really being helpful really am I? 

[ronin]

I suspect much of an answer depends on what or which objects are higher priority.

If you want planets primarily then perhaps something that will deliver a higher magnification may be more appropriate. Will say there are in effect 3 planets to view and a lot more DSO's.

If therefore you decide to bring DSO's up in priority then aperture becomes more relevant. You do not need the magnification adn you do usually need the light collection.

A nice 80mm ED should allow say 150x without compromise and so show Jupiter well and Saturn pretty good. Mars is a problem and as it is not going to be around for another 2 years then not really worth considering at this time.

If you go DSO hunting then unfortunatley bigger is better, although an 80mm ED will show a lot. Lets face it you are going to need a BIG scope to see anything of the galaxies in the Coma Cluster. Where a 80mm loses is extracting detail in something like M33 in visual.

If you are looking at the TS site as an example and source of information:

The 80mm f/7 would seem to be a good all round option.

The 90mm f/5.5 a bit nicer for DSO's but I suspect little advantage for planets over the less costly 80mm.

Better for both is the 102mm f/7 but that means an additional 200€ on the top budget.

[Liquid360]

At this point there really isn't much left to say except specific recommendations which now seem a bit superfluous and so I'll keep this brief.

I've a SV90 APO Raptor. I like it lots. Very pretty.

The End [emoji16]

[Mark at Beaufort]

I had a William Optics SD66 refractor which I had as a  travel scope. It provided excellent wide field views and gave me one of best views of the North American Neb using a 26mm Nagler and UHC filter. I also had a 2.5mm EP which gave me a 155x mag suitable for double stars. I regret selling this scope.

Yes I did occasionally view the Moon and Planets and it was okay. I also had a thousand oaks glass solar filter which again gave reasonable views in white light.

I think if you want to view just the Planets then you would be better buying a Mak/Cass - something like a Skywatcher 5".

Like Shane has stated there are so many options - I have a 180mm Mak/Cass solely for the Moon and Planets, a 10" Dob for DSOs, a 6" f5 Newt for quick grab and go and a 4" Astro Tech APO for solar (Herschel Wedge) and double stars.