Galaxy clusters on a 6"?

[Uranium235]

Hiya,

Im just thinking what to have a go at next (once these damn clouds clear!). Up until now ive just been picking on single targets but after finding the perseus cluster on stellarium the prospect of imaging a fistfull of galaxies in one go is a tempting one.

I know theyre going to be pretty small, and a few of them are approaching mag 15 which i know is outside the range of my 6" newt.

I will be using the 314L+ on this, so has anyone else had a go at this cluster? What sort of exposure lengths will I be looking at to pick these up on a 6"? And would narrowband be of any use on these?

Also, are there any other clusters that are worth a go at?

Thanks!

[ollypenrice]

How deep you can go depends on f ratio, not aperture. A Newt is usually fairly fast so you should get nice and deep with long exposures.

Have a look at NGC 7331 in the top of Pegasus, still well placed. It has lots of little friends around it and Stephan's Quintet nearby as well. I first imaged it in a 4 inch f5 and later in a 5.5 inch f7. The later image gave larger galaxies but needed aout twice as long as the fast 4 inch.

Galaxy Cluster season is really in the spring when the plane of the Milky way is on the horizpn and we see 'out' into deep space. Classics are M106 and friends, the Virgo Cluster with Markarian's Chain etc.

Olly

[Uranium235]

Cheers Olly, I will be using the f5 newt for this so plenty of speed Will 10min subs do the job?

Rob

[mdstuart]

I am a visual observer and I can only pick out the brightest objects with my 10 inch in these clusters. I have seen 1275 in Perseus. The densest cluster if I recall is in coma B which will be up in a few months. I can see 4874 but there are dozens for an imager like you.

Good luck, I would love to see the results.

Mark

[Uranium235]

Cheers Mark, ive just checked out coma b on stellarium.... darn.. looks like ive missed it for this year. NGC1275 is the one I will be going for first as its well placed, all I need now is for this flamin' rain to sling its hook

Its getting to the point where I cant remember the last time i was out!

[RobH]

10 minute subs will do very nicely.

Don't bother with NB filters, you won't get anything useful on such small targets.

I've gone down to mag 22 with my 6 inch F8, so mag 15 will be a breeze

There are some great galaxy clusters in Pegasus too.

Cheers

Rob

[dph1nm]

How deep you can go depends on f ratio, not aperture. A Newt is usually fairly fast so you should get nice and deep with long exposures.

Sorry Olly - I can't let this pass. Imaging faint galaxies has been my profession for the last 25 years. You need aperture, and lots of it! f-ratio really won't help you.

NigelM

[FraserClarke]

Well you're going after surface brightness, so really you need both aperture and f-ratio -- and ideally a nice dark site to reduce the sky background. Should be possible on a 6-inch scope with enough (total) exposure time though.

I don't think you'll need to expose as long as 10 minutes, unless you are somewhere really dark. You'll likely be 'sky limited' in less time than that using broad band filters (agree NB will be of little use to you, mainly because for these clusters the emission lines will be redshifted out of the narrow band pass!). You just need to expose long enough that the sqrt(sky level) is greater than the noise in your bias frames. Then, instead of exposing for longer, you might as well take more images and combine them later. If you need 10 minutes -- you're a very lucky man with a very nice dark site

[ollypenrice]

Sorry Olly - I can't let this pass. Imaging faint galaxies has been my profession for the last 25 years. You need aperture, and lots of it! f-ratio really won't help you.

NigelM

In the context of the question the aperture was a 'given' at six inches. Surely in that context my reply is correct? The OP would have a thin time of it with a six inch f15 Mak but do fine with an f4 or f5 Newt? Of course, there is 'amateur faint' and 'professioanl faint' and here we are disussing the former, more's the pity!!

But it would be good if you'd expand on your point. I would have thought that aperture would contribute light grasp plus resolution and focal length would contribute image scale plus resolution and that a favourable relationship between the two would provide speed. Am I missing something? 'Professionl faint' galaxies are also likey to have small angular size so will need focal length as well as f ratio to provide any useful data? The professional telescopes are all very fast, I thought.

To return to the OP, I think 10 mins at f5 would be okay if the sky is dark (and if Rob says so 'cos he knows his onions!) I have only imaged at a dark site so have no experience of the effects of UK skyglow. It's an easy matter to come back down a bit if necessary.

I haven't many images of faint galaxy clusters but as an indicator of what to expect, this was taken in just a 4 inch f5 Petzval using 5 minute subs a few years ago. Lum totalled a couple pf hours, I think. It seems to have reached glaxies in the mag eighteens.

Olly

[red dwalf]

and a very nice image olly,

wish i could do that !

[RobH]

Thanks Olly

I would think that, if your sky is dark, the only trouble you'll have at F5 is that you may burn out your brighter stars, and end upo with no star colour in the images.

Considering that, you may be better off with 6 minute subs, just more of them.

I have mag 5 skies at the very best and can shoot 20 minute luminance subs with no fogging issues.

Cheers

Rob

[Uranium235]

Awesome pic olly

It shows that size doesnt really matter (..lol). But despite being not far from a town centre, the skies here can get surprisingly dark (easy to pick out the MW).

But I still think I will have to employ a filter to get up to and over 10min in L, the only LP filter I have available at the moment is the Neodymium filter, whether thats good for 10min with a CCD I dont know. Its a shame i cant bodge the CLS clip filter onto the Atik.

I will have a go with just L next time (last night was first light, so im still getting a feel for it), which reminds me.... it was so sensitive in L with a 0.5s loop that I actually confused it with stellarium! .... Clicking on stars thinking "why wont it select???"...dope

[dph1nm]

My point about f-ratios and aperture is simply that what really matters is not the f-ratio, or focal length, but the aperture/pixel size combination.

So for a given CCD camera, a 6" F4 will be the same 'speed' as a 6" F8 with 2x2 binned pixels, or a 6" F12 with 3x3 binned pixels. You won't get the same field of view of course, but you will get the same image scale off the camera. The other option is to get a CCD camera with larger pixels in the first place.

An 12" F4 with 2x2 binned pixels will also give the same image scale, but will be 4 times 'faster'. A 24" F4 with 4x4 binned pixels will be 8 times 'faster'.

It makes a difference - here is Stephan's Quintet taken on an F4 scope - happens to be a 1.8m. Exposure time to get this shot - 2mins per colour.

For film photography you cannot do this change of pixel size - so f-ratio does matter.

NigelM

[ollypenrice]

Good post, Nigel. Thanks.

Olly