Equipment Recommendations

(for a light-polluted city balcony)

 

Not all telescopes, cameras or equipment are suited for a light-polluted city balcony. From experience, here are my recommendations...

 

Telescope Type

• Conventional eye-to-eyepiece telescope viewing is not recommended in badly light-polluted skies. It's fine for the Sun, Moon and planets but little else. This is mostly because of the telescope's limiting magnitude. Replacing the eyepiece with a digital camera will allow you to go so much fainter. In my case, seven magnitudes fainter than using my eye at the eyepiece.

• Because of limited balcony space I suggest a compact telescope design such as an SCT, a Mak or one of their variants. These are “folded light-path” telescopes. As such their tube length is about one-third that of conventional design. A 15-25cm objective is a good size. These designs usually run at F10 or F12 thus keeping the image's background darker in a light-polluted sky.

 

Telescope Mount Type

• I strongly suggest an alt-az (altitude-azimuth) mount. These mounts will move the telescope the least physical amount during slewing and tracking. Your telescope will probably be at the balcony railing so you'll want the least telescope movement or else it will strike or even pull the 'scope right over the railing. Though a equatorial mount is best for astro-imaging due to its long exposure time capability and lack of field-rotation it is not recommended on a balcony. Your light-pollution won't allow long exposures anyways and the equatorial mount has a lot of motion for a telescope close against a balcony railing.

• The big disadvantage of an alt-az mount is that you'll be limited to 5-10 minutes of total integration time due to “field rotation”. For many targets, this is sufficient exposure time. However, processing software that corrects for field rotation can extend your total integration time significantly. I have successfully exposed for up to an hour (total integration time) and corrected it for field rotation.

 

Telescope Tracking

• There are two telescope tracking types, manual and GoTo. Always choose GoTo in light-pollution. In a light-polluted environment it is frustratingly difficult to manually “star-hop” to find a target. Most ordinary deep-sky targets are not even visible in a light-polluted finder-scope so how could you find them in your telescope? GoTo, a computerized navigation of the sky, is the only real way to finding urban deep-sky targets.

 

Astro-Filters

My recommendations include...

• Red Filter – This will moderately improve atmospheric seeing. It is only really useful on brighter objects like the Sun, Moon and planets and only when using a monochrome camera.

• “Light-Pollution” Filter – It will moderately reduce sky background brightness at the cost of somewhat longer exposures.

• Dual Band Emission Filter (Ha/OIII) – This filter has the greatest effect on light-pollution reducing it by ~95%. You can even use it in full moonlight. Its disadvantages are that it works mostly on emission sources (planetary nebula, emission nebula and supernova remnants) and increased exposure times are needed.

 

Digital Camera

There are two general types, color and monochrome...

• Color Cameras – the only way a color camera can be effective in a light-polluted environment is with certain Dual or Tri-band filters. Otherwise these cameras pick up more light-pollution than monochrome cameras do. Their quantum efficiency is also lower thus requiring longer exposure times.

• Monochrome Cameras – These cameras are less sensitive to light-pollution owing to their wide spectral response. Their quantum efficiency is usually higher so exposures are moderately shorter.

• Higher mega-pixel cameras are recommended. Depending on your telescope optics and camera pixel size a lot of mega-pixels may allow the use of a focal reducer that will effectively reduce the telescope's F-ratio and/or “bin” the camera's pixels thus significantly reducing exposure times without sacrificing image resolution. Your combination of telescope, camera and focal reducer will all have to be “matched” to ensure that the resulting images are not under or over sampled. You will typically want a final image scale of 1 to 2 arc-seconds per pixel.

• Moderately small camera pixels are desired. Smaller pixels decrease the field-of-view making larger targets not possible. However, these smaller pixels decrease the target's background brightness thus reducing the effects of light pollution. A larger image scale for larger targets can be obtained by using a focal reducer.

• Choose a camera with a higher quantum efficiency (QE). These cameras will reduce the needed exposure times.

• For less noise, choose a camera with a low Read Noise value.

• Deep-sky imaging in light-pollution limits single exposures to at most 30 seconds. To reduce tracking issues most exposures will be 1 to 10 seconds at a time, and imaging the Sun/Moon/planets even less.  Because of the short exposure times, and unless you live in a hot climate, there is no real need to purchase a “cooled” camera.

 

Other Equipment

• Bahtinof Mask – an inexpensive and easy to use aid that allows for very precise focusing. Highly recommended.

• Filter Wheel – this allows easy changing of filters or a focal reducer without losing your target and often, the telescope's alignment. My filter wheel actually leaks so much balcony light-pollution into the camera that I had to build a shield to keep it out.

• Focal Reducer – a 0.5x focal reducer is recommended. Because eyepieces are not used when imaging this is the only way to change the telescope's “power”. This way you can switch the 'scope between a small and large field-of-view. For small-sized targets I typically select the 0.5x focal reducer to give me a larger field of view for finding a target, slew the telescope to it, center it, swap out the focal reducer to enlarge the target and then refocus. Otherwise, you may have a lot of frustration trying to find targets using the 'scope's small native field-of-view. Make sure your reducer will actually fit inside your filter wheel. I had to modify mine to fit.

• Optical Finder with Lighted Cross-Hairs – Find a right-angle finder with 10-15x power. When accurately aligned to the 'scope itself, it becomes easy to align the 'scope to the sky using GoTo giving a more accurate alignment thus making finding targets easier.

• Large Plastic Tub With a Lid – This will safely store much of your observing equipment 24/7 right there on the balcony. Just make sure you anchor it down for windy days.

• Table or Shelf – I use a shelf at standing height for my laptop and a separate nearby table for miscellaneous items.

• “Dew Shield” - not for dew, as you'll never have any, but to keep stray city lighting from entering the telescope tube and lowering your image contrast a little.