Scope info - SW 8" & 10" Newts

[Gina]

Yes, the C6 looks very interesting - could well be a possibility. I like the look of Hyperstar as a future upgrade. Of course a C8 looks even more interesting but would take another month or two to save up for.

[JamesF]

Given the requirements, perhaps taking a couple of months longer to come to a decision may not be at all unhelpful. It's not like you're missing the best of the dark nights, after all.

There are a few issues with the hyperstar that would warrant further investigation before committing yourself. It's far from cheap, but even ignoring that I believe it's exceptionally sensitive to focus and collimation (AFAIR you're actually imaging at f/2) and routing of cables to the camera is also a pain. It may not be possible to use a DSLR either.

However, it may be perfectly usable as a longer focal length DSO imager with a reducer and/or external focuser. Both together would probably still cost less than the hyperstar setup.

Of course, if an SCT is under consideration and you decide that the hyperstar is not the way to go then the Maks may have to come back into play as well. There's no 8" model, but there is a 7" and the central obstruction is smaller than the SCTs.

James

[Whippy]

SteveL had a Hyperstar on a C8 and he was explaining to me that because of the steep light cone that a f2 focal ratio produces, he believed that it adversely affects filters. If you look at filter promo blurb, they tend to say that the filters work down to f4 so I believe what he says. A DSLR would work but that's a big camera, I think you'd be better off with a dedicated CCD with a smaller, round body if you were thinking about that route.

You can buy 8" Mak-Casses (and Mak-Newts) and larger, Intes Micro make them up to 14" and beyond.

Fact is when you get into longer focal lengths, there's always an element of compromise even with larger budgets. IMO, as you're starting down the imaging road Gina, I'd hold fire for a while and nail down your setup you have now before attempting longer focal length imaging. An ED80 is a great starting point, I wouldn't make it any trickier by adding more potential issues to the mix.

Tony..

[JamesF]

You can buy 8" Mak-Casses (and Mak-Newts) and larger, Intes Micro make them up to 14" and beyond.

I stand (sit) corrected. I thought the larger ones were all SCT variants.

James

[Gina]

Thank you both It will be a month or two anyway before I can accumulate the funds but, yes, this is something to think about for some time.

I think a camera tele lens would be a better bet than the Hyperstar though under f4 is expensive.

[Dipper]

Skywatcher 250/1200 PDS is 1130 mm in height and about 11-12 kg in weight (only OTA, no accessories).

[Gina]

Skywatcher 250/1200 PDS is 1130 mm in height and about 11-12 kg in weight (only OTA, no accessories).

Thank you That confirms that it's too big for my obsy.

[Chris]

Its a bit disappointing about the 6" RC, I'm also after a compact long FL imaging scope for the same reasons as you Gina, the 8" RC still sounds great for this criteria, I can't remember the price difference between the f/10 C8 and an f/8 8"RC but all I know is that I wasn't at all impressed with the mirror flop with the 16" SCT's I used at Uni, it really was a major source of error for my photometric study and a big pain in the bottom with the need to refocus all the time:( As mentioned you'd definately need to lock the primary mirror and retrofit a focuser plus reducer as imaging at f/10 can't be fun.

[Gina]

Agreed!

C8 - £790 - First Light Optics - Celestron C8 XLT Optical Tube Assembly

8"RC - £899 - http://www.altairastro.com/product.php?productid=16462&cat=268&

page=1

The RC does include a normal focuser and no mirror flop is a BIG consideration. Expensive though and would probably take until mid-summer to save up for. Maybe something for the darker nights later in the year.

Biggest Mak that FLO sell seems to be the 7" at £715. That's really got some magnification at 2700mm FL and f15 great for planetary but rather dim for DSO use. I think that's going too far probably. Anyway there's still the mirror flop problem.

[Dipper]

Thank you That confirms that it's too big for my obsy.

It wasn't too big for my room!

[JamesF]

Looks like there's no other answer, Gina. You're just going to have to build your own 10" RC

James

[Gina]

Looks like there's no other answer, Gina. You're just going to have to build your own 10" RC

James

[Mr Spock]

I used to have a SW 250 Newt. I sold because of the size more than weight. I replaced it with a C9.25 which is less than half the length and weight with no loss in performance.

[htj]

Hi

Actually, I'm by no means sure of which type of scope to go for next. I guess the ideal would be a nice big APO but that's precluded by the price unfortunately. I could do with a longer focal length for the smaller DSOs and for planetary for that matter - which I have some interest in.

I think a large part of the problem is contained in the first sentence here .

Something to consider:

For planetary you need a fairly large aperture - say 200 mm or more - to get a good resolution and lots of light grasp in order to do really short exposures in order to freeze the seeing. A Schmidt-cassegrain like the C8 would be an obvious choice and seems to be very popular for planetary imaging. The 9.25 would probably be better, but price is starting to ramp up considerably here.

The F/6.3 reducer is an option, but I share your scepticism for focussing with mirror movement.

For DSO speed (focal ratio) is more important then raw aperture, but regarding reach there are some important things to consider, i.e., how good resolution you can actually get.

With planetary imaging the goal is to find the best moments in seeing, however due to the long exposures done in DSO, that is not an option.

The maximum resolution is bound by seeing, and assuming the seeing in the UK is similarly to Denmark, 3-6 arcseconds is probably the maximum you can hope for. Factoring in nyquist, you should aim for a pixel resolution of roughly half of the seeing-limited resolution.

Lets say the seeing resolution is around 4 arcseconds, meaning you should aim for a pixelresolution around 2 arcseconds/pixel. With an 8 inch GSO RC (focal length 1624) and the 1100d (pixel size 5.2 micron), you would end up at a pixel resolution at (5.2/1624)*206.27 = 0.66. This will give you big pictures, but the _actual_ resolution of them will not be better than what the seeing can provide.

Throwing in the Astro-Physics 0.67 reducer (which supposedly goes well with the GSO RCs), you can down to a F/5.3 focal ration which is much faster, and a focal length of 1088, and a pixel resolution of .99, which is still way more then you would actually get out of it.

The short version: In order to get the best out of an 8 inch RC telescope you need a imaging chip with rather large pixels (can be achieved by binning on a monochrome CCD), but unless the chip itself is rather large, one often ends up with quite small pictures (which might be okay).

All of this also assumes that you can guide at the really long focal lengths (high pixel resolution actually), which is a challenge in itself.

Short version: A bigger telescope will not necessarily give you better DSO pictures.

Your current setup have a pixel resolution of 1.79 pixels/arcseconds which is quite sensible and already pushing the maximum resolution you can get in the UK. A reducer-flatner for your ED80 will give you a faster scope which can give you a better signal-to-noise ratio, and give you much nicer stars in the corners. It will lift the pixel resolution to 2.15, but that isn't necessarily a bad thing, as it makes it easier to fight bad seeing.

Sorry for the bucket of cold water.

But figure out what you want, then the list of possible equipment will hopefully be reduced.

/Henrik

[g0ibi]

Fascinating thread, been looking at the 6" RCA and a C5 for the future, so very helpful input, thank you all. I'm also looking at Altair Astro's MAK 150-1800 as a possible contender??

[Gina]

Thank you Henrik That's a very useful analysis of the situation - much appreciated

[Chris]

So if going for a long focal length scope in order to get up and close to galaxies and planets in more detail is off the table due to what the seeing will allow, is Gina better of concentrating on light grasp and fast optics therefore something like an f/4 Newt? e.g. the SW Quattro or the GSO one, both of which would have a slighlty shorter tube than the 200pds which is good for space saving and f/4 would be nice, how difficult are f/4 newts to collumnate compared to f/5 Newts, are they much more fiddly?

[RikM]

So if going for a long focal length scope in order to get up and close to galaxies and planets in more detail is off the table due to what the seeing will allow, is Gina better of concentrating on light grasp and fast optics therefore something like an f/4 Newt? e.g. the SW Quattro or the GSO one, both of which would have a slighlty shorter tube than the 200pds which is good for space saving and f/4 would be nice, how difficult are f/4 newts to collumnate compared to f/5 Newts, are they much more fiddly?

I have heard quoted that f/4 is twice as sensitive as f/5 when it comes to collimation. The only person I personally know well with an f/4 Newtonian calls collimation...a word we're not allowed to use on this family friendly forum

[Gina]

I've been getting some extremely useful information in this thread - thank you all

All this is pointing me towards getting the FR/FF for the ED80 before thinking about buying another scope. I know that will improve my DSO imaging and it seems I'm unlikely to do much better for DSO than that.

[fatwoul]

I'm beginning to think the 10" is a bit too big.

The Quattro is shorter, isn't it?

[Gina]

The FF/FR will give me f6.3 with my ED80 giving me almost one stop more light gathering so f4 would give me another stop more light and about a quarter of the exposure time for any given situation, of my present setup. Worth considering though more problem with collimating is also important. Is the difference between f4 and f5 in light gathering worth the extra effort in betting good collimation? That's the question.

[Chris]

thanks Rik, well answered:D

Gina- this is another good question. The FR is another good option I know QM is happy with his:)

200pds is looking like a good compromise so far with its FL of 1000mm and f/5 optics, Well to put things into perspective you could get both an FR for your ED80 and a 200pds for a lot less than some of the OTA's considered on this thread so far:)

[Whippy]

Don't forget to factor in a coma corrector into the optical train too with a reflector.

I guess it depends on what you want, a plug & play system or do you like to tweak and play about? Personally, I wanted to spend my time actually imaging so I stuck with refractors, less time to setup and get going but generally a bit slower.

You can buy faster refractors though, my old Zenithstar 66 in combination with a .8 reducer was running at f4.8 so it's possible to get a fairly fast system without having to collimate everytime you setup. IIRC, SteveL has his little Borg refractor running at f3.8 generally.

Tony..

[Chris]

Hey whippy my only scope at present is a ZS66 and I tried a while ago to get hold of a suitable reducer/flattener and things didn't work out:( Can you recommend a reducer for this scope please:)

I don't know about Gina but I'm leaning towards getting a 150pds plus coma corrector and cheshire, as my obsy isn't going to be that big, but its tempting to go for the 200pds as its only 80 quid more, it might fit and should still be ok on my old HEQ5 on a pier:D

[Whippy]

The specific reducer/flattener for the 66 was the FFII, although any flattener/reducer that works for a 400mm scope should be alright.

Tony..