Seeing low SB objects

[glenc (Glen)]

Objects with low SB (surface brightness) are difficult to see.
Some examples are Barnard's Galaxy NGC 6822, the Helix planetary nebula NGC 7293 and the Running Chicken nebula IC 2948.
http://antwrp.gsfc.nasa.gov/apod/ap020123.html
http://antwrp.gsfc.nasa.gov/apod/ap070803.html
http://apod.nasa.gov/apod/ap080418.html
A UHC or OIII nebula filter greatly enhance the last two objects.
I find that sweeping the telescope or eye slowly back and forth helps with all of these objects. The eye detects movement very well. Also I look at all the faint stars around an object one by one, and use averted vision to look in the 2 o'clock position next to an object.
Nights with a breeze and no dew are better for seeing things with low SB. I saw Comet Boattini's tail on a recent windy night by sweeping back and forth at 90 degrees across the tail.
Please add your hints for seeing low SB objects.

[Rob_K]

Good advice Glen, thanks! I view through a small aperture scope so need all the help I can get – filters are no real help because they kill too much of what little light I get. For the really LSB objects that have to be teased out from the background, I suppose that first requirement is a good dark night with excellent transparency, followed by detailed charts that absolutely nail the position of the object in the FOV. If it’s a big object, work out the position of the brightest bit/s that you are most likely to see. Ensure that you set up so that no extraneous light will interfere with your viewing (eg a distant streetlight that hits your retina every time you take your eye away from the eyepiece). Then a long period of light adaptation. Always do a tour of brighter objects first, working to dimmer. Only when your eyes are working really well do you go to the field.

If you don’t see anything first up, identify the position, and tease out fainter & fainter stars in the field using averted vision. Movement is important – move the scope backwards and forwards to see if anything ‘jumps out’ at the position, however faintly. Use averted vision in the 2 o’clock position. Staring helps but your eyes can invent things too! If you think you’ve got it in vision, move the scope – if it disappears you haven’t! Once you have it, work on it, using averted vision – can you get more of its extent? Is it oval, roundish or indeterminate in shape? Is there a hint of central brightening? Going away from the field for a while and coming back helps too. This refreshes your eyes and gets rid of IFOs (Imaginary Faint Objects!).

There are all sorts of advanced techniques that I’ve never bothered to try – blanket over the head to kill extraneous light, even something called "double-averted-vision", LOL…

And when all else fails, buy a bigger scope!

Cheers -

[glenc (Glen)]

Well said Rob.
It is good to compare the sky on one side of a star with the other side to see if there is any difference in sky brightness. If you do that at low power with Antares you will see the nebulae IC 4606.

[CoombellKid]

I agree with what has already been said, some good tips in there. I also
think just spend as much time as you can at the eye piece training your
eye, the more you do it the more you will see/detect.

regards,CS

[Starkler (Geoff)]

All good advice, especially movement and covering your head to block out extraneous light. Even at a dark site ambient light from other sources can be a distraction when hunting very faint objects.

[Chippy (Nick)]

I was interested to read the bit about nights with a breeze make it easier to see LSB objects. What is the reason behind this? Is it so that particles in the atmosphere aren't in static positions and therefore scattering light in constant positions?

Originally Posted by glenc

Objects with low SB (surface brightness) are difficult to see.
Some examples are Barnard's Galaxy NGC 6822, the Helix planetary nebula NGC 7293 and the Running Chicken nebula IC 2948.
http://antwrp.gsfc.nasa.gov/apod/ap020123.html
http://antwrp.gsfc.nasa.gov/apod/ap070803.html
http://apod.nasa.gov/apod/ap080418.html
A UHC or OIII nebula filter greatly enhance the last two objects.
I find that sweeping the telescope or eye slowly back and forth helps with all of these objects. The eye detects movement very well. Also I look at all the faint stars around an object one by one, and use averted vision to look in the 2 o'clock position next to an object.
Nights with a breeze and no dew are better for seeing things with low SB. I saw Comet Boattini's tail on a recent windy night by sweeping back and forth at 90 degrees across the tail.
Please add your hints for seeing low SB objects.

[AlexN]

The way I was taught is to spend more time at the eyepiece.. and essentially allow the photons to build up.... I've found it very useful information.. When i first got my scope, if i peered into the eyepiece and couldnt immediately see the target, I assumed that I wasnt pointing in the right spot or that it was out of the mag limit for my scope... After learning that more time at the eyepiece would help... i would gaze into the eye piece for sometimes up to 10 minutes..

This helped greatly, and I've now seen quite a few fairly dim objects from light polluted skies.

[ngcles]

Hi All,

I was going to add to this but much of it is good advice and I'd only add what I normally preach about -- Exit pupil.

Think about the exit pupil of the 'scope eyepiece combination. Small LSB (small galaxies and PNe) think about an exit pupil around or just under the 2mm mark, for large LSB a bit more, say around the 4mm exit pupil mark.

One real challenge to test all this advice coming up in a few months time is the galaxy NGC 45 in Cetus (not too far from NGC 247 & 253). It is one of the lowest surface brightness galaxies around and a serious challenge in 12" 'scopes under a very dark sky even though it has an integrated magnitude of 10.5v the SB mag is in the low 14s because it is 8 arc-mins across. It has almost no core and the disc is thin and wispy enough to see background galaxies through (in the DSS).

This is how I saw it a couple of years ago in the 18" under a superb rural sky at Mudgee:

x139 35' TF NGC 45 Galaxy
*RA: 00h 14m 04.1s Dec: -23° 10' 54"
Mag: 11.3 (B) S.B.: 14.6 B-V: +0.71Size: 8.5'x5.8'
Class: SA(s)dm P.A.: 142
Inclination: --- R.V.: +468

This is a very, very LSB eg found to the ENE of a mag 6* which is not far outside the halo on the SW flank. There is also in the outer halo of the eg a much fainter mag 10 * almost directly south of centre. Fairly large, probably 4' - 5' diameter, appearing round with a very LSB halo rising weakly, if at all to the centre without any sign of structure or nucleus. Would be very easily passed over in sweeping.

Many experienced observers feel 12" is probably the minimum aperture, but who knows? For those in the 10-12" range it is a really testing LSB challenge to try-out the above advice. Good luck with it!

Best,


Les D

[jjjnettie (Jeanette)]

You can also draw out more detail on Jupiter cloud belts and surface detail on Mars by tapping the side of the scope just enough to make it vibrate.
Works well on galaxies too.

[Chippy (Nick)]

I sometimes find that just by touching the focuser (without even adjusting it) I get a clearer image. As soon as I take my finger off - the image deteriorates...

Originally Posted by jjjnettie

You can also draw out more detail on Jupiter cloud belts and surface detail on Mars by tapping the side of the scope just enough to make it vibrate.
Works well on galaxies too.

[glenc (Glen)]

Nick wrote "I was interested to read the bit about nights with a breeze make it easier to see LSB objects. What is the reason behind this? Is it so that particles in the atmosphere aren't in static positions and therefore scattering light in constant positions?"
I think it is because transparency is better, and their is no haze or less haze. On the other hand seeing is usually worse when it is windy.
The Fornax dwarf galaxy (PGC10074) is a good example of a LSB object. It is visible in 20x80 binoculars when conditions are excellent. http://en.wikipedia.org/wiki/Image:Fornax_Dwarf.jpg

[CosMos (Rich)]

I have found NGC 7293, the Helix Nebula to be an easy object in 7x50 binoculars from a reasonably dark site. Definitely rocking the telescope gently will help as a moving object is better detected by the eye. Keeping away from watching TV, no smoking or drinking and possibly wearing sunglasses during the day may help in the gathering of photons to the eye.

[Starkler (Geoff)]

Originally Posted by CosMos

and possibly wearing sunglasses during the day may help in the gathering of photons to the eye.

I have read that exposure to a lot of bright sunlight during the day can impact on night vision for a few days afterwards. Wear those sunnies!

[hector (Andrew)]

Proper dark adaption: it takes about 30 minutes for the human eye to adjust to the dark to really start to observe LSB objects. The average observing site requires no more time than that. For quite a long time I had my 20" set up in a dome observatory in Coonabarabran and I found that I was able to dark adapt much better than normal due to the restricted view of the night sky. I would be able to navigate the observatory with no problem after a while usually 15-20 minutes having come in after just being outside. once out side the sky would look murky and terrible, then when i went back into the observatory I would bump into things for the next 15 minutes while my eyes adjusted to the conditions in the dome. The reason is that I had to adjust (which I did) to the "darkness of the dome". remember this dome is in rural Coonabarabran where it is DARK by normal standards.
Time to really observe your object as the more you look and observe the more you will see. Not talking about a 2 minutes glance or even a 5 minute consider. When I am trying for FAINT objects I will take up to an hour to decide if I have seen the object or not. I use a hood and an eye patch to keep the observing eye from being effected by the brightness of the Night sky let along any other stray light. This method has alowed me to observe things LSB objects like the galaxy eso270-17, Terzan 9 & 10, Palomar 14, the local group dwarfs AND1 and 2, or S147 the SNR in Taurus.
Try and try again.
Andrew

[glenc (Glen)]

Andrew, that is an impressive list of objects that you have seen. Well done.