1.1

Market Development

Satellite based remote sensing was an integral part of space business from the beginning. The first military systems were followed by governmental systems (none military). Both military and institutional systems were designed from engineers with nearly unlimited budgets. The problem is that nearly all commercial remote sensing systems derive from these designs and design strategies. Subvention which has made space business possible in the 20^th century has become the biggest flaw of space commercialisation in the 21^st. In fig 1 an overview about the existing commercial remote sensing systems is given.

Figure 1

commercial HR Systems

Even today’s “fully private” systems cannot survive alone. Massive state support is needed to keep them alive. This has two negative outcomes

To access new markets the price for the end user has to drop significantly. Therefore the number of attendees to the market has to increase from a handful to some ten competitors. With conventional satellite design this cannot be done.

1.2

Cost reduction strategies

Micro satellites can be the key to success. In over 120 missions during the last decade they have shown their ability to handle complex mission scenarios [4]. But not only miniaturisation is needed to build satellites for a reasonable price also some sort of “mass production”. This has to be done to reduce the enormous price per kg for space hardware. Another needful thing is to abandon the “space proven technology only” design strategy. With 5 successful TUB Sat’s the TU-Berlin has proven that COTS and reduced redundancy does not necessarily decrease the satellites reliability.

1.3

Challenges

Despite a favourable optic system all technologies needed for state of the Art remote sensing have been demonstrated on small satellites. Yet the main challenge remains: Micro satellite does not only mean weight but also volume limitation. In the typical 60 x 60 x 80cm Box no telescope with an aperture greater than 12inch will fit in [5].

3.1

Scalability of telescope design

It was part of our studies to evaluate the scalability of this technology in order to find out which satellite class to concentrate on. In fig. 3 the outcome of this evaluation is shown: the advantage of deployable structures increases with an increased aperture. It is clearly visible that the capabilities of the foldable telescope technology are between the classical solid telescopes and the foldable mirror technology. The bigger a telescope gets the more vulnerable it is. While a 4inch telescope may not need any mechanisms at all bigger telescopes need at least an adjustable focus to compensate thermal effects. The biggest solid telescopes even need in-orbit collimation, too (mainly astrophysical instruments). Due to their sheer size and the increasing structural stress during launch increases. Stable collimation cannot be assured.

Figure 3

mass of satellite vs. telescope aperture

3.2

Applications for foldable telescopes

Although the Dobson Space Telescope is designed as a micro satellite for earth observation and NEO survey the technology of foldable telescope design offers advantages for other satellites classes and missions, too. For any satellite system needing apertures greater than 12inch while lacking appropriate mass, volume and funding foldable telescope technology is useful. Possible missions may vary from a successor of the mars express orbiter to low-cost astronomy missions.

Especially astrophysical missions are an interesting application for the foldable telescope technology. The main problem of Hubble and its successor the James Webb Space Telescope (JWST) is that there is only one of them. As a replace for the two cancelled Hubble service mission it would be possible to build a fleet of low-cost sky observatories carrying 2m mirrors optics. This would enable much more astrophysical research. Furthermore this opens ways for specialized or long duration missions without blocking the workhorse of all other astronomers.

3.3

Process of unfolding

The telescope will be unfolded with the deployment of the booms. After this the secondary mirror is about 1,5m depart from the main mirror but not exactly in the right position. This will be done during collimation. To deploy the booms many different technologies already space proven can be used. From booms with hinges to inflatable structures everything is possible. The low cost approach in mind our team tries to avoid technologies that need special operation conditions during ground testing (such as simulated zero g or vacuum).

3.4

Collimation

Secondly micro actuators will fine adjust the position of the secondary mirror and thereby collimate the telescope. The fine adjustment can be seen as a mediator between the requirements of the optical system and the abilities of the booms. The more precise the booms work the less difficult is collimation. Our team develops an automated collimation mechanism based on lasers and reflectors as sensors and piezo elements as actuators. The mechanism is sketched in one of our studies [7]. The last step is the focussing of the camera system. After this the Dobson Space Telescope is ready. In order to compensate thermal deformation of the telescope structure the collimation and focussing can be performed at any time during the mission to insure stable image quality. The boom deployment itself will be irreversible.

4.1

Mission

The DST satellite will have two different missions: remote sensing during sun phase and NEO survey during earth shadow. This dual mission will increase the duty cycle of the satellite by expanding its exploitation into the former useless eclipse time. There will be two cameras. One camera is placed in the primary and one secondary focus to fulfil the different requirements of the mission. Switching between the two modes can be done by flipping the secondary mirror in and off the optic path. In and fig. 4 the main features of DST and general procedure for readying in space are shown.

Figure 4

DST in Orbit

4.2

Foldable vs. solid

The best way to figure out the advantages of a foldable telescope is to compare it with a classical one. We have chosen Orbview III (OV3) from Orbital [9] as the reference for commercial remote sensing with small satellites. In the group of three commercial satellite systems with a GSD of 1m or better is it the most comparable to DST. Tab. 2 compares the two satellites. In order to insure comparability the DST is designed to have the same technical abilities as OV3. The most obvious change is the renunciation of an orbit control system. This prohibits the use of an orbit lower than 550km.

Table 1

Comparison OV3 vs. DST

To compensate the reduced spatial resolution (a result of the higher orbit) the aperture and the focal length of DST’s main mirror is 17% larger.

The biggest advantage of the DST System is that it only weights about one third of the OV3 Satellite. This has mainly 3 reasons

The increased number of mechanisms and that the telescope needs to be unfolded in orbit seams to be a slight disadvantage but this is compensated with the lower risk of vibration damage.