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M51 & M101

greglloyd

By greglloyd
in Observing - Deep Sky

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greglloyd

greglloyd

Posted
Posted

I had the first chance to get in moonless skies the past few nights with my new TeleVue plossl eyepieces. So with the 8" Dob in hand, and with Ursa Major high overhead, my targets of choice for more prolonged study were M51 - The Whirlpool Galaxy, and M101.

Firstly let me say I'm very pleased with the new eyepieces, they give nice sharp images. I wouldn't say pin sharp to the very edge of the field, but I do suffer from an astigmatism in my eyesight and I think it is never possible to get the very sharpest images right across the field with my eyes the way they are. My scope is an F/6 and I'd say that at least 85% of the field of view was pin sharp or very close to it. Nice concentric diffraction rings.

Ok, so on to the galaxies! The skies have been surprisingly transparent here this week and Wednesday and Thursday nights in particular were extremely clear with a limiting magnitude approaching +7 and absolutely no haze up there whatsover. Swinging the scope over to M51 showed it to be bright and very obvious with direct vision with the companion galaxy also showing up very bright. M51 itself had a large halo at 48x with hints of the spiral structure trying to reveal themselves with adverted vision (an even some hints with direct vision after an hour's worth of dark adaption).

Increasing the magnification to 110x showed off M51's structure fairly well with 3 foreground stars clearly visible in the disk and just to the side of the disk. I could frequently get glimpses of darkness appearing between lanes with adverted vision and clear signs of a bright arm to the left and another to the right of the main disk. Moving the field back and forth slightly, and using adverted vision, I could detect an overall spiral structure circling the entire way around the disk. Very nice.

I shifted my focus over to M101 and was surprised to find it very obvious to pick out this week. It was clear to see in the FOV - though no structure was detected other than an obvious increase in brightness at the core. But again, I was surprised just how well this galaxy looked this week which clearly was down to the very transparent skies. At 48x it filled the entire eyepiece view.

All in all, a really enjoyable viewing this week!

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  • 2 weeks later...
John

John

Posted
Posted

Good report Rich. I was amazed how much more of M51 I could see at the dark skies of the SGL5 star party compared to my back garden which has some light pollution issues. I was even more amazed when I saw the same galaxy through the 20" dobsonian at SGL5 ;)

It's a really nice object even in modest apertures. I must have another go at M101 as well - I've not had a really good view of it for ages.

Thanks for posting :)

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acey

acey

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Great report - and interesting to see your comment on higher magnification yielding greater detail on M51. A lot of people think you've got to look at galaxies at low power but I've found you should always try pushing as high as possible to bring out detail. The same point is made by Roger Clark - his website has a lot of interesting stuff on deep-sky observing (including dark adaptation), and the following link has his views of M51 (through a 12.5") at various magnifactions:

ClarkVision: Visual Astronomy: M51 and magnification

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Talitha

Talitha

Posted
Posted
hi,

ive been trying in vain to spot them over the last few days, with no success.

tried tonight but there was way too much moonlight, does this affect the chances of veiwing them a great deal?

thanks

rich

Hi Rich. ;) Yes, moonlight really bleaches these two out. Try again when there's no lunar interference, and let us know how you did.

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DarkerSky

DarkerSky

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Posted
Great report - and interesting to see your comment on higher magnification yielding greater detail on M51. A lot of people think you've got to look at galaxies at low power but I've found you should always try pushing as high as possible to bring out detail. The same point is made by Roger Clark - his website has a lot of interesting stuff on deep-sky observing (including dark adaptation), and the following link has his views of M51 (through a 12.5") at various magnifactions:

ClarkVision: Visual Astronomy: M51 and magnification

Definately agree with that. Most of my galaxy observing is done at moderate powers. Quality of seeing is generally a limiting factor, but you do start to tease out more detail the higher you go

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acey

acey

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There's always a limit to how high you can go before a galaxy disappears: Clark's book (and website) discuss the "optimum" magnifcation, which is dependent on aperture and background sky darkness. Basically his idea is that the galaxy has to be made a certain optimum apparent size for best viewing (in the case of galaxy detail, this means making the details the optimum size). At the same time, you want to optimise the darkening of the background sky with magnification, as against the weakening of the object itself through magnification.

With my 8-inch I used to find that many if not most of the galaxies I viewed would disappear at x150 (at a dark site). With my current 12" I do most of my viewing at x250 and sometimes go up to x375. Apart from the effect of larger aperture, I think this is also because I'm often going for smaller (more distant) galaxies, or else for small details within larger, closer galaxies.

The main thing, as ever, is the darkness of the sky. Clark's calculations mostly assume a background sky quite a bit darker than my limiting mag 6. In a light polluted sky it can become impossible to get the optimum magnification because the background remains too bright for the object to be seen. The result is often that you have to view only the bright central core, at low to medium power.

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