My new 300mm dobson from Orion Optics UK
I have been observing the deepsky visually for a few years with my 8-inch TAL 200K mounted on a Synta EQ6. What I didn’t like about this setup were two things: it was a lot of work to take it apart for transport when I wanted to go to a dark sky site and the aperture was just a little too small for deepsky observing from my light polluted backyard. A lot of objects just stayed out of reach.
So I wanted a telescope that could be setup and taken apart quickly and I wanted a telescope with a little more aperture. Set-up should only take a minute or two. I also wanted to be able to put it into my small family car without any help and carry it around on my own. My lower back problems limited the weight of each component to 20 Kilograms maximum. Because of these back problems, I also wanted a telescope that allowed sit-down observing. I decided to buy a closed tube Dobson with the largest aperture I could manage on my own.
After reading many reviews, test reports and discussing equipment with fellow observers, I decided to go for the 300mm f / 5.3 Dobson from Orion Optics UK. The OTA (with cradles) and the rocker box both weigh about 15 Kilograms (without the alterations and additions that were made). The tube is about 155 Centimeters long and is ideal for sit-down observing.
I ordered the telescope from the factory with four upgrades:
- Hilux coating
- Primary mirror 1/6 lambda (in the end I got almost 1/9 lambda)
- 9 x 50 finder scope
- 2-inch Crayford focusser
After 8 weeks the telescope arrived. It came in two boxes, one small box containing the rocker box, and a big box with the complete tube assembly. The Crayford focuser was slightly damaged, probably during transport. It was replace by Orion Optics UK immediately. (You can enlarge all images in this article. Just move the mouse-pointer over the image and click)
Rocker Box and tube assembly and tube-rings
The rocker-box is made of aluminum about 10mm thick. The setup (only Allen screws) took me about 3 minutes. It looks very stable and sturdy. The tube-cradles are attached to a pair of large rings that fit into the rocker box. The rings can be adjusted in all directions using setting screws. This way the rings can always be adjusted to fit the rocker-box. The rings rotate in the rocker-box on four pieces of Teflon. The base of the rocker-box rotates on three Teflon disks that are fitted to the three legs of the base.
The tube is made of aluminum and sprayed with bright white paint. On both ends there is an aluminum black ring, held in place by a few small screws. The finder scope is mounted with a dovetail and bracket. The bracket and the finder scope are in the same black and white color as the tube. The 2-inch Crayford focuser is black. The finishing of the whole tube-assembly looks very good. All in all, the entire telescope looks nice (aesthetically). The initial setup of the rocker-box and tube-assembly takes only 5 to 10 minutes. To give you an idea of the size and the looks of the telescope, here it is, a proud owner included
The mirrors / collimation
The secondary mirror is held in place with a 4-vane spider. You can center the secondary in the tube by tightening and/or loosening 4 large setting screws on the outside of the tube. On the back of the mirror cell you will find 4 screws. With the central screw you can move the secondary “up” and “down” through the tube. This way you can center the secondary in the focusser. When the central screw is loosened you can also rotate the secondary. Once you centered the secondary ( in the tube and in the focusser), you can fasten the central screw, and collimate the secondary, by using the other three screws. (For an image of the secondary and the 4-vane spider see image below under "flocking paper in the tube")
The primary mirror is held in place by a three-point mirror cell. Collimation of the primary is very easy, and must be done manually with the help of some collimation tool (laser, Cheshire, sight tube etc.). You simply loosen three Allen screws by hand. You then “collimate” the mirror using three large setting screws. This is also done manually. When the collimation is finished, you simply fasten the three Allen screws “finger tight”. These three screws hold the collimated primary in place.
As for the quality of the mirrors, I can only tell you about the primary. You get a detailed test-report with your mirror, telling you the different values like Strehl, PV wave front, RMS etc. as well as the serial number of the primary. I do not know a lot about optics, so I rely on others if it comes to explaining the graph and the data on the datasheet. According to Harry Rutten (author of Telescope-optics), whom I showed the test-report, the mirror is of an excellent quality. The Strehl ratio is 0.986, the PV wave front is 0.115 wave and the RMS is 0.019 wave. To see the test report, click on the image below.
I have tried the finder-scope a few times. I always wear glasses (astigmatism) when observing, and I noticed that the eye-relief is far from sufficient for me. I only see a very small part of the field of view. If I take of my glasses, the eye relief is ok. The finder scope will be replaced with a Telrad or some other one-power finder in the near future, because I will use it only for aligning the telescope on two or three bright stars. The rest is up the Argo Navis Digital Telescope Computer.
The Crayford focuser works fine. Focusing is no problem, even at high magnifications. It runs very smooth, and the friction can be adjusted simply by turning a setting screw. With a second screw the focuser can be locked. The only downside is that I have to use an extension with all eyepieces I tried (2-inch and 1.25-inch).
In the last few weeks, together with Leo and Joop from the local astronomy club, we managed to make a few alterations and additions to the telescope and mount:
1: a fan for creating laminar airflow in the tube;
2: digital setting circles DTC Argo Navis;
3: power supply for computer and fan;
4: tube extension to keep out stray light;
5: flocking paper in the tube.
Together with these alterations and additions, the tube assembly weighs 18 Kg and the rocker-box 17 Kg.
1: the fan
I added a fan to the backside of the tube, which covers the complete aperture. The fan draws in the air from the top of the tube. The air goes through the tube and past the mirror, and exits behind the mirror, through the fan. This way a laminar flow is created within the tube, and turbulences due to difference in temperature inside the tube disappear within a few minutes. I tested the fan 4 or 5 times until now, and even when I moved the scope from a heated room into my backyard, just a few minutes after the fan was switched on, the turbulences inside the tube were gone. It works perfectly. Leo had tested this system on two other Newtonians, and it worked, so we expected it to work just fine on my new scope.
How did we construct this fan? Leo used a piece of very sturdy foam about 1 inch thick. We got a fan that is used for cooling PC’s with the option to have variable speed. We cut an opening in the centre of the foam in which the fan fits nicely. After that Leo connected an external pot-meter to the fan so that the speed can be varied from absolutely zero to fast. I finished the fan with flocking paper and 4 strips of Velcro to attach the fan to the backside of the tube.
2: digital setting circles DTC Argo Navis
On my EQ-6 I used Argo Navis digital setting circles for locating objects. Because the 300mm Orion Optics UK will be my deepsky telescope from now on, I decided to transfer the Argo Navis from the EQ6 to the Orion Optics. I ordered a set of new (10.000 steps) optical encoders and a few cables from Wildcard Innovations, the company that produces the Argo Navis. Joop Wiersma managed to attach the decoders and the Argo Navis to my new dob just wonderfully. He already had some experience because he build the Argo Navis onto his own Orion Optics 300mm and with my scope he even made a few improvements to the whole construction. He needed some small parts like bolts, nuts and a few pieces of aluminum (total price 10 Euros) and what he constructed in a few hours is amazing. It not only looks very good, it works 100%!
Below you will find some images. From left to right you see the Argo Navis, the altitude encoder and the azimuth encoder. If you like to know about the complete construction in detail, you just have to wait a little. It will be published in my blog, as a separate item. Anyway, the construction is made in a way that when I want to separate the tube from the rocker box, loosening one screw (manually, no screwdriver needed) and disconnect one encoder is enough. A very convenient and user-friendly solution. However, for transport I remove the bracket that secures the altitude encoder to the rocker-box completely to prevent it from getting damaged. A few yards of Velcro were used to keep all the cables in its place.
And here's a shot of the complete construction...................
3: power supply for computer and fan
Of course the Argo Navis and the fan both need some power. I chose to attach two sealed led-acid batteries to the floor of the rocker-box again with some Velcro. One is connected with the fan, the other with the Argo Navis. I used cigarette lighter plugs to connect the fan and Argo Navis to the batteries. This makes it very easy to unplug everything after an observing session. Because of the weight of the batteries, everything feels even more stable. The rocker-box is the best place for the batteries. You can turn the scope in whatever direction, all the cables stay in exactly the same place. A simple but perfect solution.
4: tube extension to keep out stray light
Another addition I made is a tube extension that both keeps out stray light and prevents the secondary from dewing. The tube was made from 1mm thick cardboard and yes, a few feet of Velcro again. I did not have a chance to test it because the extension is a bit heavy, and I need to attach a counterweight to the tube to keep it balanced. I simply will add another cradle to the tube to act as the counterweight. It can easily slide up and down the lower half of the tube, to adjust the balance.
5: flocking paper in the tube
I added some flocking paper to the inside of the tube, but only to the part of the tube that I see when I look through the focuser directly without using an eyepiece. I hope the flocking paper prevents reflection of incoming stray light directly into the eyepiece. I added the flocking also to the inside of the tube extension.
To be continued………..
So far for the hardware. My next article will be about “first light” and my experiences with the telescope under the stars. Will the additions and alterations work? I hope that the weather will help a little in the next few weeks, so that I can give you some detailed observing reports. Until then I wish you all clear skies!
Posted by Math
on 12/19 at 01:02 AM