Quick Look at the "Roegger PEGASUS" 150/750 Newton

On Sunday I bought another scope: An Newton reflector, called "Roegger PEGASUS", with 150mm aperture and 750mm focal length (which works out to f/5 focal ratio) – a big mirror in a cheap package. 

Prices range from 90€ (used on eBay) to 140€ (new on eBay), and up to 230€ (if you buy new from an "proper" scope shop on the internet) so I was lucky to get one for 70€. With prices about twice of an 70/700 refractor, will this be twice as much fun? It is cheaply made, but has some potential as a fun scope. :-) I will use it less on planets (my long focal length refractors are probably better suited here) but more on Deep Sky objects.

My scope was branded as "Roegger PEGASUS", but can be found here under various other "brand-names" (and color schemes). There are different versions floating around:

• The version without an corrector (focal length 750mm), available as an "Roegger PEGASUS". Seems very similar to the "TS Optics Starscope 1507", and somewhat similar the "OMEGON N 150/750 EQ-3".
• The version with an corrector (focal length 1400m), available as an "Seben Big Boss", "TS Optics Megastar 1550" or "Bresser Pollux N 150/1400 EQ-2".
• And then there are hugely better versions: Three is e.g. the "Celestron OMNI 150XLT", or the "Sky-Watcher Explorer 150P EQ3" (and possibly the GSO). These have an parabolic mirror and much much better EQ-3-2 mount (aka "NEQ-3"). These telescopes are for all practical purposes completely different scopes.

Beware: Even if the numbers are the same, there can be differences between the brands!

What was included with mine:

• The OTA with 1.25" focuser (rack and pinion)
• A corrector lens ("Bird-Jones") was not included
• An EQ-3-1 mount An EQ-2-2 mount
• Two mounting rings and one "screw on" rail (that almost looks like a thin dovetail rail)
• One Plössl 25mm eyepiece (1.25", non-standard filter size) 
• One Plössl 6.5mm eyepiece (1.25", non-standard filter size)
• One 2x Barlow (plastic housing, singlet glass lens, lots of stray light, same Barlow as the one supplied with my 76/350 table-top Dobson)
• One "1.5x erecting eyepiece" (ignored)
• A "6x30" finderscope (which I ignored mostly, for now) with 30mm aperture and achromatic lens
• A flabby finderscope-bracket

Some of the things worthy of notice:

• The primary is in all likelihood spherical.
• [Update 2013-11-01] The diameter of the primary is 153mm (6 inch), it has a grinded bevel of about 1mm width, and the edge of the primary is 14.5mm thick. (BTW: I used a black marker to darken the bevel.) 
• [Update 2013-11-01] The adjustment of the primary cell is done via three sets of push/pull screws, but no springs – longer screws and a set springs will greatly benefit the collimation process. BTW: The scope was only "screwed together" (one could see that all three "pull screws" were screwed in first, and the "push screws" were barely threaded out) – but collimation was somewhat OK, I have seen worse.
• [Update 2013-11-01] The secondary needs slight adjustment – I will have to do a proper collimation.
• [Update 2013-11-01] The full diameter of the back of the primary is covered by a foam pad, a piece of cardboard and on the outside a piece of camera leather (together about 6mm thick). I removed all three and put in some small DIY pads underneath the clips of the mirror cell.
• The secondary mirror is rather small (roughly 38mm, about 25% central obstruction) and does not fully illuminate the primary. I suspect that the effective aperture is more likely to be 120mm to 130mm (very rough estimate). [Update] I've done the numbers and a 38mm secondary (at an distance of roughly 560mm from the mirror) will fully illuminate 150mm (on axis, that is). Some off-axis vignetting will occur however: to illuminate a diameter of 20mm at the eyepiece one needs an secondary of about 52mm diameter. (If used with a Barlow the vignetting will be reduced. One needs however to make sure that the Barlow itself does not add vignetting…)
• The spider holding the secondary has three vanes which are rather thick (5.5mm). But people who done tests write that it won't hamper contrast much (if it is noticeable by me anyway) – collimating (and a good figure of the mirror) is more important.
• The diameter of the OTA tube seems to be a bit too small.
• At the outside of the OTA one can see that the scope is cheap: The seam of the OTA protrudes a bit, and when you rotate the OTA in the mounting rings you the OTA seam will "bump" against the hinges of the rings… But that is only a minor problem.  
• The OTA tube is deformed at the end of the mirror. O.o This is caused by the OTA end ring being too large, and the screws connecting the end ring with OTA tube cause the tube to be deformed. (Furthermore some light can get inside the tube through the gap between end ring and OTA tube.) [Update] For each screw holding the rear cell to the OTA, I put some one washer inbetween the OTA and the rear cell.
• There is a thread at the inner end of the focuser's tube, probably for an Bird-Jones corrector to double the focal length to 1400mm. (With this corrector the secondary might fully illuminate the primary – don't know, sure would like to know.)
• The focuser itself is quite nice. Only major problem was that the focuser's tube was loose! The tube had at least a couple of degrees play! However with a bit of flocking material I took take care of it. Otherwise the focuser's tube does protrude a bit into the OTA, if fully racked in (but not too much). The eyepiece retaining ring is made of metal and quite solid (nice), the focuser's tube is made of metal (nice), the focuser's casing is plastic (but OKish), the tube has lot's of travel (nice), the knobs are large enough (nice).
• The filter threads of the eyepieces is non-standard. (At least the barrels are interchangeable with the eyepieces from the 76/350 – those have a standard thread at one end and a "coarse" thread at the other. So I could exchange the 1.25" barrel from the useless H20mm eyepiece and put in on the PL25mm – now I can use standard filters with PL25mm)
• The supplied "Moon filter" is the most nonsensical piece of astronomical equipment I have ever seen. It is a (heavily over-engineered) disc of solid aluminium painted in black, with about 8mm thickness (!) and 30mm diameter. The filter is held by an screw-in plastic retaining ring with an clear aperture of about 12mm. The filter itself has about 15mm diameter. And now comes the real kicker: The green plastic filter has curved surfaces in the center and flat on the periphery of the filter! This thing looks like the bottom of a green Coke bottle… With the non-standard filter-thread, it doesn't even make sense to replace the filter glass, if you happen to have an 15mm diameter filter lying around.
• The PL25mm eyepiece has an "additional" section between the 1.25" barrel and the optics. This section seems to be designed to hold an reticle (again heavily over-engineered).
• The finderscope bracket is very awful.
• An EQ-3-1 EQ-2-2 mount (and tripod) was not made for such heavy scopes. The problem is that the axis have to much play and I haven't found a way to reduce the play any further. My EQ1 (adjusted, with DIY wooden legs) seems more up to the task… But the legs of the EQ-2-2 are better than those of the the EQ-3-1.
• The counterweight rod is 10mm (like my EQ-1/EQ-2) and not 12mm (like my other EQ-3-1).
• The counter weight is one piece of about 2.9 kg.
• The counter weight rod does not rotate (like my other EQ-3-1 mount).
• The "north adjustment" of the EQ-3-1 EQ-2-2 is slightly different than my other EQ-1/EQ-2/EQ-3-1 mounts: Normally there is a machine screw at the bottom (that you don't need to touch), and a tommy screw at the side to fix the north position. But here there is only one tommy screw at the bottom. The advantage is that you can easily separate the mount from the tripod.
• The tripod legs are more stable than those from my EQ-3-1 (from my 70/700).
• The mounting rings seem quite sturdy. Opening the rings requires completely unscrewing two screws, which is a pain in the proverbial behind.
• I have mounted the scope on the dovetail and mount from my 70/700 – but without a supplied quick-release the combination of scope and mount is quite unwieldy.

  Enough written today, hopefully more once I played around with it a bit.

Serendipity – Broken Glasses and the Pleiades

Yesterday I bought an used 150/750 Newton telescope, of course I got it rather cheaply (this *is* Cheap Astronomy, after all). The 150/750 telescope is everything it promises: a large mirror in a cheap package, with quite a few quirks – it will be fun playing around with it.

I choose a scenic route back from the seller (having no chance to do otherwise an vacation, this was quite nice), and while I was exhausted from the journey I had to try out the new scope at the night sky. And then it's when it happened: I had the glasses rather carelessly on top of my head, they fell down and broke (right in the middle) into two pieces. Though I don't need them to look through the scope (and prefer rather not to wear them at the eyepiece) they are necessary for me to look with the naked eye at the night-sky. Without glasses, the stars are quite blurry – which proved to be a fortunate accident. When I later scanned the night sky with the truly naked eye (after having possibly seen the California Nebula in the 150mm Newton) I stumbled without glasses upon the Alpha Persei Cluster, (aka Mel 20, Melotte 20 or Collinder 39) the Pleiades.

[Update] I am such a noob. I got side-tracked in my star-charts and can't properly tell altitude. The object I saw was much lower in the sky (about 20 degrees) than Mel20 (about 45 degrees) – it was the Pleiades! In a 70mm scope at 12x magnification they look a little bit different than the photos: much more lively. Though I should have recognized them in the binoculars. Doh. m( Thankfully nobody reads this. :-) [/Update]

For me, without glasses, the Pleiades seem like a longish blur, a bit reminiscent of an elongated galaxy – standing out quite clearly against the night-sky. There is something there! I grabbed my 7x60 binoculars, and sure enough there is a nice star cluster there.

Next my modified 70/300 refractor came in rather nicely. With a 25mm Plössl (at 12x magnification) it was splendid. The refractor revealed more than my (bad, bad) 7x60 binoculars, showing the brilliance of the stars in that cluster rather nicely.

What is important to have a large field of view to reveal the contrast between the star-rich cluster and the "normal" night-sky with a lower star density – if you yank up the magnification then the star cluster will look like any other star-field… Though (at 300mm focal length) a PL30mm, PL32mm or PL35mm eyepiece would probably be nice, as with the lower magnification (10x, 9.4x or 8.5x respectively) it might reveal more of the surrounding sky, and frame the cluster better.

I must say, the Pleiades are a night-sky target well suited for binoculars and small telescopes with low magnification (in the range between 7x and 15x) – highly recommended eye-candy!

[Update 2013-10-05] Now that I know what to look for I took another look a the Pleiades yesterday. Some cloud-ish something was hanging in the East and the view of the Pleiades wasn't nearly as vivid as I the first time I saw them. I could have sworn: the first time I say the Pleiades I thought I could discern different star colors. Mainly blue for the brighter ones, but some orange-ish dimmer ones interspersed. (I think my memory is playing tricks on me).

No colorful stars yesterday, and much less vivid – the weather was not as good last night. At both nights I tried the Pleiades with the 70/300 Refractor and a PL25mm (12x magnification) and my new 150/750 Newton and the same eyepiece (30x magnification). The 5.9 inch Newton is significantly brighter, as was to be expected. The images at that magnification are good in the 5.9 inch Newton, but of course the Pleiades now fill the entire FOV of the Plössl. A PL32mm or maybe a PL35mm would be a welcome addition to my eyepiece collection – maybe for Christmas. :-)

I am very happy to have bought that cheap 150/750 Newton. The EQ-3-1 mount and tripod isn't great, but I got it to behave with some small mechanical modifications.

On the first night I saw the Pleiades I tried the little 76/350 Newton as well (which has an effective aperture of about 60mm) and I must say the images are not good compared with the 70/300 Refractor (which is a badly collimated instrument). Viewed through the focuser's tube it is obvious that the secondary of that little Newton is too much to the front. The primary is not adjustable (at least without some DIY modifications) – but adjusting the secondary might help a lot.

Telescope Basics: Aperture Size – Pros and Cons

Probably the most basic telescope parameter is aperture (no, not that one). The thing about aperture is: The size of the aperture is the area that collects light. As the night is usually quite dark, you want to gather more light.

The question is: all other things being equal, what are the "pros and cons" of smaller versus larger apertures? Usually, "More Dakka" is the way to go – most of the time there is never enough aperture. Want to see M31 (the Andromeda Galaxy) in its full glory, then a 24-inch Dobson is your friend.

However real world limits (like available funds, available space, available means of transport, or even mundane things like astronomical seeing at your observation site) usually can make smaller apertures sensible and worthwhile.

Some things are quite fine in small apertures. E.g. a cheap used 60/900 refractor is quite OK for first views of the Moon, Saturn, Jupiter or even Mars – though the number of "small telescope objects for beginners" is somewhat limited. Most things (like planetary nebula or galaxies) start to make fun in somewhat larger apertures – especially if you are an beginner.

With experience and good seeing, one can get by with smaller apertures.

	Advantage	Disadvantage
Smaller Aperture	- Less suspectable to bad seeing - Easier to manufacture - Easier to curtail aberrations - Cheaper - Lighter - (Usually) easier to handle - Faster cool down time	- Less light gathering - Less resolution
Larger Aperture	- More light gathering - More resolution	- More suspectable to bad seeing - More demanding to manufacture - More demanding to curtail aberrations - More expensive - Heavier - (Usually) more cumbersome to handle - Longer cool down time

Telescope Basics: "Fast" and "Slow" Focal Ratios – Pros and Cons

I wanted to go through some "telescope basics" and thought that gathering "pros and cons" regarding focal ratio would be a good start. This table applies if comparing scopes that differ only in focal ratio, but are of the same telescope type (Newton, Fraunhofer, and so on) and have the same aperture – e.g. comparing a 60/350 refractor with a 60/900 refractor, or comparing an 114/500 Newton with an 114/900 Newton.

Judging by the number of points, the result look like they favour "slow" telescopes – and sure enough if you want to cheaply get a good scope, it is easier to get a "slow" scope (e.g. a 60/900 refractor or a 114/900 Newton) which is cheap and good. Getting a "fast" scope (e.g. an 60/350 refractor or an 114/500 Newton) which is cheap and good is a bit harder. Either you pay more for a faster scope, or you (usually) have to accept less quality.

The main point against slow telescope is length of the OTA, with the problems following from this (needs better mount, more difficult to transport, takes up more space when stored).

	Advantage	Disadvantage
Faster	- Shorter tube (less demanding on the mount, easier to transport and store) - Easier to achieve wide-field and rich-field views	- More aberration (chromatic, spherical, coma, and so on) - More expensive solutions needed to combat aberrations (e.g. parabolic primary mirrors for Newtons, or APO lenses for refractors) - More demanding to manufacture - More suspectable to collimation errors - Selection of "proper" size of focuser (for refractors) or secondary mirror (for Newton scopes) is more challenging (vignetting and/or reduced aperture can easily become a problem) - More stray-light problems - High magnifications are more difficult - More demanding on the eyepiece  - Usually more expensive
Slower	- Less aberration (chromatic, spherical, coma, and so on) - Less demanding to manufacture - Less suspectable to collimation errors - "Correct" size of focuser or secondary less critical - Less stray-light problems - Easier to achieve high magnifications - Less demanding on the eyepiece - Usually cheaper	- Longer tube (more demanding on the mount, more difficult to transport and store) - Wide-field views are more difficult to achieve

One more point arises when one wants to do prime focus astrophotography, which is that the faster scope allows shorter exposure times (being "faster") – with an increased FOV, but decreased resolution (less magnification). When using eyepiece projection or afocal astrophotography, this advantage is no longer of relevance and then the slower scope usually is better suited (with less aberrations).

(If you compare a catadioptric Newton with an non-catadioptric Newton – e.g. an 114/1000 with an 114/900 – then usually the catadioptric version has most of the advantages and disadvantages of a fast Newton. In this example the catadioptric 114/1000 is in most regards more on par with an Newton 114/500.)

Welcome!

So I thought I move over my "Cheap Astronomy" stuff to a dedicated blog. Let's see were it goes.

(I am BTW in no way associated with the wonderful Cheap Astronomy at www.cheapastro.com.)