ED120 Magnitude Limits - point sources

[John]

Had some fun under decent skies with my ED120 refractor tonight.

Managed to see with direct vision supernova 2020hvf in NGC 3643 which is currently reported at magnitude 12.4

With averted vision I caught, a couple of times, the magnitude 13.15 (Stellarium figure) star just north of the supernova.

Very nice view of Messier 57 - the Ring Nebula.

With averted vision I managed to spot the star close to the edge of the nebula which is magnitude 13.0 according to the chart below.

Looking at the theory, the limiting magnitude of a 120mm aperture telescope is given at between magnitude 13.1 and 14.1.

I found that using high magnifications enabled me to see close to the magnitude limit of the scope. Tonight 200x - 300x were used with the ED120 when observing these faint point sources of light.

My skies are probably average for a location on the edge of a large town. Limiting naked eye magnitude at the zenith tonight was around 5.5. According to "Clear Outside" I am in a Bortle Class 5 area.

I probably have not quite reached the limit of the scope on point sources but I reckon I'm coming quite close to it

What I could not see however, was the magnitude 11.4 central star in the Cats Eye Nebula (NGC 6543) in Draco. I think this is possibly because the star is set against a nebulous background which reduces it's contrast and therefore it's visibility in smaller apertures.

On the next clear night I will use my 130mm refractor and see what, if any, differences there are in it's limits on the same targets

 

[Doc]

Nice report John. My 120ED Esprit can also see the mag 13 star just outside M57 but it can be difficult and looking back at my notes it normally says with "averted vision".  I think your skies are better then mine as I cannot see as low as 5.5 mag as a limiting magnitude I would estimate nottingham is about mag 4.6. A nice read thankyou for posting.

[chiltonstar]

Nice report John. I've found with my scopes the Limiting Magnitudes calculated using www.cruxis.com are much closer to my own visual limits than some of the textbook values, giving me quite often four Uranian moons for example, rather than the three that some LM values would suggest.

Chris

[Shaun VS]

Great report. A number of years ago I remember reading an article about pushing the limits of limiting magnitudes. ASI seem to recall there were examples given of very faint DSOs and Halley’s Comet. The basic principle is that for very faint objects photons appear randomly, even as a cluster, meaning that an object can be seen fleetingly. Observe it several times and later check on the position and it could be confirmed. The principle is the longer you observe the more chance you have of detecting it. Takes a great deal of practice to master steady,  constant averted vision. 

Anyway I tried it out and glimpsed a 16th mag star in the field of AY Lyr. Was it worth the 30 mins, I will leave you to decide?

Edited May 26, 2020 by Shaun VS

[John]

6 minutes ago, Shaun VS said:

...Anyway I tried it out and glimpsed a 16th mag star in the field of AY Lyr. Was it worth the 30 mins, I will leave you to decide?

With what aperture scope ?

 

[Shaun VS]

Sorry missed the important bit......10”Dobsonian.

[John]

18 minutes ago, Shaun VS said:

Sorry missed the important bit......10”Dobsonian.

The faintest that I have got with my 12 inch is mag 14.7 from my back garden.

Mag 16 with a 10 inch is amazing - I think the theoretical limit for that aperture is mag 15.2.

 

 

[Merlin66]

Back in the 1980's I did visual SN searching to verify the charts being prepared by Greg Thompson (The Supernova Search Charts- CUP)

Using a 12" f5 with both a UO 12.5mm and 25mm ortho eyepiece, from a site 15 Km from Melbourne CBD I could regularly pick up and identify stars down to 15.5 mag around the target galaxies.