TheSkySearchers

21 deepsky observing tips
by John Baars

A list of 21 deepsky tips that will make you a better observer 1. You have to make sure that the viewfinder and telescope are aligned with each other. But almost everyone will undoubtedly have done that already and we assume that. There are also amateurs who swear by a Red Dot Finder: a small red dot...
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Thanks for posting this article John!

Lot's of great information!

Thanks John, excellent tips and tricks.

Thanks John. Appreciate it very much.

An excellent article of practical viewing tips John, thank you for posting it.

Nice refresher John!

Much appreciated! Thanks very much.

smp

Great write-up, John!

Definitely a recipe for success.

Thanks for putting some good advice out there for the beginning observers John. Lots of info to consider and put into practice in order to become a better observer. I would like to make a couple of comments if I might, for your paragraphs 13 and 14.

"13. You can also think of a deepsky filter, but do not expect the difference of day or night. Usually it is a subtle difference. And sometimes surprising. UHC filters and OIII filters are commonly used. Be sure the exit pupil of your telescope to be at least 2-3 mm. Those filters eat light. Sometimes even 5 mm is necessary."

Having narrow-band UHC or O-III filters of both narrow and wider bandwidths can be useful. Something in the range of 12 to 15nm bandwidth are excellent for use at lower magnifications as you indicate. They dim the field noticeably but at the wider exit pupils enough light is gathered to compensate. When using small apertures or high magnification then a wider bandwidth filter in the 20 to 30nm range can be beneficial. These do not reduce field brightness as much while still boosting contrast of the target object. For instance, if you are observing a planetary nebula using a narrow-bandwidth O-III and want to increase to high magnification, the field severely darkens due to the smaller exit pupil. By switching out the narrow-bandwidth filter for one of larger bandwidth, this will counteract the impact of an overly dimmed field and still yield visible contrast increase.

Also, thank you for pointing out the issues of the wide-band deep-sky filters often marketed as light pollution reduction filters (LPRs). Visually the impact is subtle as you stated. Having used them in both light polluted conditions and dark sky conditions, they can indeed boost contrast, albeit minutely. It takes a trained eye to really see their benefits, and even so, it is not significant. In fact, in areas with a lot of LP, they are pretty much useless because high levels of sky glow will swamp the filters rendering them totally ineffective. I have never seen an object using the wide-band LPR filters that I could not already see without it. However, imagers can get good results from the use of LPRs because imaging sensors are far more sensitive than are our eyes.


"14. The best deepsky filter is the jokingly called "gasoline filter": a long drive to a place where it is really dark. After several hours in complete darkness the sky looks less dark than when you started. It is not the sky. It is you own completely adapted eyes that take care of that. A look at the limiting magnitude of your telescope will prove it. A long forgotten ability of our eyes from the times we were hunters."

I am glad you mentioned the gasoline filter axiom. I definitely subscribe to this thinking. Darker skies are the great equalizer, more so than aperture alone. For instance, when comparing our suburban backyard to our dark site house, I have found consistently that the 10 or 12 inch dobs used at the dark site keeps pace with the 17.5 inch used in our more light polluted home backyard. It simply is a day and night difference between the two locations, and that truly makes the bigger difference.

Rreally sound advice John.

Excellent article.

Joe

Good information as always JB, especially for beginners like me. Worth reading over and over.

THP