Lowrider - Dobsonian Evolution
Comments on the optical layout of big Dobsonians

Aperture fever leads to Dobsonian telescopes that are bigger and bigger and therefore higher and higher. Most oberserves want to have big aperture in a small, lightweight and easy to handle package for a given aperture. The typical answer of todays ATM'ers seems quite always the same. They start to cut the focal length of the primary mirror. "I have chosen F4 instead of F5 because otherwise the telescope would have become to high for me" or "Despite my fear of coma I have chosen a faster focal ratio to avoid the ladder". If you discuss with other Dobi-owners, or if you look at the websites of various ATM'ers you will find statements like this all over. As bigger as the mirror gets as more coma and a host of other problems become suddenly acceptable for the observer. It is like a miracle. In these days were 20 inch telescopes are a popular size for the deep sky enthusiast, it is modern to discuss F4 or even F3 projects. A healty aperture fever turns to a trendy f-ratio fever. As long as these observers are happy, let them be happy.

I want to show and hopefully convince you that a better and very simple solution for this aperture versus size dilemma exists. Tilt the secondary less than 45 degree so that the light will bounce back down to you. Most people visualize a Newtonian as a telescope with a 45 degree secondary. That is because 99.99% of all Newtonians are build this way. With an angle of 45 degree, the central obstruction could be kept to minimum and the image quality could be kept to maximum. This logic is valid for all telescopes were size isn't a problem for the observer. But when ATM'ers are going to cut the focal lenght in order to get a low eyepiece hight the strict 45 degree logic becomes problematic. Here you should trade between cutting the focal lenght and the tilt of the secondary in order to optimise overall image quality.



Comparison Classic/Lowrider
Classic -cut the focal lenght- approach

Results in less than achievable mirror quality. Making a good Mirror at F4 is harder than making a near to perfect Mirror in F5. Even if the mirror would be perfect heavy coma will takle you. As long as your dobsonian is driven by yogurt power coma is a much greater problem than you might expect. Such a scope has a very small sweet spot. You have to keep your hands permanently on the secondary cage to catch Jupiter in those fleeting moments of perfect seeing. On the long run planetary observing can be frustrating as long as you do not have a tracking solution. Dont ask me for the weight and bulk for a tracking platform that can support a 25 inch telescope. If you want to keep the total weight of what has to be transported to you oberserving site low, you are better of if you look for deep sky objects. With special eyepieces such as Naglers on axis resolution is lower due to big amount of glass in the optical path. Same problem holds for coma correctors, which can also ad false colors. The scope is more critical to focusing errors and is extrem critical in collimation. Possibly not feasible with a transportable instrument unless main and secondary mirror cell as well as truss-tube are extremly solidly and heavily built. All this contradicts to a tranportable telescope. In practice collimation would be not as good as in a scope of moderate focal ratio. This results in less then perfect resolution and contrast. Steep light cone jeopardizes effectiveness of interference filters like O III or UHC. Both filters are working no longer in the right light. The stated effects are all summing up. This concept might be an appropriate workhorse for an photographic instrument which is permanently and equatorially installed. But we are discussing here pure visual, transportable, leightweigt giants.

Lowrider -tilt the secondary- approach

It does away with all the above disadvantages ! But, there are also some drawbacks to consider:

If you do not observe at a really dark site, straylight would be a problem, since the eyepiece looks towards the skies. For the Lowrider effective baffling is more difficult than in traditional design. I recommend baffeling in the focuser tube. This holds because the focuser tube can be a bit longer because of the tilted angle. The Lowrider has a similar problem as a Cassegrain. In addition baffeling shoud be extended on the secondary mirror holder, so the baffels on the secondary cage can be kept to a minimum. Wind can cause here a problem. A baffle on the secondary might cause vibrations. A solution can be a baffle made like a ladder. Were the rungs are tilted 90 degrees againts the light path (as seen from the eypiece).

The quality of the secondary must be better. The light travels a longer way from the secondary to the eyepiece. Small errors in the secondary can show up in the eyepiece which will be not visible in the classical concept.

Third a slightly bigger central obstruction compared to traditional concept. The difference is not very much because at F4 or F3 the obstruction in the traditional concept becomes also bigger. Below you can see a comparison between both concepts. The Lowrider links a probably good F5 Mirror with a 26.7% obstruction, while the traditional approach combines a questionable F3.2 Mirror with a 21.3% obstruction. It should be clear, that the quality of the primary has a much bigger influence on image quality than the 5 percentage points difference in linear obstruction. The eyepiece hight was set for 2 meters on a 30 inch telescope.



As a further disadvantage was the unusual eyepiece position discussed. The observer must look toward the heaven. Well - I cannot say that for an astronomer this is unusual. But indeed it can be tyring. How much this argument counts cannot be answered in a general manner. If the observer can stand straight under the eyepiece it might be not so tyring.
Don't compare this with an oberver who is looking into an refractor. The eyepiece 40 inch above the ground, when observing at the zenit. Without an rectangle mirror you will be off in notime, due to a broken neck. In an well built Lowrider the observer stands straight under the eyepiece.

ATM point of view

I only know very view Dobsonians on the net (eg. Ed Taychert) which has used this concept to a good advantage. The secondary mirror can be kept round. So its easier to make. Thinking about grinding a good flat becomes more vivid, instead of grindig again steep slopes. Ambitious mirror grinders could be tempted to grind a good and big F5 mirror instead of stucking longer with a smaller mirror, because they feel not prepared to make an excellent F4 or even F3 mirror of the same diameter. So this concept gives bigger aperture in the hands of an ambitious grinder. From an optical point of view this concept is interesting because of its superior overall performance. The only real disadvantages is a slightly bigger central obstruction and the need for a higher quality at the secondary. I guess in practice the big plusses from the relaxed focal ratios are fare overcompensating this two disadvantages. The higher expenses for an F4 or F3 mirror can be invested in a good optical flat and in a more effective baffeling (or in an even bigger Mirror!). Cost could be saved due to thinner (an lighter) mirror blanks, cheaper eyepieces and/or no need for a coma corrector. Dollar for Dollar. A Lowrider will buy you more aperture an sharper pictures as the classic approach can.


First Scopes in Germany
Probably the first Lowrider built in Germany was made by Werner Reimann. This 22 Inch Scope was sold and replaced by another Lowrider-Dobsonian. (Sadly this picture does not show the eyepiece of the scope.) Despite it's visionary optical layout it remained quite unnoticed at the international telescope meeting in Vogelsberg (ITV) in 2001.

Mr Reimann says " I am not afraid of climbing a ladder but it makes a difference to climb 3 or 5 steps. The eyepiece hight in this case is 50 cm lower than it would be in the classical concept. This results in a lower center of gravity of the whole scope. Drawbacks ?. Well, you are looking inclined above, therefore you need a longer light shield opposit of the focuser. This scope has an angle of 30 degree. A relativ small lightshield is sufficient. A viewing position of 30 degree is not inconvenient. Collimation is the same as for a classical dobsonian. In total, the advantages are overcompensating the disadvantages on telescopes with 20inch and over."

In 2004 and 2006 the concept was adapted by Timm Klose on a beautyfull 20inch mirror and later on Roland Hermann on a 24incher. Meanwhile a lot of new Lowriders are going to be built in Germany. The discussion about grindig super fast mirrors and the early sceptisism have become less since these very active telescope builders have shown the "proof of concept" to the interessted public. How I built my 20 inch F5 Dobonian (not yet a Lowrider)
Building Parts for ATM'ers.
(The Site were I sell parts for ATMers is only in German language, Sorry !)