Help choosing a scope for Imaging

[Sp@ce_d]

With all the price increases going on and maybe the events in Japan slowing availability of optics etc. I'm thinking of bringing my plans forward and looking for an imaging scope to add to my setup now.

80mm is normally recommended as a first scope for imaging, but I was wondering if this is still the case when I already have lenses that cover 10mm - 300mm with my DSLR. I can piggy back the DSLR on the ST80/NEQ6 and guide.

I'm quite interested in imaging galaxy and planetary nebula as well as the wider field stuff so would something with a slightly longer focal length complement what I already have or would sticking to 80mm still be the advice?

I do aim to progress to CCD but again, I understand I could get an adapter to use my DSLR lenses on that too?

I'd be grateful for some help in deciding how to go about this. If the advice is still to go for an 80mm then I had originally though of a budget of £500 for a ~80mm scope. Of which the Equinox was top of my list, which included:

Skywatcher Equinox 80 APO

William Optics Megrez 72 FD DDG APO

Skywatcher Evostar 80ED

However with the price increase the Equinox is now nearly £600 so puts it up against :

Skywatcher Equinox 80 APO

Meade 5000 80mm ED APO Triplet

Orion 80 ED CF (Carbon Fibre) Apochromatic Refractor (who actually makes this, is it a real triplet for £615)?

Now If I were to go for something over 80mm then I should restrict my budget to £800 as I need to think about the additional cost of Focal Reducers/Flatteners.

That seems to cover things like:

IKHARUS 102mm F7 ED APO Refractor

William Optics Megrez 90 APO

Skywatcher Evostar 100ED

Of course I could try second hand but having kept an eye out there doesn't seem to be much in this range and if there is.. it moves quick..

So,

A) do I go for an 80mm

do I go for greater than 80mm

C) Whats the recommendations within the budgets, this can include scopes I haven't listed

D) none of the above spend it on Fridays 90 million rollover :(

[ollypenrice]

You can use something like the Geoptik adapter to couple CCDs to camera lenses but without a one shot colour CCD you would have an almighty palaver taking an LRGB picure. There is no filter wheel option so it would mean taking off the camera or lens. Think about the flats situation!!

In choosing an imaging refractor the key things are f ratio and flatness of field, then control of bright stars mainly in the blue. The Equinox or the new offerings from TS seem to be the business. New lenses are coming out all the time so the market is red hot, but some are coming out too fast to be properly sorted.

Don't buy a lottery ticket because doing so has no significant effect on your chances of winning. Dr Johnson described them as a tax upon imbeciles. Look at the presenters and concur.

Olly

[Sp@ce_d]

You can use something like the Geoptik adapter to couple CCDs to camera lenses but without a one shot colour CCD you would have an almighty palaver taking an LRGB picure. There is no filter wheel option so it would mean taking off the camera or lens. Think about the flats situation!!

Ah yes of course.. well I was thinking of going down the one shot colour route first, before adding the complication of UK weather and filters.

I guess I'm trying to get this right first time as I'll probably have to stick with my choice for some time by all accounts of astro market forces..

So, in terms of choosing a scope. OK, I understand (I hope) that the faster (lower f ratio, I take it depth of field is out the window here) the better. i.e. more light in less time needed and flatness of field in relation to the area covering the sensor at least, is good and what we want.

Presuming that I can cover objects with my lenses that go up to 300mm focal length. Therefore to obtain more than a dot of the smaller DSO's I'm going to need a longer focal length scope.

What ideally would that be in the next step up from 300mm?

Is my thinking correct that I'll get a larger & better image of smaller DSO's with a longer focal length regardless, than say the 500mm of the Equinox 80 or 480mm of an APM/TS triplet?

Or is it better to have a higher quality smaller focal length scope. Would I be able to push the resulting images further in processing to gain more detail/size than from a longer focal length of lesser quality. i.e. does size matter?

Of course I understand the camera/sensor and pixel size plays an important role and for now lets assume I will use my DSLR.

... unless that ticket comes up on Friday..

[ollypenrice]

I'd check out some simulations on CCD calc or a planetarium software to see what different focal lengths offer you on your chip, remembering that your chip is likely to get smaller if you go for CCD! You can then see what pictures you could take with what FL.

Remember that f ratio determines the speed and that longer FLs require more effort on the polar aligning/guiding/keeping out of the wind side of things. This affects productivity...

For a rule of thumb, very rough and with exceptions,

450-650mm large nebulae, M31,M33.

About a metre, galaxy pairs/trios, M101, globulars, some smaller nebulae.

Around 1.8 metres, galaxies, planetaries and oval stars from guidng issues!!

Our smallest refractor, the Tak FSQ85, gets the most use other than in the springtime when a longer FL is needed for the galaxies.

Depth of field is not much of an issue as long as you use a Bahtinov mask or FWHM measurement for focusing.

All things being equal it is best to try to get your object large on the chip, sure, but if that comes at the expense of a slow f ratio and not enough time then the noise will mean that a shorter, faster scope getting better SNR will be able to present its image fullsize because of lower noise. But the better scaled image may be so noisy that it can only be shown half size. You do see this quite often on the forums.

On the guiding/learning the ropes side of things I would not go beyond a metre myself.

To give you an idea of scale here are two images of the same region, the first taken at 328mm in the focally reduced Tak and the second at 980mm in the TEC.

Olly

http://ollypenrice.smugmug.com/Other/Nebulae-and-clusters/M8-M20-NGC6559-2010/935757461_PCNBS-X2.jpg

http://ollypenrice.smugmug.com/Other/Nebulae-and-clusters/trifid-finished/928841121_dugLY-X2.jpg

[martin_h]

I use the ED80 and 100, here is a couple of images of the same subject. Note - not taken with the same CCD but close.

[Sp@ce_d]

Thanks Olly and Martin.

I appreciate the help. Your examples were what I needed to see to get an idea of how things scale over that focal range. I've also been off checking that with Stellarium and the FOV calculator here http://www.12dstring.me.uk/fov.htm

Of course, you realise you've added to my dilemma as you've both convinced me I need 2 scopes now!

As spring viewing time is supposedly on the way I'm thinking of perhaps going for a longer focal length now, (perhaps up towards the metre). Although that seems to imply a slower f ratio (or sillier money) than a 500mm.

How slow is considered practically acceptable?

Should I make sure whatever I go for has a matching focal reducer & flattener available. Or is mixing other manufacturers OK.. or even better?

Is it always the case to use a focal reducer to gain the fastest possible speed out of a scope?

The trouble with all this is I thought I'd got it sussed out. But the more research and discussing I do the more I worry about making the wrong choice... Henry Ford had the right idea.. "any colour you want, as long as its black" !!

Graham

[martin_h]

LOL. Not easy is it?

I use the ED100 pro but at f9 is slooooowwwww The dedicated reducer brings it down to f7.65 which is not to bad, I've only used this scope so know no different and I get reasonable images from it - it just takes a bit longer, or longer subs.

[dmahon]

Your Nikon has a crop factor of 1.5. Assuming the lenses you have were built for the camera and already take this into account, then even the ED80 telescope will be a big step up from 300mm to (500x1.5)mm.

I have an ED80 and an ED120. I have reducers/flatteners for both. If you only have one, you will end up significantly cropping some images or having to take mosaics of others. If I had to choose, I would take the 120 as that is a better focal length for my camera for the things I want to shoot at the minute (galaxies). If I had a CCD rather than a DSLR then that might change.

[ollypenrice]

The more imaging I do the more I want, above all, a fast f ratio. My slowest is the TEC at f7. Sometimes I would die for f5, which is nearly twice as fast, remember. 8 hours of Luminance at a top class site and still hardly any integrated flux around M81/82 in the TEC. But I would still consider the 8 inch RC at F8 for galaxies because they tend not to be all that faint.

I would say that speed is more imortant in shorter FLs because the faint dusty nebulous targets need the most time. Still, F8 is really stretching my tolerance and life is sweet at f5. And remember, I get maybe 300 clear nights per year. Imagine the frustration of UK weather at fF8!!!

If you are dead set on a metre the SW MN190 is fast and has great optics, but it is a beast of a thing. Don't underestimate the practicalities in imaging. There are nights when shooting at a metre is out (wind, grumpy autoguider, bad seeing) but on which a short FL will soldier on.

Over a metre is getting serious on the obstacles front. Be aware of that.

You need to post specific questions about flattener reducers and which work with which. Respecting the chip distance is very important but usually easy with a DSLR because that is what the designers usually have in mind (the pests!!!)

Olly

[martin_h]

I agree, f7 is about as slow as you want to go.

[Sp@ce_d]

If you are dead set on a metre the SW MN190 is fast and has great optics, but it is a beast of a thing. Don't underestimate the practicalities in imaging. There are nights when shooting at a metre is out (wind, grumpy autoguider, bad seeing) but on which a short FL will soldier on.

Over a metre is getting serious on the obstacles front. Be aware of that.

Olly

Funny enough the MN190 was on my list of scopes to consider for progressing on to a bit later in my original plan. But you're right, I should remember I need to learn to walk first. I guess I'm trying to flesh out the results I'm actually out to achieve and understand the reasons why the experts do or don't do things in a particular way. I see what you're all saying and I should keep it simple for myself to get up to speed first. After all, getting decent subs is one thing but I'm still getting to grips with processing them after anyway!

And another thing. Having been comparing scopes and sensors on the FOV calculator I realise how CCD and pixel size comes into play. When I move from DSLR to CCD it's likely that my sensor size will drop too which seems to mean I may have to think about mosaics for the larger objects too… and then I start to read about narrowband… ahhh make it stop !!

So, really then it seems I'm back where I started and I should stay with the lower focal length and keep it simple and sensible for now then, right? I mean it's been nearly two weeks since I had any clear night sky here anyway

Without being able to actually try anything out before buying it's quite a difficult decision choosing a scope. Whats a good choice similar to the M190 on a lower focal length (and budget of course)?

The workhorse ED80, Equinox, what about the WO 72 or 90 or the TS 65mm Quad APO Astrograph Refractor/TSAPO65Q or TSED8053. What about the IKHARUS 102mm F7 ED APO?

So guys.. I guess what I'm asking is that age old question: if you were in my position knowing what you do now..

(ok I know.. but I've asked now.. )

[ollypenrice]

The ED80 and Equinox are not workhorses in any derogatory sense. I have seen, at my place, SO many killer images knocked out by the ED80 that I hold it in the highest respect, though I have never used one.

The TS65 Quad is embroiled in QA issues with a large number showing astigmatism due, so they say, to an over tight rear element that can be eased by dismantling. My view on that? Forget it. You are not a beta tester.

There was a very good first light from the Ikharus posted on the forum recently.

The real thing is to decide which FL to go for. Honestly I think shorter is better when you are starting out, but you need to think that one through for yourself, as we all do.

Olly