Amateur astronomy history does not have many milestones. John Dobson mount was probably one of them. Massive production of compact SCT telescopes may be as well. CMOS sensors introduction should also be considered as an important step. Should we look at Seestar S50 as an amateur astronomy milestone as well? Do we live in a Seestar era?
Seestar does not introduce anything new – all the components and ideas were already there. Fork alt-az mount, CMOS imaging chip, different sensors (GPS, accelerometer, compass) that describe the device orientation, live stacking – we already know all of them. However, we did not have so far a complete observation setup that could be used both to preview and to stack deep sky and Solar System targets that weigh 2.5kg and cost like 500$. This idea is very new and was noticed by complete beginners, but also advanced astrophotographers and visual observers as well.
What is inside? Technically speaking Seestar contains the following components:
- alt-azimuth fork mount with GoTo capability
- 50mm aperture f/5 triplet lens with low dispersion (ED) element
- optical train switchable elements: IR cut for regular imaging, dual-band filter for emission nebulae, and solid metal cap for dark frames collecting
- Sony IMX462 color sensor (not cooled) with focusing motor
- electronics that control the whole process
Additionally, there are two flat mirrors inside the Seestar that reflects the optical path and keep the Seestar size under control. The whole device is powered with an integrated (but easily replaceable) 6000mAh battery. Seestar is built mostly with plastics, however, in both axes there is a single metal bearing. The worm and worm gear in both axes are made of hard plastic.
There is a noticeable play in both axes worm gears, but there is no easy access to cancel that play. Seestar can be disassembled to reach the worm gears and make there some adjustments, but that is not a trivial task, especially for the azimuth axis. Canceling the backlash must be done very carefully because these plastic worm gears have some eccentricity that causes the play can be different at the different points of the worm gear. When we cancel the play to zero in the slackest position, then the worm gears can wear out quickly or even be damaged.
The amount of the play is different for each Seestar, but when combined with an automated and frequent dithering process it needs to cancel the play after half of the dithering moves. And that process causes the rejection of the frames due to the elongated stars. I hope ZWO will address this issue at some point because it can be easily reduced or eliminated in software.
Functionally Seestar is quite a versatile tool. It can be used to watch nature during daylight – just put it somewhere outside, sit in a chair, and look for birds or other animals on your tablet’s screen. It can also be pointed to the Sun (don’t forget to attach a solar filter then) and you can monitor and image sunspots. Once the Sun is down and the Moon is up, you need to take off the solar filter and point Seestar to the Moon. For the Sun and the Moon you may either take some photos while previewing or record a video that later can be stacked and processed to get a higher resolution image.
Finally, once the Moon is low or below the horizon, you can do stargazing 🙂 Due to the integrated GPS module, compass, and accelerometer, Seestar quickly finds a proper view of the sky, and after plate solving the selected target lands in the center of the screen. Many objects are already visible in the preview mode when the screen is refreshed every 0.5 seconds. But when the target is faint, it may not be enough. Then enhancing serves for help – that is nothing else but live stacking of the subframes with the exposure time 10s by default. This 10s can be adjusted to 20 or 30s, but then the tracking capabilities of Seestar may be a bottleneck and more frames will be rejected due to the elongated stars. After turning on enhancing the process begins with horizontal calibration, when the Seestar plate solves three frames separated by about 15 degrees and determines the exact axes orientation. After that several dark frames are collected to calibrate the subframes. And then live stacking of subframes begins and you may see on the screen how the target image getting better and better.
The image above is Mizar and Alkor stars recorded by Seestar under a light-polluted sky. It is only 6x10s of exposures, but stars 400,000 times fainter than Mizar were captured, and using short exposure capability (200ms frames) Mizar double system separated by 14 arc seconds was split easily.
The IMX462 color sensor in Seestar is not actively cooled, so the number of hot pixels increases when the ambient temperature increases. However this effect is limited by using dithering and pixel rejection during the subframe stacking, so the quality of the final image is decent. It can be even better when we select all subframes to be saved and then stack them using dedicated software, like AsiStudio or Siril. The live stacking process in Seestar is not perfect in removing the trails from satellites, and sometimes in the final image, there are few faint lines. Stacking with an external program removes them completely.
Frequent dithering moves in combination with significant backlash in both axes cause a long settle time, so the amount of rejected frames may be large. You can find on the Internet reports of more than 50% rejected frames due to elongated stars. During some sessions, I sometimes observed up to 40% rejected frames – during the 40-minute session I was able to collect only 25 minutes of data. It is not acceptable in my opinion, and should be addressed by the manufacturer. The backlash can be canceled mechanically by slight tension applied to the axis, like in the example image below with the help of the thin rubber band. This simple solution reduced the rejected frames problem by 80-90%.
IMX462 color sensor uses an IR cut filter in the optical train that is switched on by default. When imaging emission nebulae under a light-polluted sky you can select an internal dual-band filter. I could not find any detail on the bandwidth of this filter, but I assume this filter just passes the lines Ha, SII, and OIII and it is quite wide. The example image captured with this filter is presented below – this is the M16 Eagle Nebula. Data was collected under the dark sky, so the filter was not quite necessary for this case. The side effect of using this filter is false colors of the stars, so it is sometimes better just to desaturate stars, so they do not sting the eyes.
Collecting a larger amount of data requires more time. Seestar is an alt-az telescope, so its field of view slowly rotates while tracking the sky. The actual amount of rotation depends on the location of the target, but it is noticeable after about 30 minutes and any sessions longer than 1-2 hours will result in the final image with evidently noisier corners due to the missing data.
This problem can be addressed in two ways. The first one is to collect the data for several nights. Start each night when the target is at the same location, so orientation will be the same, and collect subframes. You can later stack subframes from several nights easily. The second method is to use a polar wedge or similar solution to tilt the Seestar in the way its azimuth axis will point to the pole. There are several descriptions on the net on how to do this. I can imagine that works, however, I would be very careful with that kind of operation, because Seestar is not designed to work in an inclined position. The azimuth axis contains a single metal bearing, but the enclosure base is made of plastic and noticeably bends when you mount the Seestar tilted. That movement transfers then to the worm gear and may cause excessive wearing or grinding.
Seestar also has the ability to record images of planets, but the 50mm aperture will not resolve any valuable details on the planet’s surfaces. Jupiter’s belts and Saturn’s rings as small ears – that will be probably all to see. But field of view and pixel scale in Seestar are selected reasonably – many medium-sized targets fit well into the sensor’s area. Even smaller objects, like most galaxies from the Messier catalog and many from the NGC/IC catalogs, are also clearly visible in the captured images.
Who will be interested in having Seestar? I think very many of us. I already know a few visual observers who purchased Seestar to get into astrophotography quickly and painlessly. I also know a few people who were not interested in astronomy much, got Seestar, and were quite happy about it. I, on the other hand, consider myself an advanced astroimaging amateur – and also bought my own Seestar to have it as a standalone traveling setup. This is quite a versatile tool with a lot of capabilities, but also several limitations, for sure. Most of these limitations are by design – to cut the cost and weight/size of the device. A small aperture, a small sensor without active cooling, plastic worm gears, single bearings, mostly plastics – all these details are for the reason, and the reason is money. But there is of course room for improvement for future versions, so I am waiting for the Seestar S50 successors. However, I am almost sure that every piece of upgrade will raise the price. Seestar popularity is also proven by the presence of many 3rd party accessories. The most desired ones are a dew cap, a 2-inch filter holder, and a solar foil holder. You can find accessories set in our store https://shop.astrojolo.com/product/filter-holder-system-for-seestar-s50/
