Type of scope

[Mak the Night]

Thanks.

Mak the Night I am looking at the SW 130 explorer and just wondered if the parabolic mirror was worth the extra money and if us beginners would even notice at this stage?

There are a lot of repeated rumours and falsehoods in astronomy that get re-repeated with the telling. The alleged inferiority of spherical mirrors seems to be one of them. Spherical mirrors are easier for manufacturers to mass produce cheaply. In telescopes of around f6.9 with an aperture of 130mm (5.1") there should be no discernible difference between a spherical or parabolic, apart from the fact that the spherical mirror in the Explorer has a longer focal length effectively making it more powerful. Plus the OTA is slightly longer. I also believe spherical mirrors suffer less from problems like coma. 

I have no compunction to swap my spherical mirror OTA with a parabolic one. When you get a telescope of 150mm or more I would opt for a parabolic mirror, most telescopes above 6" have them anyway. 

The only advantage to the SW Explorer with a parabolic mirror is that the OTA is more compact. The disadvantage with the parabolic mirror is that it only has a focal length of 650mm.

It depends what you prefer.

[Highburymark]

I really wouldn't go for an EQ mount. At this end of the market they tend to be flimsy and wobbly added to the fact that they are more complex than a dobsonian.

Agree with this. My advice would be to keep things simple. simple to set up, operate and store.

[Mak the Night]

I tend to agree that EQ mounts can be difficult at first to set up. I'm partially paralysed in my right arm and I learned how to set up the EQ 2 mount on my Newtonian and Maksutov predominantly with one arm. Adding the counterweight was the biggest challenge lol!

Once the EQ mount is set up though it works fine and it is relatively easy to track objects especially with the slo mo controls. The disadvantage to EQ mounts IMO is that they are non-intuitive and the eyepiece on a Newtonian can end up in odd positions depending on what direction (and elevation) you are viewing. Some of the angles of the RACI on my Mak when it is aiming at a low elevation can be difficult for me. I can't wait to get a GOTO with an alt-az mount!

The EQ 2 mount supplied with the SW Explorer can make the OTA wobble a little if you bang into it, but it settles down soon enough. Ideally though an EQ 3 would have been a better choice.

I do believe some SW Explorer variants are sold with alt az mounts.

[Alan64]

Between these two in question...

http://www.firstlightoptics.com/beginner-telescopes/skywatcher-explorer-130p.html

http://www.firstlightoptics.com/reflectors/skywatcher-explorer-130.html

The 130mm/5" f/7 has a longer focal-length, than that of the 130mm/5" f/5, and therefore a longer optical tube.  900mm for the f/7...650mm for the f/5.  Take a low-power 30mm eyepiece, for instance.  With the f/7, the magnification would be 30x; with the f/5: 22x.  A 30-32mm eyepiece helps to serve as a "finder", there in the sky.  A larger portion of the sky would be visible with the f/5.  After a search, and an object of interest is spotted, one may then pop into place a high-power 9mm eyepiece: f/7(100x); f/5( 72x), and for a closer inspection.  The f/7 is somewhat better at higher powers, whilst an f/5 is somewhat better for lower powers.

Both telescopes, however, are equipped with same mount.  With the longer tube of the 130mm f/7, there is an increase in the "moment-arm" effect; a flexing of the tube, particularly in the wind.  The tube would therefore be more "shaky", for the lack of a better word, unstable, and therefore more difficult with which to observe, especially at higher magnifications.  The 130mm f/5 would be nonetheless more stable upon the EQ-2 mount.

As a bonus, with the 130mm f/5, you get a parabolic primary mirror.  The Heritage 130P also comes with a parabola, but I'd prefer a solid-tube myself.

That said, with the equatorial, the telescope can follow and track any object there in the sky; and if the RA axis is motorised in future, the tracking would be automatic, with an object practically motionless there in the eyepiece, standing still, for long-term observing, and without having to touch the telescope or the mount.  The telescope demonstrated within this video is the shorter, parabolic f/5 variant...

In future still, if a simpler alt-azimuth mount is desired, the tube can easily be transferred to it... http://www.rothervalleyoptics.co.uk/skywatcher-az4-1-alt-az-mount-with-aluminium-tripod.html

Keep in mind that with the 130mm f/5 on the EQ2, you won't be paying as much for the mount as you will be for the optical tube itself.  The Newtonian is of very good quality; and, if cared for, will last for many years.

Whilst Jupiter and its moons are a sight in and of themselves, Saturn, its "father", with its imposing ring system and its largest moon, Titan, is even more wondrous, and should reappear well into the night sky sometime in the spring.

I enjoyed observing Saturn last late-summer and fall, and with my 150mm f/5 on a traditional alt-azimuth mount...

Great fun, that was!

[Mak the Night]

I tend to agree with Alan64 that the EQ 2 mount may be more stable with the shorter 650 f/l OTA. I never swapped mine for the shorter and faster OTA though as I wanted the more powerful magnification for my primary interest in lunar and planetary observing. A 10mm wide angle eyepiece would give me 90x which easily 3x Barlows to 270x for viewing the Moon. I personally tend to prefer slightly slower scopes. 

[John]

On the spherical v's parabolic mirror issue, a spherical figured mirror can produce decent images if it is of a reasonably "slow" focal ratio. If a 5" / 127mm mirror is to perform to diffraction limited levels I understand that it would need to be around F/7.5. I believe these figures are derived from Jean Texereau's well respected book "How to Make a Telescope". This assumes that a well figured and polished mirror.

Thats not to say that a faster spherical mirror of that aperture can't produce "pleasing" images at faster focal ratios but diffraction limited is the minimum standard that we should be aiming for if the scope is to have potential performance it deserves for it's aperture.

In faster focal ratios than the above, a spherical mirror will exibit, not surprisingly perhaps, spherical aberration as it's figure is at variance from the ideal parabolic figure.

Personally I would always go for a parabolic mirror in a newtonian. With the chinese so good at producing decent parabola's at very reasonable cost, why not ?

[Alan64]

It seems that manufacturers possess the inexplicable notion that an equatorial is best for beginners, and thereby proliferating and inundating the market with the EQ-2 as an all-around mount.  Of course, the AZ-3 is much worse...

http://ca.skywatcher.com/upfiles/en_mounts_caty01317065198.jpg

...so much so that I'd rather have the EQ2 and wing it.  I had one very similar to the AZ3, and loathed it, and for a very brief time before I sent it and its defective Meade 390 refractor packing.  The kit was brand-new.  The 90mm refractor's doublet appeared as though it had fallen out of its cell into gritty, damp dirt, then placed back into its cell.  I had never seen anything like it.  As a result, to this day I own not a single Meade product.  First impressions are important.

I once had a Japanese-made version of the EQ2, and for a Parks 80mm f//11 refractor, and that which replaced the defective Meade and its mount, and I liked it a lot... http://www.astromart.com/images/classifieds/370000-370999/370322-1.jpg

I gave it to a close relative over ten years ago, and I don't believe he has used it even once.  I wish I had it back.

[John]

Meade should never have put a 90mm F/10 refractor on their AZ-3 type mount

Put a short tube scope on one though and it's a decent and very portable mount for a low price.

[Alan64]

Meade should never have put a 90mm F/10 refractor on their AZ-3 type mount

Put a short tube scope on one though and it's a decent and very portable mount for a low price.

Yes, it would work with a smaller telescope; fancy that.  The Meade's was a bit larger, but of the same design, incidentally. 

[John]

Yes, it would work with a smaller telescope; fancy that.  The Meade's was a bit larger, but of the same design, incidentally. 

And both copied from the much better engineered Vixen Custom D mount.

[Mak the Night]

On the spherical v's parabolic mirror issue, a spherical figured mirror can produce decent images if it is of a reasonably "slow" focal ratio. If a 5" / 127mm mirror is to perform to diffraction limited levels I understand that it would need to be around F/7.5. I believe these figures are derived from Jean Texereau's well respected book "How to Make a Telescope". This assumes that a well figured and polished mirror.

Thats not to say that a faster spherical mirror of that aperture can't produce "pleasing" images at faster focal ratios but diffraction limited is the minimum standard that we should be aiming for if the scope is to have potential performance it deserves for it's aperture.

In faster focal ratios than the above, a spherical mirror will exibit, not surprisingly perhaps, spherical aberration as it's figure is at variance from the ideal parabolic figure.

Personally I would always go for a parabolic mirror in a newtonian. With the chinese so good at producing decent parabola's at very reasonable cost, why not ?

I suppose f/6.9 is just skirting the ideal in that case. I'm not arguing that the parabolic mirror isn't superior, but I've used an AstroMaster 130 with some of my own eyepieces (19mm Panoptic, 15mm Celestron Kellner & 10mm Luminos) and I couldn't detect any difference other than the magnification difference with the Explorer. 

Admittedly the telescopes weren't side by side, but I've viewed the Moon and Saturn with both amongst other things. My eyesight is quite good for distance although recently I have needed to wear glasses for reading. I am also slightly photosensitive (an inherited condition) which means I normally wear shaded glasses during the daylight (which is definitely not due to me being a vampire or my making a fashion statement lol) which has the interesting effect of giving me very good night vision as my pupils naturally dilate easily.

So, all things being equal, I wonder just how much worse the SW Explorer spherical mirror is compared to the parabolic version. I personally could detect no difference. If I had the same choice of getting a 130mm telescope I'd probably still opt for the f/6.9, 900mm spherical. Although I'll bet the f/5 parabolic is better for viewing DSO's.

[Alan64]

If only the manufacturers could come up with an inexpensive one-arm or fork-type alt-azimuth, and a bit more robust than that which came with this 60mm...

...and one that would accommodate tube rings. 

I imagine all of that extra cast-metal would make it cost-prohibitive, however.  

It would be a noble effort, and in the spirit of outreaching.

It's as though simpler must be more expensive; illogical.

[Mak the Night]

Spoon01,

In terms of mirror shape, the general opinion seems to be parabolic = good.   Spherical = maybe, maybe not good.    So, parabolic is the safer choice.

For the Celestron AstroMaster and many 130mm scopes parabolic mirrors seem to be the norm. But I couldn't detect any difference with the Sky-Watcher 130mm spherical and the Celestron parabolic apart from the better magnification of the spherical. I think when you get to 6" and above the parabolic is better. So, I'm not totally convinced a 130mm parabolic mirror would be the 'safer' option as it would almost certainly have a faster focal ratio. Which in itself can bring problems with some inexpensive eyepieces.

[Alan64]

I suppose f/6.9 is just skirting the ideal in that case. I'm not arguing that the parabolic mirror isn't superior, but I've used an AstroMaster 130 with some of my own eyepieces (19mm Panoptic, 15mm Celestron Kellner & 10mm Luminos) and I couldn't detect any difference other than the magnification difference with the Explorer. 

Admittedly the telescopes weren't side by side, but I've viewed the Moon and Saturn with both amongst other things. My eyesight is quite good for distance although recently I have needed to wear glasses for reading. I am also slightly photosensitive (an inherited condition) which means I normally wear shaded glasses during the daylight (which is definitely not due to me being a vampire or my making a fashion statement lol) which has the interesting effect of giving me very good night vision as my pupils naturally dilate easily.

So, all things being equal, I wonder just how much worse the SW Explorer spherical mirror is compared to the parabolic version. I personally could detect no difference. If I had the same choice of getting a 130mm telescope I'd probably still opt for the f/6.9, 900mm spherical. Although I'll bet the f/5 parabolic is better for viewing DSO's.

I wouldn't mind having a 130mm f/7 myself.  It's not really a slow telescope, when you compare it to others. 

The 130mm f/5, with its 650mm focal-length, can most certainly accomplish the higher magnifications with a 2x barlow.  A barlowed 9mm, for instance, would realise 144x; a barlowed 8mm: 163x.  Hmm, we're still not over 200x yet...

A better-quality 3x barlow combined with a 9mm would enable: 217x, and considerable.  I routinely 2.8x barlow a 9mm, with my 150mm f/5, and for 234x.  With said combination, I have witnessed the Airy discs and the shimmering first-diffraction rings of the stars of the Trapezium within Orion, beaconing in the night, and with the ghostly nebula swirling around and about them; surreal. 

A 5mm, like the BST Starguider, with its larger and more comfortable eye-lens and eye-relief: 260x, and with but a 2x barlow.  Of course, the seeing conditions would have to cooperate.

An f/5 can be quite versatile, with the aid of a barlow or two.

[Mak the Night]

I wouldn't mind having a 130mm f/7 myself.  It's not really a slow telescope, when you compare it to others. 

The 130mm f/5, with its 650mm focal-length, can most certainly accomplish the higher magnifications with a 2x barlow.  A barlowed 9mm, for instance, would realise 144x; a barlowed 8mm: 163x.  Hmm, we're still not over 200x yet...

A better-quality 3x barlow combined with a 9mm would enable: 217x, and considerable.  I routinely 2.8x barlow a 9mm, with my 150mm f/5, and for 234x.  With said combination, I have witnessed the Airy discs and the shimmering first-diffraction rings of the stars of the Trapezium within Orion, beaconing in the night, and with the ghostly nebula swirling around and about them; surreal. 

A 5mm, like the BST Starguider, with its larger and more comfortable eye-lens and eye-relief: 260x, and with but a 2x barlow.  Of course, the seeing conditions would have to cooperate.

An f/5 can be quite versatile, with the aid of a barlow or two.

f/7 seems positively sprightly compared to my f/12.7 Mak lol. I had the same spherical/parabolic  f5/f7 quandary when I bought my Explorer, in fact I very nearly bought the Celestron AstroMaster and I think it was the better Sky-Watcher mount that convinced me to buy the Explorer (although I believe the f/5 Explorer has the same EQ 2). 

I've read on forums where people have returned 130mm Sky-Watcher Explorers that had spherical mirrors and exchanged them for the 'superior' parabolic ones. This would make sense if there really was a noticeable difference in viewing between the two. I just can't see any discernible difference myself.

I think the f/5 scope is easier to manipulate and the AstroMaster seemed easier to slew than my Explorer which seems much larger. Although the sheer physicality of the f/7 Explorer always impressed me. 

I prefer the 900mm f/l as it is easier to get the magnifications I like with my eyepiece collection. Lunar observing at 270x was easy with my 10mm Celestron Luminos and a TeleVue 3x Barlow. The Luminos tends to whiteout/flare a bit sometimes with faster scopes though with the Moon. I haven't tried my 10mm Delos with the Explorer as I'm not too sure about the weight balance with my disability.

I don't envy anyone trying to choose a budget telescope of around 5" aperture as there seems to be so much choice today. If I had my choice all over again I'd still go for the f/7 Explorer.

[Spoon01]

Thank you very much everyone that commented. I decided to go for the SW explorer 130. It's being delivered tomorrow so fingers x for clear night as my son and I are quite excited!

There's obviously a lot for us to learn and I'm sure I will have more questions at some point.

Thank you all again for taking the time to share your knowledge

Jodi

[Ruud]

Hi Jodi. Congratulations. I hope you and your boy will make many exciting discoveries with the telescope. 

[Alan64]

Whether you chose the f/5 or f/7, the telescope itself will be of very good quality.  We'll want to see a pic of the kit, if possible, and hear of your and your children's experiences, please.

...well done.

[Mak the Night]

Thank you very much everyone that commented. I decided to go for the SW explorer 130. It's being delivered tomorrow so fingers x for clear night as my son and I are quite excited!

There's obviously a lot for us to learn and I'm sure I will have more questions at some point.

Thank you all again for taking the time to share your knowledge

Jodi

Good luck. You won't be disappointed with the Explorer 130. It's basic but quite effective. The included eyepieces are quite usable and are a 10mm and 25mm Modified Achromat.

The 25mm is probably the most comfortable to use. The 10mm , although often maligned, is not bad for a giveaway and will give you a 90x magnification if you have the 900mm f/l, f/7 OTA. 

You may be disappointed with the supplied Barlow however. Mine's unusable and most people have the same experience with it. I think the Barlow lens element is plastic as it doesn't focus properly and degrades the image.

Even the draw tube is only a metal band over a plastic body. Here it is connected to the 25mm (above).

I recommend this Celestron Barlow from FLO: http://www.firstlightoptics.com/celestron-eyepieces/celestron-2x-universal-125-barlow.html