Favourable Conditions, Large Exit Pupil

[scarp15]

The car gingerly negotiated the final stretch of narrow road, compacted with snow and ice, climbing then descending, to reach with some relief the intended location in the Northumberland countryside. 

My plan was to capitalize on using a large exit pupil and widest possible field of view to both encapsulate and in some instances, enhance selective seasonal objects. Setting up a TV85 alongside my 14" OOUK dob in the snow, this equated to 4.4mm and 6.7mm exit pupil respectively. Conditions looked to be promising and steady with clear skies, although transparency was best described as good rather than excellent, SQM readings ranging between 21.1 and 20.96, a little short of expectations for this location, based on previous occasions. The air temperature was -9c and my mittens were later called for, interesting conundrum when changing filters.

The Merope Nebula in M45 was quite apparent although faint and subtle in the dob not seen at all in the refractor. This was not such a surprise considering the disparately between each instruments exit pupil size. The Pleaides was culminating in the south and I would repeat observations throughout the session. Andromeda was the next wide field, large exit pupil target and for which the dust lanes just seem to go on for ever, expansively filling each TFOV, with the TV85 having much advantage conveying a pleasing 4.24 degree field. The Orion Nebula was of course repeatedly visited, filtered and unfiltered and at varied magnifications. In the widest possible field, nebulosity seemed to drift outwards reaching further bounds. At higher power, filtered and a bit more contrast, the Trapezium appears as an illuminated greenish tone, a shade of burnt orange surrounding the outer reaches and fine rippling textures abound. If there is one justifiable reason to go visit a dark sky location on a good night, the Orion Nebula is surely it, by now that wobbly ride in a vehicle more in keeping for the urban commute than an icy hill, was entirely forgotten.  

Dark nebula patch B34 using the TV85 would be my next target. Both M37 and M36 are almost sharing the field of view, with B34 situated two degrees west of M37. As with a previous occasion, I could not be sure I located this, perhaps transparency being not quite good enough for dark nebula searching. Undeterred, I tried again later on returning to Auriga as it ascended higher still in the South. Another large target is the Flaming Star nebula in Auriga IC 405. Its huge, with a UHC and wide field low power, you get a sense that you are in it, but I cannot gain the structure, not unlike close by IC 410, which I visited next The Tadpole, with an OIII filter and 21E can be bright and shapely. The California, an emission nebula, is a fine H-beta target, benefiting with the large exit pupil, applied to both scopes. In the TV85 it is encompassed comfortably inside the field of view. In the dob the profile was easy to assume and follow, subtle yet with much clarity, a satisfying object to observe at this time of year as Perseus is elevated high. Next followed Barnard's Loop and by following a familiar procedure, I was able to drift across this fine subtle veil starting at refection nebula M78 and using again my H-beta filter and advantageous 6.7mm exit pupil, arriving at the other side to open cluster NGC 2112. Nudging the scope a little north the Loop is also 'bright' dipping down south and it becomes invisible. Yet again I had become familiar with the characteristics for this direct observation and knew where and what I was looking at, which incidentally isn't much. There was no sign of the Southern Arc starting from Rigel.

The Flame Nebula, the 21E worked best tonight readily profiling the dark central lane and filament structure, however drifting down in an attempt on the HH, this became a little scrappy, unclear unconvincing. Small reflection nebula NGC 2023 around one of the stars near to where B33 is situated was though apparent.

Many more observations followed, the Monkey Head Nebula, The Eskimo Nebula, NGC 1514 Crystal Ball Nebula each with an OIII filter, particularly to resolve the Crystal Ball fuzz around a central star. I always seem to struggle though with the Collarbone nebula in Orion for some reason.

Yet further fabulous wide field showcase objects were to follow in both scopes, M35 and NGC 2158 an uplifting spectacle in the dob and the double cluster sensational in the frac. M81 M82 and back to the many open clusters in Auriga concluded the session. Low power wide field large exit pupil observing in the right circumstances takes some beating.

 

       

[Guest]

Hi Scarp

Very interesting account. Could you elaborate a bit more on the effect of exit pupil diameter and how to achieve a desired configuration.  I observe with an OOUK 8 inch f6 Newtionan,  using Vixen 6mm, 12mm and 25mm eyepieces.  How does exit pupil serve to enhance viewing of the various DSOs ?  Up until now the achieved magnification had been my primary concern.  Would there be an additional eyepiece that could be recommended. 

Many  Thanks

 

 

[John]

Lovely report Iain and the photo captures the scene really well

[scarp15]

1 hour ago, AstroCiaran123 said:

Hi Scarp

Very interesting account. Could you elaborate a bit more on the effect of exit pupil diameter and how to achieve a desired configuration.  I observe with an OOUK 8 inch f6 Newtionan,  using Vixen 6mm, 12mm and 25mm eyepieces.  How does exit pupil serve to enhance viewing of the various DSOs ?  Up until now the achieved magnification had been my primary concern.  Would there be an additional eyepiece that could be recommended. 

Many  Thanks

 

 

To begin with, a large exit pupil applies best to a dark and transparent sky i.e. NELM 6+ or SQM mag 21. It requires the observer to become dark adapted, enabling your pupil to open to its widest potential, therefore no stray light.

A large exit pupil has then two advantages. Combining with a filter, particularly the more aggressive contrast gained using a H-beta filter, the lighter background produced at such low power can gain to draw out very subtle structures and profiles within large faint and diffuse emission nebula. In my report I make mention to the California as a fine example, I would add that large faint emission nebula such as this will require the observer to become accustomed to and familiar with this type of observing, that is when there is little to distinguish the object from the background. Repeated observations will gain and reveal more, that are increasingly satisfying.

The Pleiades is being discussed in another thread currently in the Deep Sky Section. To add though, the absolute subtlety of extensive reflection nebula associated within this cluster is being washed out somewhat by glare. There is little again to contrast the nebula with the sky background, so increased magnification and contrast may wash these features out. To draw out such as the Merope nebula, requires an eyepiece that when dark adapted produces a more grey sky background. It makes it easier to pull out the subtlety of the nebula embedded within.     

I hope that this makes a bit of sense, others may add comments to. I also use the same scope as yourself on the same subjects I have just referred to. A 30mm eyepiece i.e. Explore Scientific 82mm, will be beneficial and provide you with a 5mm exit pupil.

[Guest]

An excellent summary and much appreciated.  I have an observation site in the Mourne Mountains where sky glow is completely absent,  ideal for tracking down those faint nebula you have indicated.  Cheers. 

[mapstar]

Brilliant read and a fantastic remote location by the sounds of it. 

Crystal ball neb. Must try that one 

[scarp15]

Thanks for all the comments. 

Dark nebulae, there are a few of us on the forum who have taken to pursuing some of E.E. Barnard's list of famous dark nebulae. Mel Bartels conveys a good account that can be sourced online for approaching and pursuing dark nebulae and the conditions required. Belt of Venus: beltofvenus@perezmedia.net (sorry for some reason I cannot seem to upload these websites onto the page) presents some very interesting selective observers sketches and detailed accounts. There is also a wealth of information for the observer, upon specific observations on the Cloudy Nights forum. The session on Saturday I was interested to discover B34 near to M37. I think that I was searching for an oval dark patch and I was unsure if I was looking at it or something else. I hadn't become fully informed in terms of scale which did not help. Since looking at a drawing of B34 and account on Belt of Venus, it does appear familiar. B34 is considered to be one of the easier dark nebulae observations, but they are highly sensitive to transparency, which really has to be very good. I think that I had also misjudged on eyepiece choice, opting for a widest field largest exit pupil, I recall that to successfully observe Barnard's E (B143, B142 in Aquila) this was accomplished with a 21mm eyepiece in the F7 refractor, 3mm exit pupil. I passed on B37 in Monoceros and B29 (which I believe requires some aperture) in Auriga, thinking that transparency wasn't quite up to it, I now think that exit pupil / contrast might play a role to.

Apologies for rambling on a bit, if anyone has anything to add concerning winter dark nebulae observing, that would be great.