Cheap Astronomy – A Quick Look At The Bresser 76 / 350 Table-Top Dobson

A supermarket chain here has been selling off the "Bresser 76 mm (3 inch) f=350mm table-top Dobson telescope" at a very very low price. So naturally some of those sold wound up at eBay, with an surcharge (naturally), and naturally I had to buy one. (Had I known of this offer by the supermarket chain in the first place, I would have obviously bought directly there, saving myself a bit of money – but I digress, back to Cheap Astronomy).

The scope has arrived this morning, so I hadn't had a chance to test it on the night sky. I however played around a bit with it, and I thought I write down what I learned so far (and as I doubt I have the energy at the moment to do a proper review of it anyway). [Update] And of course the weather turned bad the afternoon… [/Update]

What's in the box:

• The table-top Dobsonian telescope (Art. No. 88-43201)
• Nominal aperture is 76 mm (3 inch)
• Nominal focal length is 350 mm (13.8 inch)
• The nominal focal ratio is f/4.6
• An H20mm Huygens(?) eyepiece (1.25 inch), giving a nominal 17.5x magnification
• An SR6mm Ramsden eyepiece (1.25 inch), giving 58x magnification
• An 2x "shorty" Barlow (1.25 inch), increasing the nominal magnification to 35x and 117x respectively
• And blue "Moon filter"
• Caps for focuser and Barlow, canisters for the eyepieces
• An planisphere
• An CD with the Stellarium software
• Some documentation
• A finderscope is not included

The telescope (and mount) features:

• An 1.25 inch focuser 
• A single-arm secondary mirror support
• The OTA has an outer diameter of about 95mm (the end caps are about 1mm larger in diameter)
• Two setting circles, one for azimuth and one for altitude
• The tension of the altitude axis can be adjusted with a large knob
• An wobbly compass in the base

A quick estimate regarding the secondary and effective aperture:
The secondary mirror has about 19mm diameter (small axis, +/- 1mm) and is about 235 mm (+/- 10mm) away from the main mirror.

If I did not make an mistake:
To illuminate an 20mm diameter field at the eyepiece, the secondary diameter would have to be about 40mm – for 5mm this comes out to about 29mm diameter. With an 19mm secondary, the on-axis aperture becomes around 60mm.

Taking into account the (arguably small) obstruction of the secondary with sqrt(60*60 - 19*19) sqrt(30*30-14.5*14.5)*2, we end up with about 57mm (2.25 inch) effective aperture. (For comparison a 29mm secondary would yield an effective aperture of about 70mm)

Blunders, big and small:

• The main mirror is probably spherical (will see how the stars look)
• The main mirror is not adjustable (adjustment on mine seems fine though)
• The secondary is (probably) too small
• [Update] After a short visual inspection I can confirm that the secondary is definitely too small.
• [Update] Furthermore the position of the secondary is not centred when viewed from the focuser (the secondary is moved too much towards the front). The collimation screws of the secondary are too short and make achieving a proper collimate impossible! One has to replace the screws with longer versions!
• [Update 2013-10-06] I replaced the secondary's screws with a bit longer versions – now I could put the secondary were it belongs. (And I need to buy some long M3 screws.) I collimated the secondary (no Cheshire, no laser) and I hope it should now be better – a daytime test was promising. Before collimation the image from the Pleiades was a bit disappointing compared with my 70/300 refractor – let's see what it does the next time at the nightsky after collimation.
• [Update 2013-10-07] With longer screws and after collimation the scope is much improved. Looking at the Pleiades the view is almost like that from my 70/300 refractor – the Pleiades were not a pretty sight before collimation in the little Dobson. Maybe with proper collimation tools (Cheshire and Laser) I could improve it a bit more beyond that – but this has to do.
• The focuser shifts quite a bit. 
• The eyepieces are not parfocal (which is aggravated by the focuser shifting)
• Using the eyepieces with/without the Barlow is not parfocal (again bad due to focuser shifting)
• The altitude setting circle has some +/- 5 degrees play with respect to the telescopes altitude (with 0 degree deviation from true altitude being on the end of the play!)
• The altitude setting circle furthermore is black on silver
• The altitude tension is very difficult to adjust
• The azimuth tension seems a bit to light (and is not easily adjustable)
• Already during the daytime the planisphere is impossible to read!!! Seriously, WTF???
• The mate black paint in the OTA is not fully dark. It seems however to be somewhat OK (and I refuse to flock anyway).
• There are no provisions to add an finder 

Other

• The H20mm eyepiece has its field stop before the field lens – this indicates that it is in fact not a Huygens type eyepiece. (The optics looks the same as those from the H20mm eyepiece supplied with my 70/300). Whether this eyepiece is a Ramsden or a Kellner eyepiece, I don't know.

What's nice:

• The focuser has enough travel (especially compared to my 76/700 Newton)
• The focuser position is located at an angle (some people like that, some people hate that…)
  
• The focuser retention ring can be (with some fiddling) exchanged with my 70/300 refractor, making it possible for me to use both 0.965" and 1.25" eyepieces – though this fiddling will be a bit hard under night-time observation conditions…
• The telescope vibrates when touched, but being so lightweight the vibrations die down completely within an second (give or take).
• The setting circles, are both quite large – during the daytime they helped me measuring out the local horizon at one observation site. Whether they are helpful during the night is doubtful though.
• The azimuth setting circle is printed white on black, which is good (though the font and scale used is a bit "thin", which is bad).
• The compass looks like a toy (and behaves like a toy compass), but it is a nice touch.

tl;dr

• Focuser shifting is awful. Fixing the shifting is difficult, as the space between drawtube and focuser is small – the "two rail" solution I employed at my 70/300 refractor is not easily possible here…
• The secondary mirror is too small (which is probably for the better, considering the probably spherical main mirror): The effective aperture is probably about 57mm (2.25 inch) according to my calculation (which might be wrong)
• The actual focal ratio therefore is probably closer to f/6.1
• But other than focuser shifting, the telescope seems usable to play around – though I have not tested it properly yet.

[Update 2013-08-17]
I shortly tested it on the Moon tonight. The phase was 45° (waxing gibbous) and the Moon was already very bright. I forgot to bring the "Moon filter" with me (nor any of the other filters I have) which was a big oversight – after the session my observation eye was in photopic vision. o.O

Both the H20mm (which is really a Ramsden or Kellner) and the SR6mm performed admirably when observing the Moon in this table-top Dobson. I brought my other 1.25" eyepieces with me (an PL10mm, an SPL12.5mm and an SPL25mm among others) and they too provided very nice views of the Moon.

I then tried the supplied 2x Barlow and the result was quite a bit visible "false color", blue and orange/purple fringes – regardless of which eyepiece. Furthermore the field seemed a bit tilted, which made focusing the entire field not really possible, plus the color seemed not the same on all edges. Possibly the Barlow's lens is tilted. So at least in this fast scope such an cheap supplied Barlow is not a good choice. However with an good Barlow on the Moon this scope should be able to provide more than 120x, or so I would guess. I have ordered (hopefully better) Barlows and I will see how they do in this scope.

This observation of the Moon was followed by a serendipitous naked eye observation of an ISS pass (As Tony Darnell says: "Keep looking up!"). It was quite a sight! But the ISS's magnitude of -3.2 was, with an waxing Moon, "only" comparable to one magnitude less without the Moon in the sky… Still, it was very nice.
[/Update 2013-08-17]

[Update 2013-08-20]
I took a good look at the Barlow and found that the retaining ring (coarse plastic thread) was threaded in tilted. I removed the retaining ring, removed the white goo from the lens (what seems to be a singlet(!) lens, but at least a glass lens), blackened the edge of the lens and put it all carefully together again. Then I made another threaded hole for the retaining screw, as the original was slightly angled in such a way that it pushed out an eyepiece if you tightened it down.

Then tonight I made a short test at the Moon. This time I had my filters with me. I tried them all. The supplied blue "Moon Filter" has a nice color, but is not nearly dark enough. Of all my other filters only the "B+W #103 8x" (optical density 0.9, adds 3 stops) brought down the light level enough, and just barely at that. A combination of filters would have been interesting, but the reflections on two adjacent filters meant a stark degradation of the image – so only the ND filter.

First of all: the "strange color" which I saw is only visible in the supplied H20mm – whatever it is, it seems to be a problem of the eyepiece design. Otherwise the H20mm eyepiece seems sharp. Will see how it does with longer a focal length telescope.

I again tried the Barlow with my H20mm, PL10mm and SR6mm eyepieces (as the other eyepieces have no threads for filters). It was not possible to focus the combination of SR6mm and 2x Barlow – there were always weird halos and everything was washy. Both the PL10mm and the H20mm gave an somewhat sharp image, but it was discernibly worse than without Barlow – not but a large margin, but noticeably.

So as expected the Barlow is not something one should pay money for.

And while the focuser shifting is somewhat reduced with the "two rail" measure I took, it is still awful.
[/Update 2013-08-20]

[Update 2013-08-24]
After fixing one last problem, I can say that the Barlow is not so bad after all!

I found one more spot in the Barlow were there was a black shiny plastic surface causing an internal reflection: The inside of the lens' retaining ring! Small, but vicious. After I put a little strip of black flocking material on the inside of the retain ring I was rewarded with an hugely improved contrast! All my eyepieces now produce a (almost) sharp image. It is not "tack sharp", but it is not so bad compared with my other (more expensive, and better) Barlows.

It is of critical importance to track down and remove all sources of internal reflections! This is especially important in fast scopes, as their divergent light path is much more susceptible to cause internal reflections.
[/Update 2013-08-24]