Quick Look at the "Bresser Pluto" 114/500 Newton

I must admit it, I'm a scopeaholic. With the weather usually bad here, my interest in optics, and anyway my health not a friend of the cold, I'm inclined to stay insides and play around with scopes. So I got another used scope today…

I had bought it two weeks ago and I told myself that it will be the last (for now) – then I bought the 150/750 Newton, as it was soooo cheap… So the Bresser Pluto 114/500 was late and arrived only today, but I swear it will be the last scope for months to come. I'm going to play around with the scopes, keep the scopes I like, sell the rest ASAP.

The scope came with:

• Newton OTA (Art.-Nr. 45-42000) D=114mm and F=500mm (f/4.4)
• 1.25" focuser on the OTA
  
• EQ-1 mount with aluminium legs and plastic leg joints
• One counterweight with about 2.3 kg
  
• Two mounting rings
• 5x13 finderscope

I knew when I bought it that some things were missing, but what I didn't knew was in what horrible condition the mirrors were: both of them were covered with goo, some mix of dust and cooking(?) oil turned into some sort of resin. Serves me well, as I could have guessed the condition: the main cover was missing – doh.

"In order to protect your mirrors from dust, an layer of goo has been applied."

But that gave me another chance to learn how to clean mirrors. If I would have failed, then nothing was lost – but if I would succeed, then well, I succeed! And here's what I did:

• I ran hot tap-water across the surface – this took care of most of the goo and dust. This was quite amazing how easily the stuff came off!
• Then immediately after that I ran de-mineralized water across the surface to avoid water-stains.
• After that I used an old (and freshly washed) T-shirt and "breath-mist" to take care of the the remaining residue: I wiped from center to edge, always taking a fresh spot on the T-shirt – this almost cleared both mirrors.
• There were two blobs of goo (1mm by 2mm) that the water didn't remove. The T-shirt removed both blobs, but a streak of goo remained. For these streaks I used a lens-pen and "breath-mist". This took care of the last remaining goo.

And what can I say? Success! The mirrors now look quite usable. Sure, one can see that these are not new mirrors, but there are no striking issues with the surfaces (unlike my 76/700 scope).

Next I tried to collimate the scope by eye, without any aiding tools. I think I managed to somewhat collimate the scope, but only after I lengthened the three M3 adjustment screws of the secondary, so I can put the secondary further in. The secondary still sits not quite right – I suspect I still don't have enough travel to adjust the offset for secondary – but collimation is now "close enough for rock'n'roll"!

Some of the things worthy of notice:

• Primary mirror is probably spherical (there are supposedly versions of this scope with a parabolic mirror floating about, but nobody knows for sure).
• The scope is compact and relative light.
• The secondary has an diameter of about 40mm, central obstruction is about 43mm (38% aperture diameter). The mirror is larger than that of my 150/750! The distance between primary and secondary is about 280mm (I measured it three times). The distance from the secondary to the focal point is about 170mm. So the actual focal length is more likely to be about 450mm. And due to the size and position of the secondary the scope is probably stopped down to about 100mm diameter. Including the obstruction the effective aperture is about 90mm. (Again: one can recover some of the lost aperture by using a Barlow).
• The secondary is held by a three vane spider (3.5mm rods, threaded at the ends). 
• The collimation screws for the secondary have a burr.  The burr of the screws eats into the metal (on the back of the secondary's holder) during collimation. This leaves a trench in the metal and little metal fillings in the OTA… 
• The collimation for the primary is done via three very nice knurled nuts that can be used without tools. Only the screws for fixing the primary's position need a screwdriver.
• Taking out the primary mirror (and putting it back in) is a bit of a hassle: you have to reach into the OTA, dismount the primary cell, and pull the cell out through the OTA – more below.
• The 5x13 finder is the usual fare: cheap plastic housing, non-achromatic singlet lens stopped down to 13mm – what a joke, these things should be outlawed.
• The focuser is a horrible mess. There was some thought put into it: It had three "rails" put into it. However the placement was not good (90° apart) and there was still too much play. Putting the right amount of felt into this focuser was really difficult, as the tube is rather narrow at the eyepiece end and widens up towards the secondary. And the two screws from the rack and pinion do not allow for much adjustment. (Funny how no two cheap scopes are alike, even so they all probably come from the same factory…)
• Otherwise there is on the focuser a nice stable eyepiece retaining ring with two(!) setscrews. The focuser's drawtube protrudes somewhat into the OTA when fully retracted (as always with cheap scopes), but as the position of the secondary is "too short" you usually need to rack it out anyway. The drawtube is made of plastic. The travel of the focuser seems enough (but not much).
• The mounting rings can easily be opened, so a dovetail-plate is not necessary (but I added one nonetheless, to mount it to my EQ-3-1).
• There is one 1/4"-20 screw on one mounting ring, allowing to piggy-back a camera or somesuch.
• [Update 2013-10-19] After testing on an almost full Moon: The 114/500 has awful straylight problems. It depends a bit on where the Moon is in the FOV, and at what angle one looks into the eyepiece. I then looked into the focuser's tube without an eyepiece: The body of the secondary is made from shiny plastic – what could possibly go wrong with such an design??? Well for one thing: the light passing the plastic obstruction before hitting the primary is glaring, which is visible through the focuser tube (when viewed without an eyepiece). And secondly the area around the secondary is visibly shiny when viewed through the focuser.

With the focal length and the size of secondary: What a weird scope. It is probably the corrector-less version of the catadioptric Bird-Jones 114/1000, where the dimension and position of the secondary would make sense. Who knows. Probably no one.

I am going to play around with the scope, test it a bit on the night sky – but I will probably sell it again.

[Update 2013-10-06] Taking out (and putting in) the primary is a bit of a hassle, as you have to reach into the OTA and pull out the cell through the OTA.

Here's what I did:

• First I removed the secondary (with spider vanes and all).
• Next it is advisable to remove the OTA's front ring, as the fit between the primary cell and the front ring is rather tight. Though it is not necessary to remove the front ring, it makes the removal of the cell a bit easier.
• Now I find it helpful to put the OTA on a mount that allow you to rotate the OTA so that it points directly up to zenith (mirror at the bottom, the open end up) and directly down to nadir (mirror at the top, open end at the bottom).
• Drive out the focuser tube so that it does not protrude into the OTA.
• Point the OTA up to zenith and remove the three large knurled nuts. The cell now sits loosely on on the OTA's back ring.
• WARNING: the cell would fall down if you now rotate the OTA to point down !!!
• With the scope still pointing up, reach with one hand into the OTA. Slide with your fingers along the OTA. At the end of the OTA carefully touch the cell, best at two or all three points where the cell holds the primary. If you can't avoid touching the primary itself, try to touch it only at the edge of the mirror. Take your time, this is a bit tricky, but possible with patience.
• Now, while holding the primary with one hand, rotate the OTA to point down. Again, this is tricky but possible.
• With the OTA pointing down, carefully lower the cell with your hand. Be advised that there are three springs sitting (and one piece of cardboard) on the back of the cell.
• If you have removed the OTA's front ring, you can simply slide the cell out of the OTA.
• If you have not removed the OTA's front ring, then you need to carefully tilt the cell. This enables you to slide the cell through the front ring (you might need a little bit of force – but not much – for this).
• Once you have the cell out remove the springs (and cardboard) and put the cell down – done!

Before you reassemble: I found it helpful to use a black marker and put some black on the three highly reflective chrome plated screws.

To reassemble you basically need to follow this in reverse order. Take care to put all three springs on the back of the cell (and the cardboard if you are so inclined). Once you reach the OTA's rear ring with the cell, slide one screw of the cell through the rear ring, and put on one knurled nut to keep the cell (with the springs) from falling down. Now you can (without sweat) put on the other two knurled nuts (and continue your reassembly). [/Update 2013-10-06]

[Update 2013-10-27] I think I will try to lengthen the tube by adding another tube segment (a stretch-scope! ha!) – that should be the simplest solution. I will see if I can get some large enough metal-can that I can cut a piece out of. I will have to check how much to add to be able to reach focus with my eyepieces. Let's see.