Observing the planets with a "fast" scope

[Puffafish]

I am currently looking at getting my first telescope, and I'm pretty sure that I've settled for this Skywatcher 130P (and then can add a motor later if and when). http://www.firstligh...lorer-130p.html

However, this doesn't come with a barlow lens, so I shall be limited with magnifications, which might be an issue when observing the planets (apparently not so much when looking at deep space objects).

As this scope has got "faster" optics (I believe) than then 130M (600mm instead of 900mm), what difference would I see when using it? I would like to be able to see some of the bands and moons of Jupiter, and the rings of Saturn, is there much of an issue seeing planets with this shorter focal length? (I've seen suggestions that a longer 'scope is better for planets, but I'm not sure why, and I can't see a website that can show me the real difference, only ones that give scientific explanations as to what it means)

[ronin]

Right or wrong I tend to assume that a 5mm eyepiece is as small as most will go, after that they are more difficult to produce well and so more costly. So on that line of thought a 5mm will give you 130x.

Also some say that an eyepiece equal to the f number of the scope is "optimum", did see a reason for this somewhere but never relocated it again to delve through. Again though that means a 5mm eyepiece.

So 5mm eyepiece, as said gives 130x. That is enough for Jupiter and it's bands, you may find a better image at 80x, so the scope should supply a magnification and image that is adaquate. The moons remain as little dots around Jupiter.

Saturn next, I have found that somnething arond 120x is adaquate, have seen rings with a bit less but sticking to 120x again the 5mm eyepiece will deliver.

Which 5mm eyepiece? At f/5 I would avoid the plossl's, you will be wearing one as if it is a contact lens. For eye relief that means a TMB clone, BST Starguider, Celestron X-Cel, WO planetary.

I find the BST's are better then the TMB clones, as they are the same cost that drops the TMB out, for me anyway. So you have the BST at £47, and the X-Cel and WO which are both £69 I think.

Could up the budget for the ES 82's at £142 from Telescope House.

Alternative is something like an 8mm and 2x barlow but that is buying 2 lumps of optics and you need a good barlow.

Make sure it is collimated as that can "soften" the image and for bands and rings you need a shar image to deliver the detail.

[Qualia]

Good stuff, Ronin

I am currently looking at getting my first telescope, and I'm pretty sure that I've settled for this Skywatcher 130P (and then can add a motor later if and when). http://www.firstligh...lorer-130p.html

An 5" f/5 is not an ideal planetary scope. Being of such a short focal length it relies on very short focal length (FL) eyepieces to gain significant magnification. A longer FL telescope allows one to use longer FL eyepieces, which in turn would give one more eye relief making viewing a more comfortable possibility.

As you'll appreciate, magnification is equal to the telescope's FL divided by the EP's FL. So, if I wanted to enjoy Jupiter, for example, at around an average night's viewing of 140x to 160x in the 5" f/5, I'd need an eyepiece of around 4mm to 4.5mm. I often view Saturn between 150x to 200x, so if this were the same for you, we'd need FL EP's around 4mm to 3.3mm.

If you were wanting to do planetary observations, typical recommendations for the 5" f/5 might include: i) a zoom eyepiece of decent quality of between 3-6mm (a TV could be picked up for around £250) ii) a decent Barlow which in effect helps double your EP collection whilst giving you the choice of buying a little longer FL eyepiece (say an 8mm giving you a hand in viewing DSOs and lunar work) and retaining its longer eye relief; iii) a 60º field EP of around 4mm which due to its wider field will be easier to track your object and finally iv) an ortho of around these focal lengths (something which might be tricky).

It is great to see you setting your expectations a priori and a big plus-point for going to somewhere like First Light Optics for your purchase which will give you a super sound and ongoing client attention (I have no relation with the company but just admire their 100% commitment). But just looking at the numbers, I figure that if you want to do some serious planetary viewing, you will be pushing your system and eyes. This is not to dump on the scope in any fashion, it will be beautifully crafted wide-field scope and these telescopes can look at planets all the time but whatever our telescope, I think it is always helpful to the soul and mind to have reasonable expectations of what is realistically possible. Have you thought about something like the 150p which although a little more expensive might in the long run serve your better (or if you can push out the boat something like the 8" ).

Best of luck with whatever you decide.

[umadog]

I agree with Qualia. Not a problem and you won't need a barlow. You can get short focal length eyepiece nowadays with good eye relief. Like Qualia says, a 4 mm eyepiece will do what you want so why worry? The aperture won't let you go much beyond 4mm or 3.5 mm anyway and those eyepieces exist. I think a lot of this talk of fast or short scopes being unsuitable for high powers stems from ye olde days when short focal length eyepieces were horrible to work with. Nowadays that's just not the case. The only issue is the these nice eyepieces might cost a little more, so it could be more economical to buy a barlow and use longer focal length eyepieces with a more simple (cheaper) design. Either will work perfectly well, though.

The only note of caution is that an f/5 is a relatively fast scope. As a consequence, it will be picky on eyepieces at lower powers. Eyepieces that don't work well in fast scopes will show a fair bit of astigmatism and so the views will be blurry at the field edges. Also, you will have to nail your collimation at f/5. Even with good eyepieces, at f/5 there is some blurring at the field edges due to coma (Google it). If you opt for an f/6, you will get crisper views for less money. I think an f/6 is a better scope for beginners (particularly those on a tight budget).

[Qualia]

That's just brilliant, Umadog. Fantastic post!

[AlexB67]

I am currently looking at getting my first telescope, and I'm pretty sure that I've settled for this Skywatcher 130P (and then can add a motor later if and when). http://www.firstligh...lorer-130p.html

However, this doesn't come with a barlow lens, so I shall be limited with magnifications, which might be an issue when observing the planets (apparently not so much when looking at deep space objects).

As this scope has got "faster" optics (I believe) than then 130M (600mm instead of 900mm), what difference would I see when using it? I would like to be able to see some of the bands and moons of Jupiter, and the rings of Saturn, is there much of an issue seeing planets with this shorter focal length? (I've seen suggestions that a longer 'scope is better for planets, but I'm not sure why, and I can't see a website that can show me the real difference, only ones that give scientific explanations as to what it means)

Hello Welcome,

If you fish around many dealers often do a free barlow with scopes. I got mine from scope 'n' skies and the barlow I found not to be bad at all, It is actually a better one to the one they advertised, even comes with a screw on lens to do an optional 1.5x, though never found a reason to use that magnification as yet.

I can only tell from my limited experience that the F/5 scope has given me very satisfying views of Saturn and the moon, though as I learn and read a lot around here, I appreciate the arguments why a slower scope would be recommended, everything else being equal for planet use, that being said I have no complaints with the F/5 130p starter scope at all.

If you want to get a rough idea of what the magnification does for you for given eyepieces, aperture, focal length combination, you can play around with this little simulation,

http://www.telescope...id=45&Itemid=57

though knowing what I have seen, it tells far from the whole story, but it is one of the better websites I found to give a rough idea at least. Don't be fooled by how small a planet may appear in an eyepiece, given your eye will be right up against it, more or less, even at 26x magnification I can clearly make out Saturn's shape, that site would lead you believe you can't. I'd say it is an underestimation. Of course many factors are ignored in that representation, brightness, imperfections in optics and atmospheric effects and so on, so don't get to attached that calculator as a clear anwer

All the best.

[Luke]

Some great points above. Just to add that the best view I have had of Jupiter has been with an F4.4 dob, better than with an F10 SCT (I think the dob's larger aperture won under very good viewing conditions).

[Schorhr]

Just to ad my two cents ;-)

True, the 130mm with just 650mm is not the most ideal telescope for planets... BUT:

I have several telescopes, and I recently purchased a Heritage 130p.

The dobsonian is simple, and it's easy to view planets even at high magnifications!

The 2.5mm HR Planetary is sold for 32,50-45€ (I think roughly 28-40gbp) and will give a magnification of 260x.

IF the mirror is good, some h130p have had issues I heard, but I'd return it if there's problems.

260x will require good collimation (Easy but has to be done!) and good seeing (clear night, no air turbulances) and the planet should stay high.

Else a 4 or 3,2mm EP with around 200x will show a smaller but sharper image.

As conclsion, the 130mm mirror will give you very nice views as much more magnification then 300x is rarely possible anyway.

My first view of Saturn with it this year blew me away! I was expecting much less quality from such a cheap telescope, but I was wrong. Moon with 260x is stunning, too.

Planetary (30-50€) and UWA (30-40€/27-35gpp) eyepieces are great value, I do use the included eyepieces and also Plössl, but rarelly.

Two eyepieces and a 15€ achromatic barlow or better three to four eyepieces work very nice for everything (Deepsky, planet).

The explorer on EQ2 mount is a bit critical IMHO. I just mounted my 130p on a Astro3, which is a tad more stable.

So you can follow a planet by turning a knob, great, but it will shake on high magnification.

On the Astro3 i'd say 0.5-1.5 seconds after turning the telescope, longer when you nudge the eyepiece with your head :-)

AND keep in mind that both the Astro3 is more rigid then the EQ2 -and- the Heritage is a flextube, so the leaver is shorter.

I inserted my 450d to demonstrate the shaking, and it's about the same as trhough a eyepiece with high magnification:

Shaking is almost not visible in overview (LiveView without magnification, similar to it was on a visual magnification of 40-60x)

With high magnification, it does take a while to settle:

Nudging the telescope:

(The 450d has no videomode, thus filming it)

I do not own the 130 on EQ2 but it should be even worse at high magnification.

Excuse the mess :-)

I had to power my laptop and such, cable chaos.

It's possible to make a mount a little more sturdy, but it would still not be good enough for astro photography IMHO, at least longer exposure for DSO would be difficult. 650mm focal length is already a bit critical for starter astro photography anyway, especially on such a mount.

I will be using this combination visually a bit, but compared to the dobsonian, it clearly looses.

While you will have to move the dobsonian manually, it is much more sturdy, and especially over 130mm the dobsonian will be much lighter then on a STURDY EQ mount.

I like the h130p as it fits into a large backpack and is set up within seconds. On a table or Box, or with a collapsible stool and seatpad it is easy to use too.

Following a planet at 200-260x is fairly easy if you're not overly clumsy and the dobsonian mount is not overtight (and the tube centered on the dovetail mount)

On another note, I have a 76/300 and with the 2.5mm eyepiece, Saturn's rings are well visible (though no details then, due to bad seeing when I first tried).

[Mattscar]

Hi Puffafish

I have the 130p and can honestly say Iv not found an EP yet that makes a planet look bad at all. Even with the stock 25mm you should get a nice view of jupiter with its 4 galliean (I apologize for the spelling) moons strung out (although this is becoming harder as Jupiter is getting further and further away from us at the moment) and the 10mm gives a nice look at cloud bands on the planet. Iv not had chance to observe Saturn much at the moment, but when I did get chance I could certainly see the rings

If you do want a cheapish way of having a sparkling new EP quite quick after getting your scope, the BST Explorers from Skies the Limit on ebay will do the job very nicely. Iv got two of them now and love them.

[Schorhr]

Indeed, BST/Planetary eyepieces are nice. Also wide angle eyepieces in general with long eye relieve. I have a 4mm Plössl and you have to crawl into it to have a view, very exhausting and makes the telescope shake a lot :-)

Also a achromatic barlow and a 6mm UWA eyepiece makes a great & inexpensive combination. Along with a 20mm UWA, that's all that's necessary at the start.

In the long run a 24/25mm wide angle EP would be nice as it gives the maximum view with a 1.25" focuser, but a 10gbp/15€ 40mm Plössl can do that too. The red-dot-finder is somewhat limiting (but IMHO better then a chap-ish optical finder, especially with a overview eyepiece)

I usualy tale along a 20mm Erfle, 8mm Planetary and either 2.5 or 3.2mm Planetary (or both). Usualy that's a good compromise for DSO and Planets, without having to organize too many eyepieces.

[michael.h.f.wilkinson]

One thing to bear in mind is that given the same optical design slower scopes are better than faster scopes on planets due to better control of aberrations, and in the case of Newtonians smaller central obstruction. This does not mean faster scopes are bad. For those who have never seen Saturn through a scope, a view through an ST80 with 5mm EP is still mind-blowing, because it is the best they have seen yet, for all the chromatic aberrations the scope has. By contrast, a fast scope is handier for wide field observing, simply because more of the sky fits into the barrel size of a given EP. This does not mean that a slow scope is bad for DSO observing, as is often heard.

Any scope is a compromise, and the 130mm F/5 dob is a pretty decent all-round visual instrument, especially for a beginner

[Schorhr]

I second that :-) Though f/5 is somewhat critical when it comes to sharpness of the outer field with inexpensive eyepieces.

But really, it's a matter of taste and at the beginning not much of an issue, especially when keeping in mind that a very good eyepiece can cost more then that telescope :-)

[Matt Scunthorpe]

Just going on from above, i have an F5 scope, albeit of 200mm mirror. I use the Celestron x-cels and i've never had a bad view of a planet.

[Schorhr]

Here a quick comparison, both at bad seeing with the 2.5mm Planetary eyepiece.

One in the h130p, and one in the €20/18gbp 76/300 firstscope-clone

Excuse the crude sketches, I sometimes doodle like this when I have no table around...

Better then doing no observation log at all!

Also it does represent the visual impression of objects better then photographs.

h130p (260x)

It was getting a little foggy, there was a bit of a glow around the planet, this and the seeing made it hard to observe, but a few glimpses of the cassini division where possible...

76/300 (120x)

Not bad for such a cheap little €20 telescope! Though it only works well with a bit of tinkering due to the lack of collimation screws, and also the eyepiece cost more then the scope itself ;-)

140x could be possible on this telescope, but due to it's cheap design and components, this is about the good it gets.

Jupiter in the h130p from the balcony

Sometimes shows more then two bands, sometimes doesn't.

I'd say the h130p does show a little more then a 102/1300mm Mak I occasionally use, but the Mak is a nice device for planets and can be used on a stable camera mount... It higher focal length and aperture ratio make it easy to use with all sort of eyepieces and does not require collimation (as frequently as a f/5 newton).

Though it now costs about 200€/180gbp and up, without a mount. So for the 130mm telescope's price it's great, and will show more then a 80-114mm starter scope when it comes to star clusters and deepsky object, at least it's closer to a 6" then it is to a 4" visually imho...

[umadog]

One thing to bear in mind is that given the same optical design slower scopes are better than faster scopes on planets due to better control of aberrations, and in the case of Newtonians smaller central obstruction.

Sure, slower means fewer aberrations, but a fast scope doesn't have to mean a large central obstruction. My f/4 has a central obstruction of 20%, so it's negligible. At f/5 the coma won't be severe enough to cause issues with high power viewing. At f/4 it's possible one would want a tracking platform or a Paracor even for planetary work, but that's not an issue for the OP. I suppose what I'm saying is that you can make a fast scope work well on planets, but it's not cheap to do so (e.g. fancy apochromat in the case of a refractor).

[michael.h.f.wilkinson]

Sure, slower means fewer aberrations, but a fast scope doesn't have to mean a large central obstruction. My f/4 has a central obstruction of 20%, so it's negligible. At f/5 the coma won't be severe enough to cause issues with high power viewing. At f/4 it's possible one would want a tracking platform or a Paracor even for planetary work, but that's not an issue for the OP. I suppose what I'm saying is that you can make a fast scope work well on planets, but it's not cheap to do so (e.g. fancy apochromat in the case of a refractor).

Small fast Newtonians need comparatively large CO compared to a 10". My kids' 114mm mini-Dob has a 40% CO, which is not negligible. My 6" F/8 had something like 20% CO, vs 30% for an F/5 design I was contemplating.