Big SCT or Big Dob & EQ Platform?

[SnakeyJ]

I've been looking at Emil Kraaikamp's website for tips on imaging Venus, having had my first stab at this.    He's got some stunning pictures, taken with IR and UV Pass filters, with his 16" F5 dob.    This has made me think that perhaps a big truss dob on an eq platform might be a more affordable route to better planetary imaging, than the C14/CGE Pro that I have been considering.

So, how do you think the  C14s F11 @ 3910mm focal length would compare against a 16" F5 @ 2000mm or even an 18" F4.5 @ 2025mm?

Assuming both were permanently sited in an obs and mated with the ASI174, and imaging around F25-30 - the difference in focal length accomodated with barlow/powermates.

Both SCT and Truss would be dew traps, though the Truss would probably cool faster and could be kept clear with a hairdryer.

As a side advantage a dob/eq platform would not require a pier and foundations, though I suspect a driven EQ platform would be great for PI, but not great for longer exposure deep sky with 2m of focal length at native.

Current pricing for the C14XLT and CGE Pro is around £ 10,600 - though perhaps for planetary the CGE Pro could be replaced by an EQ8 for £ 7,940 - or possibly marginally on the AZEQ6GT for just under £ 7K (might be really pushing this mount, though might cope for planetary requirements).    Adding mirror locks and a decent crayford probably adds another £ 500!

A decent truss dob with OO or Nichol mirrors at 16" would probably be around £ 3.5K new and argo navis + good EQ platform might add a further £ 2K.

Although I've listed new prices, I'd be more inclined to look to the second hand market and suspect there's a better chance of finding the dobsonian or decent optics at a good s/h price, than finding a reasonable C14.

So my questions are:

1.   If price was not an option would you plump for the C14 or a 16-18" dob?

2.   Does the dob/eq platform offer potentially more bang for the buck, or are there other potential drawbacks that I've missed?

[damnut]

Hi Sanakeyj,

Colleague had the same dilemma, nice dilemma to have, and plumped for a used Orion Xxi goto i14" truss dob (£1200). Used with the 110 deg es ep's it is nothing less than brilliant!. He has added wheelbarrow handle slots underneath a new base, multiple cooling fans, but still can be broken down for transportation. This is a purely visual dso hunter setup.

For me its a no brainer both scopes you are looking at are magnificent. Bang for buck its the dob.

[JamesF]

It's a question I've idly asked myself every so often, not that I'm likely to have the cost of either lying about in loose change in the near future.  As ever it seems to be a question of where you'd like to make your compromises.

The C14 achieves f/25 to f/30 quite comfortably and though large is compact for what it is so wouldn't really be a problem to have set up all the time in an observatory.  And on a suitable mount there's probably always going to be the possibility of having the imaging application track the planet to stop it drifting off the sensor.  But the kit is hideously expensive

A large dob on the other hand is comparatively cheap, but whilst it's far from impossible once you're trying to extend f/5 to f/30 (or potentially even more) you may end up with quite a few components in the optical train.  And an 18" f/5 is a pretty large beast.  It's not going to fit in a small observatory.  And without a permanent setup there's going to be the necessity for doing a decent alignment every time you want to use it, especially when you're going to be using an effective focal length of around ten metres.

I'll be interested to hear what those people who do have very large SCTs or dobs for planetary imaging think, especially about the apparent cons of their choice that may have turned out not to be, or the ones they didn't realise until after making their decision.  You never know, one or the other might come up second hand for loose change and pocket fluff one day

James

[cgarry]

I use a C14 on a CGEM DX mount.  Although this mount is not up to doing deep sky imaging with the C14, it is perfectly up to planetary imaging.  If the EQ8 had been available when I bought mine I would have definitely bought that over the CGEM DX.

Martin Lewis is now imaging with his 18" dob on an EQ platform (http://www.skyinspector.co.uk/) and is getting some cracking results.  Looking at his site you can see what a beast the 18" dob is!

I am interested to know how those imaging with large dobs handle the large amount of kit I consider 'essential' that needs to hang off the focuser.  I now have flip mirror, electric filter wheel, powermate/barlow, ADC, and finally a camera.  On my setup this is quite a big lump of kit but works well in the back of my C14.  I know you don't strictly need all this kit but it does make imaging much less frantic in my experience.

Cheers,

Chris

[Philip Benson]

Hi,

As a keen planetary imager myself I cannot help but admire the results from Emil and Damian Peach.  In my experience seeing conditions are everything when it comes to planets and if the atmosphere is unstable then that is going to become the limiting factor on what details you see regardless of what equipment you have.

Assuming excellent seeing conditions then aperture becomes the next most important consideration in which case both a 16" f5 and a C14 at f10 or f11 will produce amazing results as you have seen.   I have a 16" F4 dobsonian although I only use that for visual use.  Used with my binoviewers I get a power of about 150x and on steady nights I can see superb detail.

I was watching the 2nd hand markets recently for signs of a C14 becoming available but then I saw a Meade 14" SCT with UHTC coatings being offered for a bargain price and so I grabbed it.   At F10 this is essentially identical to a C14 optically and depending on who you ask a better performer.  Unfortunately since I got it I just haven't had any decent nights to really test it out.   With a 3.5m focal length and 14" aperture though I am expecting some improved results over my TMB152 F8 refractor which I have been using up to now as my main planetary scope.   Certainly the image size is much bigger owing to the Meade having a focal length around 3x that of the TMB.   My camera of choice is the DMK 21AU618 which works great with all the scopes.

[mitchelln]

Emil's images are simply stunning aren't they.

I've looked into EQ platforms, but they don't seem to be very accurate. But I guess a 16 or 18" Dob would surely grab enough light for really short exposure times for planetary on such a beast. With your current rig you seem to be getting fantastic images as it is. I would be surprised if you could improve on them massively by going this route.

I have a 12" Flextube GOTO which I use with 4x Powermate. The trickiest bit I find is focussing, even with a Moonlite focuser with motor. The required movement is minute. Dew is my other challenge. I will have to invest in some heaters at some point soon I think. Balancing is not an issue though, I just strap weights at the mirror end.

Saying that, I sometime wonder what could be achieved by somehow cobbling my flextube onto an EQ8 All that light capture with a proper guided mount would be interesting!

[Cath]

I suppose with Venus, it's not so much the light gathering ability, but the resolution (aperture) - assuming the seeing allows for those amazing images ?

[Space Cowboy]

DOB all the way! Leaves some money in the kitty for a nice imaging holiday in Barbados!

I don't want to upset C14 owners but from what I've seen and heard they are an expensive headache - certainly in UK conditions.

[Kokatha man]

DOB all the way! Leaves some money in the kitty for a nice imaging holiday in Barbados!

I don't want to upset C14 owners but from what I've seen and heard they are an expensive headache - certainly in UK conditions.

...yes, just make sure you include in that kitty reserve plenty for including the 16/18" dob + EQ platform etc freight charges to Barbados & back..!

I'm not going to say anything except maybe take up DSO imaging  or move to the Equatorial regions or Southern Hemisphere if you want to spend money on planetary imaging - considering the way most of the planets are heading nowadays...sad but true that these cycles occur. 

But £ 10,600 for a C14 in the UK...strewth, that's over $20,000 Aus - a C14 here costs £ 3,600..!

Of course I use an NEQ6 (£ 1,040) which can't carry a C14.....but the trick is not to tell it that it can't - I've been fooling one for 4 years now!

I sometimes look at that 16" OO scope sitting in the corner of the room here & think "I must do something with it" - like perhaps sell it...but then I go "naaah! Couldn't do it to anyone..!"

[JamesF]

But £ 10,600 for a C14 in the UK...strewth, that's over $20,000 Aus - a C14 here costs £ 3,600..!

I think that was for the OTA and mount.  A new C14 OTA is about £5,800 in the UK, which is still a lot more than in Australia

James

[SnakeyJ]

I think that was for the OTA and mount.  A new C14 OTA is about £5,800 in the UK, which is still a lot more than in Australia

James

We really get stuffed on pricing in UK for most things and that's a prime example!

So much to read and digest here, but some great insight and opinion to consider.

[neil phillips]

 I do wonder about somekind of optical variation with C 14s

Previous images from those using C11s seem to often look better. So the big scope doesn't seem to always perform well, But then again we see others who switch in the UK who seem to have a level of sharpness and clarity that clearly indicates a consistency, others who previously were doing ok with smaller optics. can no longer seem to reach.

I wonder if Peter Edwards has had as much thermal trouble affecting he's images as others have. Because he consistently seems less affected than some other C14 users of recent times in the UK.

Are the big scopes showing how poor some locations are around the UK

I wonder if Peter Edwards has done some major thermal stabilization of he's new C14

I personally would use a C14 But would certainly be concerned if it would actually ever do any better than a C11  and sometimes worse.

[Cath]

The conspiracy theorists I'm sure would have a field day with Emil's lovely image of Jupiter if this was about our Moon

[Blazar]

Than there is the possible 16" GSO RC...? thoughts on that?

http://astrograph.net/epages/www_astrograph_net.sf/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/ADGSO16RC

[SnakeyJ]

It's a question I've idly asked myself every so often, not that I'm likely to have the cost of either lying about in loose change in the near future.  As ever it seems to be a question of where you'd like to make your compromises.

The C14 achieves f/25 to f/30 quite comfortably and though large is compact for what it is so wouldn't really be a problem to have set up all the time in an observatory.  And on a suitable mount there's probably always going to be the possibility of having the imaging application track the planet to stop it drifting off the sensor.  But the kit is hideously expensive

A large dob on the other hand is comparatively cheap, but whilst it's far from impossible once you're trying to extend f/5 to f/30 (or potentially even more) you may end up with quite a few components in the optical train.  And an 18" f/5 is a pretty large beast.  It's not going to fit in a small observatory.  And without a permanent setup there's going to be the necessity for doing a decent alignment every time you want to use it, especially when you're going to be using an effective focal length of around ten metres.

I'll be interested to hear what those people who do have very large SCTs or dobs for planetary imaging think, especially about the apparent cons of their choice that may have turned out not to be, or the ones they didn't realise until after making their decision.  You never know, one or the other might come up second hand for loose change and pocket fluff one day

James

Interesting use of the 'compact' to describe the C14 (much more commonly associated with the Skymak 127), though compared to 16/18" dob it is a very fair point.

Hopefully a tracking eq platform would be sufficient to keep the target on the sensor once its there.   Though it's going to be difficult to get it centred on the sensor with the nudge to at long focal lengths and Chris is probably right about wanting a flip mirror to aide this.

Not sure there's much difference in the image train length with a 5x (dob) or 2.5x (C14) powermates the focus position of both would need to accomodate the Barlow, ADC, FW and flip mirror - the train will certainly look pretty ungainly stuck out of the side of the dob!     I've not used flip mirrors thus far and the spiral search feature in eqmod has been my godsend thus-far at longer focal lengths, a luxury I would definitely miss with the dob!

Pocket fluff I can do!   I have seen and been tempted by a couple of large mirror sets on ABS recently, especially the 600mm Newt and Nasmyth Cassegrain set for £ 1,000 (now sold or otherwise removed).      But have to remain reasonably realistic about what could be housed in a small and temporary observatory.

[si@nite]

Neil makes a good point not everybody seems to make the transition from C11 to C14 a success due to many different reasons, but when these beasts perform the images are unmatched in MHO, it seems to be down to cooling thermel - problems when they dont, also colimation would seem more critical with the C14, theres been many posts on here regarding the issues, cgarry only recently posting about the cooling issues & the lenghts imagers go to in there quest to stabilize them!

Also the point can be made if you cannot produce good images with a C9.25- C11 when seeing allows maybe the aperture increase is perhaps a bad idea !

[SnakeyJ]

Than there is the possible 16" GSO RC...? thoughts on that?

http://astrograph.net/epages/www_astrograph_net.sf/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/ADGSO16RC

Hmmm, quartz mirrors and very serious looking bit of kit - without the cooling problems of the SCTs, but that CO looks pretty substantial.   According to the specs at Luminos the CO is 150mm of 400, which is 14% by area or 37.5% by diameter!    The specs I have for the C14XLT are slightly better at 10% by area or 32% be diameter.    However it looks a hell of a lot more scope for your money than the C14.    Though at 36.6KG you won't be hanging this on your EQ6s    Think  EQ8/CGE Pro or big bucks

[SnakeyJ]

...yes, just make sure you include in that kitty reserve plenty for including the 16/18" dob + EQ platform etc freight charges to Barbados & back..!

I'm not going to say anything except maybe take up DSO imaging  or move to the Equatorial regions or Southern Hemisphere if you want to spend money on planetary imaging - considering the way most of the planets are heading nowadays...sad but true that these cycles occur. 

But £ 10,600 for a C14 in the UK...strewth, that's over $20,000 Aus - a C14 here costs £ 3,600..!

Of course I use an NEQ6 (£ 1,040) which can't carry a C14.....but the trick is not to tell it that it can't - I've been fooling one for 4 years now!

I sometimes look at that 16" OO scope sitting in the corner of the room here & think "I must do something with it" - like perhaps sell it...but then I go "naaah! Couldn't do it to anyone..!"

Cheers Darryl - a very fair point on moving further south and with those prices down under I might have to emigrate.    Nice to know the EQ6 can be coaxed in to handling the C14.

Did you give up using the OO 16", or never get around to it?    Which would you use from a permanent observatory if you had the choice?  - I think I know where this is going

[SnakeyJ]

 I do wonder about somekind of optical variation with C 14s

Previous images from those using C11s seem to often look better. So the big scope doesn't seem to always perform well, But then again we see others who switch in the UK who seem to have a level of sharpness and clarity that clearly indicates a consistency, others who previously were doing ok with smaller optics. can no longer seem to reach.

I wonder if Peter Edwards has had as much thermal trouble affecting he's images as others have. Because he consistently seems less affected than some other C14 users of recent times in the UK.

Are the big scopes showing how poor some locations are around the UK

I wonder if Peter Edwards has done some major thermal stabilization of he's new C14

I personally would use a C14 But would certainly be concerned if it would actually ever do any better than a C11  and sometimes worse.

All good points here Neil, consistency of manufacture is always a factor, though other than the longer cooldown times, I wonder if theses big SCTs are more affected by local seeing effects (heating, roofs et al) as few of us have good clear space around our home bases.

Neil makes a good point not everybody seems to make the transition from C11 to C14 a success due to many different reasons, but when these beasts perform the images are unmatched in MHO, it seems to be down to cooling thermel - problems when they dont, also colimation would seem more critical with the C14, theres been many posts on here regarding the issues, cgarry only recently posting about the cooling issues & the lenghts imagers go to in there quest to stabilize them!

Also the point can be made if you cannot produce good images with a C9.25- C11 when seeing allows maybe the aperture increase is perhaps a bad idea !

Useful points on collimation and cooling - this also prompts a vague recollection of something JamesF mooted recently about SCT's design sweetspot for focal length and focuser's and long image trains stretching this.

Really useful thread and some interesting views coming out here, but UK seeing conditions and the amount of use that could be made of these bigger optics are somewhat sobering and recurrent themes.

[SnakeyJ]

The conspiracy theorists I'm sure would have a field day with Emil's lovely image of Jupiter if this was about our Moon

Stunning isn't it!   I didn't know that ZWO made an ADC! Google time

[Cath]

Stunning isn't it!

It is isn't it

[DrRobin]

Hi,

If it was me I would go for an RC design. They are usually slightly lower FL than the equivalent SCT, but most are truss tube and therefore cool very fast. Plus, you can use a standard laser to get collimation, something you can't do with an SCT.

I would have a look at the ones in Altair Astro for starters, or have a look on TS.

If one would fit in my micro obs I would be selling my C11 Edge as a good RC also has a flat field so good for DSOs as well as planets. Sadly, even the 10" probably won't fit so it looks like I am stuck with a C11 for the near future. I need a bigger obs......

Robin

[Blazar]

Or one of these truss cass (which they do in apertures up to 22" o_0

http://www.altairastro.com/altair-planeta-250mm-truss-tube-cassegrain-professional-grade.html

[JB80]

This is an interesting thread and I like all of the responses.

I have given this some thought in the past and for one reason or another have always ended up siding back with an SCT, with no real practical experience of either(apart from a medium size newt) it really is tough to choose.

On the face of it and going by what others produce then you would think and SCT is the way to go but after a couple of years here in Spain I really do start to wonder if it is worth the cost for the sort of things I want to do with one due to the seeing conditions. Maybe a big dob is a better option just in case it doesn't work out the way I envisage it in my head. Then again these type of purchases are often a once in a lifetime thing and you tend to want to get it right the first time.

To me the SCT has always seemed like the less faff option, that is if planetary is your thing at least and anything else would be like fitting a square peg in a round hole.

I guess you have to take Daryl's point about the planetary cycles too, all sounds a bit depressing but it's not going to stop people imaging them, is it?

Hi,

If it was me I would go for an RC design. They are usually slightly lower FL than the equivalent SCT, but most are truss tube and therefore cool very fast. Plus, you can use a standard laser to get collimation, something you can't do with an SCT.

I would have a look at the ones in Altair Astro for starters, or have a look on TS.

If one would fit in my micro obs I would be selling my C11 Edge as a good RC also has a flat field so good for DSOs as well as planets. Sadly, even the 10" probably won't fit so it looks like I am stuck with a C11 for the near future. I need a bigger obs......

Robin

Interesting you say that Robin as when I was first looking into things it was Dob, SCT or RC and RC was the first to fall off the list. The thing that always got me with RC's is they are not renowned as being the best for visual use. They are probably fine for visual for someone like me but that is the word on the street at least.

[leelee970]

There is one point that you've completely overlooked Jake..... you've just moved house and every spare penny you thought you had will go into redecorating.

sorry to depress you mate. I'm in the same boat, except my spare pennies go to repairing my car.

Lee