Street lamps…. The bane of all urban amateur astronomers around the world. They steal our darkness, mess with the natural order of wildlife and pour their metal-halide deep into our OTA’s. The other night took the cake for me as I was searching for Neptune, which just happened to be about 10 degrees above the man-made supernova at the end of my cul-de-sac. Usually I’ll attempt to take out the lamps with my trusty green laser, but this lamp was different. The sensor on it was shielded on all sides except head on… bastards!
What I needed was something to give me another 10″ or so in front of the OTA to negate the effects of the supernova. A Light Shield! Similar to a dew shield for a Cass type scope, a Light Shield on a Newtonian allows you to block stray light from bleeding onto the secondary mirror and eyepiece. The light bleed is bad because of the proximity of the secondary mirror and eyepiece to the front of the Newtonian OTA.
My requirements were pretty simple. Flexible material at least 12″ wide, duct tape, flat black spray paint, scissors and velcro! A trip to my local Home Depot was all I needed.
1 – Frost King duct insulation $20.00 (Yes it was a bit pricey but you get enough to make at least 2 10″ Dob shields with leftovers for any smaller scopes you may have. Plus it is an insulator so it can act as a great dew shield as well!)
2 – Duct Tape $2.00
3 – Flat Black Spray Paint $2.50
4 – Velcro $8.00 (Enough for 3 light/dew shields)
5 – Construction type paper or any standard legal size sheet will do
Most of you guys/gals out here most likely have all of this in your garage or work area already so the pocket book hit should not be that bad.
Take the insulation and begin to roll it out on a table. The backing of this stuff is sticky so put a sheet of construction paper down on it as you roll it out.
The paper will be laid out end to end and the seams will be covered with strips of duct tape. The duct tape not only seals the seams but it also gives the shield some extra rigidity.
After you have enough insulation rolled out to size, matted, taped and cut, you can begin spraying with the flat black paint. We are spraying the inside of the shield black so it will absorb any stray light that makes it’s way into the light shield. I gave mine about 4 coats of paints with 2-3 minutes breaks between each coating.
Once the paint is totally dry you can begin wrapping the shield around your OTA to see where the Velcro needs to be laid out.
After I measured and marked the place where the velcro was supposed to go, I cut a 1″ strip of the paper from end to end. This exposed the tacky side of the insulation and ensured adhesion of the velcro.
While using the velcro strips sticky side to adhere it is ok, being able to sew it on would be a better choice. This worked out well for me but my next shield will have the velcro sewn in.
Tada! You know have yourself a quick and easy to make light shield.
The material had a width of 12″. I used about 1″ attaching it to the front of the OTA. This left me with 11″ of light blocking goodness which was just enough to counteract the supernova at the end of the cul-de-sac.
While this was great on a calm night I can not attest to it’s functionality on a windy evening. If I were to create another i would move it down 2″ rather than 1″. Then again, when the wind begins to howl you’ll have much more to worry about!
Thanks for reading my down and dirty light shield how-to.