Decisions, decisions ...

[Joel Shepherd]

I feel I'm at kind of an inflection point AP-wise and need a shove in some direction. I've been doing AP for about a year, with a 150mm F-5 Newtonian, an Advanced VX mount and Atik 460ex mono camera (and an Orion autoguider setup). I'll never be Mr. Penrice but I'm slowly improving and get a lot of satisfaction from being able to "see" things through the camera that I'd never be able to see visually in our very light-polluted neighborhood. DSOs are my interest, not so much planetary. Budget-wise, I'm pretty flexible but have a strong Puritan streak of guilt and aversion to excess. :-)  Time-wise, not so flexible: a 4-year old daughter and full-time job will limit my efforts. Pulling 2 1/2 hours of light in a night is about my limit. I can pull 2-6 nights/month.

I'm starting to long for color and a little more FOV.

Given heavy light pollution, limited hours, desire for color and desire for larger field of view (to capture things like the Rosette and Andromeda in a single frame), what are my best options?

For example, OSC or mono CCD + filters? Can I achieve satisfactory results with a couple of hours of light on an OSC or will a mono CCD + filters significantly outperform it under heavy LP?

Would a smaller, lighter refractor have sufficient light gathering power under heavy LP (and a little bigger FOV), or am I better off sticking with a medium-sized light bucket like I have now, even though it gives the mount a run for its money (guiding works, but works hard)? Under heavy LP, what's a good trade-off of aperture and FOV/focal-length?

Any thoughts would be greatly appreciated, and I'd really appreciate insights based on experience with AP in an urban environment. Given more time and dark skies the choice would be clearer, but ...

Thanks -- Joel.

[wxsatuser]

I know nothing about CCDs but........

Like yourself I'm lucky to get 2hours imaging time and I like a nice FOV

Must admit I like to see the objects in context to their surroundings so I have stuck with a DSLR and fast lenses.

This is 2hours under moderate to bad LP with a Canon 60Da and EF 70-200mm at f/2.8, set at 100mm.

Excuse the processing as this was a quick test to see the results of dithering 25x300sec subs, using the Tony Hallas method.

Intial processing in Camera RAW, followed by registering and combining in Registar, then a bit of Pixinsight.

A better processing will come later.

[matt-c]

For me I'm going mono ccd so I can image in narrowband because I'm sick of the mess light pollution makes of my images.

I understand your thinking in osc being quicker to produce an image but it's not worth a trade off in my book, I'd rather just shoot Ha and get some really good mono shots then add colour later.

I feel the issue of time as well I've got twin 3yr old girls but my thinking was go mono now and when I have time to use it more I don't need to upgrade.

Finally your scope, (not sure what your using, mobile website doesn't show signatures) have you considered the 130pds? SmallIsh and £159 new from flo.

Im sure more informed will be along, this hobby is a minefield of information and I'm always reading new things that change my mind.

[ollypenrice]

OSC is absolutely not faster than LRGB. It may be less frustrating but it is not faster, it is significantly slower. (Any color filter, whether in a filterwheel or fixed over a pixel on an OSC, blocks two of the three colours so passes about a third of the light. In a 4 hour OSC shoot you therefore get 4 x 1/3 of the incident light. In an LRGB 4 hour shoot you get 3 x 1/3 during the colour stage and then you get 1 x all of it during the L hour. That is to say you get two additional 'thirds.' But that's not all. Most OSC matrices are RGGB so you get twice as much of the least useful colour to astronomers. The RGGB Bayer matrix is for terrestrial photography in which green is an effective surrogate luminance. It is not an effective surrogate luminance for astrophotography. (No DS mono imagers shoot twice as much green as red and blue. It would be crazy.) AND you can save time in many cases by binning colour 2x2.  AND ( after this I promise to shut up...   )  the theory that one hour in each of R,G and B (so 3 hrs) should be equivalent to 1 hour in lum does not stand up to measurement in my case. I find that a colour filter does not drop the signal to 33% but to nearer 25%. I don't know why.

In heavy LP, natural colour is always going to be very difficult. The truth is that it is more in the processing than the capture that you can combat it.  Pixinsight's DBE is likely to be your main weapon but I don't know much about this since I only get to work on LP images brought here by guests. On emission nebulae I also think that having an LP-insensitive Ha layer ought to be a big bonus. I try to avoid using Ha as luminance but for flattening RGB it might prove effective. I'm guessing.

NB would be easier. On emission nebulae HaOIIIOIII can be persuaded to give a good approximation of RGB. Or you can explore the various false colour palettes.

When time is against you a fast F ratio has to be a good idea.

Olly

[matt-c]

What olly said.

The thing that swayed it for me is that with osc you are effectively reducing how much you can gather by having so much green on the Bayer matrix, making it actually slower.

I hope anyway this ccd stuff is extremely new to me.

[Joel Shepherd]

Olly and Matt - I've read your comments in other threads regarding the inefficiency of OSC and that is probably the main thing making me hesitate about OSC ... that and the impossibility of using narrowband filters on it. The only AP I've been able to do in the last two months has been around the full moons and the Ha filter has been a joy. Still ... I wish color was easier. :-)

I'm concluding then that the mono CCD and a decent set of narrowband filters are probably the shortest path to color DSOs under heavy LP.

So that leaves the 'scope. I know there is a lot of buzz around the 130 PDS: it looks a lot lighter than my OTA and the 2-speed focuser would be nice as well. What other options should I consider, giving a good trade-off between less weight and wider FOV? Are small Ritchey-Chretiens worth considering? Is my assumption that larger aperture (e.g. bigger than 80-100mm) will be needed to work effectively under LP (which seems to be the case visually) or is that not supported from experience?

Thanks -- Joel.

[D4N]

You don't need a larger aperture for imaging under LP, a small short focal length refractor will give you a big field of view and be much easier for your mount to handle.

/Dan

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[matt-c]

Yeah Dan is right aperture does not really make a difference for imaging, and from what I'm currently reading technically for narrowband an achromatic will do. Even more reason for mono

Depending on how wide you want have you considered camera lenses? 200mm lenses are great for certain targets and the Canon 2.8 is a staple to many here. (I opted for the poor man's version, the super takumar.)

[Joel Shepherd]

Thanks for all the advice so far ... I'm back for a little more.

First, I'm still trying to suss out the relative importance of aperture vs focal ratio. Here is an example. This -- http://www.optcorp.com/telescopes/refractor-telescopes/stellarvue-svr90t-25sv-90mm-apo-triplet-refractor-telescope-with-carbon-fiber-tube-2-5inch-sv-focuser.html-- is a Stellarvue 90mm f/7 APO frac that weighs 6.8 lbs (3.08 kg), and this -- http://www.optcorp.com/william-optics-gt81-f-5-9-triplet-apo-refractor-w-0-8x-reducer-a-f81gt-ap.html -- is a W/O 80mm f/5.9 frac weighing 8.1lbs (3.67 kg ... with rings?).

Under heavy LP with a "consumer-grade" mount (an AVX + autoguider), am I better off with a smaller but faster scope, or a larger but slower? Should I prioritize speed or aperture?

Second, I've never owned a refractor and am not entirely sure what I'd need to complete the setup to make narrowband AP practical, beyond the CCD (Atik 460ex) and mount (AVX) I already have. For example, am I going to encounter limitations -- e.g. excessive vignetting -- using 1 1/4" filters with this arrangement? If I use a focal reducer can I expect to need to use spacers for some purpose? Is there a site that describes what you'd need to set up a basic small frac + CCD AP bundle?

(Also, to clarify what I mean by a larger FOV: I'm aiming to fit targets like M31, the Rosette Nebula or the Witch Head Nebula in a single exposure ... not looking for what I'd consider "wide-field".)

Thanks again for your help -- Joel.

[D4N]

Don't get hung up on aperture.

You want FOV then look at focal length, short focal length gives bigger FOV.

Work out what focal length you need to fit your intended targets on chip, my CCD is very similar chip to yours and with my 474mm frac none of the targets you mentioned will fit on the chip. So you either need a shorter focal length than that or similar and a reducer.

The 460ex has a small chip so shouldn't vignette with 1.25" so long as the filters are right next to it. These cameras are advertised as this being an advantage.

What you need will depend on the scope you choose, some refractors will have built in flatteners. This makes spacing easy as there is no need to maintain accurate spacing as they are designed to have the correct spacing when the image is in focus. If the refractor doesn't have a built in flattener then you need to get one and then you also need spacers to get accurate spacing to the CCD. The flattener may also be a focal reducer.

/Dan

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[Joel Shepherd]

Thanks again. So I think I'll proceed with speed first, focal length second and everything else third ... after some further reading it sounds like speed-wise there is a big difference between f/5 and f/7.

Spacing worries me: I don't have a good sense of what I'd be trying to achieve with it or how I'd know what was needed. Guess I'll do some research ... an examples/explanations -- even if they are particular to one person's rig -- would be much appreciated.

[D4N]

Well to be honest that is something that bothered me so I bought a quadruplet. The scope has a built in flattener so it isn't possible to mess up the spacing between the CCD and flattener.

The only spacing I have to consider is for keeping the CCD and OAG par focal but that is pretty easy to do.

Don't go too crazy on speed though, lower f numbers mean it is much harder to reach perfect focus and any optical aberrations will be more obvious. Higher f numbers are a lot more forgiving which is why people stop down lenses.

/Dan

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[ollypenrice]

Spacing's not hard to work out. The flattener will specify a chip distance (the distance from its rear end, usually the last metal part) to the camera chip. The camera maker will tell you how far the chip is from the camera front and how thick the filterwheel is. If you use an OAG then you'll need to add that thickness as well. You add the tickness of the wheel and OAG to the camera front-to chip distance and compare them with the flattener's specified chip distance. You then buy a set of spacers or an adjustable spacer to make up any shortfall. Put these at the reducer end. Keep your filterwheel next to the camera. Baader and Telescope Service and others do all sorts of spacers.

The WO Star71 and TS equivalents are excellent but only buy from a good dealer because the QC is very dubious based on the experience of several friends.

Olly

[Scott]

mono may well be faster than colour. if you've got more than 1-2 clear nights a month no doubt this can be capitalised on but if experiences are what count then riddle me this....why have I got so many more ha only images than bi-colour or lrgb images? could it be that by the time another clear night comes around, my target is no longer in its optimal position? I know what I'm going to hear now....but you can shoot rgb,rbg,rgb instead of rrr,ggg,bbb. Well no doubt this will work fine with an electric filter wheel but whilst I'm using a manual wheel, I've no desire to change filters 10 or 20 or even 30 times in one night. 

I've said it many times, if i've only got one camera it's going to be a mono. Not because I think it's quicker but because I like NB. I still believe colour has it's place especially in the uk....Hell, even some experienced imagers own them

[ollypenrice]

mono may well be faster than colour. if you've got more than 1-2 clear nights a month no doubt this can be capitalised on but if experiences are what count then riddle me this....why have I got so many more ha only images than bi-colour or lrgb images? could it be that by the time another clear night comes around, my target is no longer in its optimal position? I know what I'm going to hear now....but you can shoot rgb,rbg,rgb instead of rrr,ggg,bbb. Well no doubt this will work fine with an electric filter wheel but whilst I'm using a manual wheel, I've no desire to change filters 10 or 20 or even 30 times in one night. 

I've said it many times, if i've only got one camera it's going to be a mono. Not because I think it's quicker but because I like NB. I still believe colour has it's place especially in the uk....Hell, even some experienced imagers own them

No, I wouldn't do this with a manual wheel either.

OSC certainly has its place and is, indeed, used by several experienced imagers. The thing is, though, to go for OSC for the right reasons rather than for the wrong ones. Speed, in my view, is the wrong one. Peace of mind and relaxed capture might be the right ones.

A question: when we shoot RGB,RGB etc without refocus it may be that we are not optimally focused either because the filters aren't parfocal or because the colour correction is imperfect. With a mono we can, though, refocus between filters if we wish. Is there any reason to suppose that the situation for OSC is any better? In a poorly corrected scope an OSC is stuck with the same focus compromise without the possibility of refocusing each colour, surely? If the lack of parfocality comes from the filters are OSC filters more parfocal than separate ones? Do we know?

Olly