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Old 08-02-2009, 10:41 AM
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telecasterguru (Frank)
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SCT v Newt for imaging

I am interested in getting into imaging so as to keep a visual diary of my observations.
I am thinking of using either a SCT or Newt on an EQ6 mount for both DSO and planetary? Would an EQ6 carry a LX200 12" with imaging accessories?
Would I be limited in the size of the Newt? Which would be visually superior, the SCT or the Newt? I have a Canon EOS 350d and a webcam that I have been using on my dob for lunar work and now it is time to get serious with imaging.
Thanks
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Old 08-02-2009, 12:34 PM
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renormalised (Carl)
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Whilst the EQ6 is a pretty rugged setup, with a 12" LX200 on it and all the sundry bits and pieces, it's going to probably tax the mount....the LX200 is a fairly weighty OTA. I'd be more inclined to go for a G11 just to be on the safe side. With a newt, I wouldn't go any larger than a 10" and that might even be too much when you add guidescopes, cameras, other counterweights, cables etc. Once again, a G11 would be better. Pricier, but better in the long run. As far as image quality goes, the newt doesn't have the central obstruction a SCT has, so the images will be a little brighter, however one of the new ACF model Meades will have superior views as they're corrected for coma right across their FoV. Plus, with an FR (focal reducer) you can make the SCT just as fast as a newt, so taking just as good widefiewld shots. Your 350d will take some pretty sweet shots with either a SCT or newt, so both will be great for astropiccies. However, for ease of use and less hassles with angular moment and torsion forces being put on the mount, I'd go for the SCT. A 10" newt has a pretty big moment arm and will induce flexure in the mount. The SCT, being more compact, won't have the same problem and will be easier to operate. Basically, if you chose either one to go with, what is the most critical thing is the mount. It needs to be strong and rock steady, especially for imaging purposes. That's why I'd go for the G11 mount. It's a lot more robust than the EQ6 and is designed for heavier payloads.
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Old 08-02-2009, 01:43 PM
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Thanks for the info renormalised.
I've had a look around the web and the G11 goes for around $7000 while the EQ6 runs out around $2000. I suppose the price justification is in the final image.
I think the C11 is a lot lighter than the LX200 and maybe that may be a viable option on an EQ6.
Thanks again
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Old 08-02-2009, 02:10 PM
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renormalised (Carl)
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You could also try the Meade 10" LX200 ACF as well.

Or, if you could afford it, you can pick up a Celestron CGEM 1100 (11" scope) for $8000....complete kit....scope, mount, goto drive, the lot.
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Old 09-02-2009, 08:02 PM
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My C11 rides happily on top of my EQ6 for deep sky and planetary imaging no worries... I did initially want the Meade 12", however its quite a bit heavier.... Even the G11 is right on the edge for the Meade 12".. Fred (Bassnut) will tell you that the Meade 12" is about as big as you'd want to go even on the G11 mount, on an EQ6, its just a bad idea...

A quick word on SCT's... The front corrector plate, a large, thin piece of glass.. cools down very quickly, and when it does, dew will rapidly form on it... Newtonians generally do no suffer dewing problems except in extreme conditions, and in most cases, a simple dew sheild will stop it. The SCT **WILL** require a dew heater... No If's whats or buts about it...

I find the EQ6 easily handles the C11.. You have to be picky about what accessories you decide to hang of it though... I strongly recommend an off axis guider for your deep sky photography.. With a seperate guide scope, you are pushing the limits of the mount, as you're adding quite a bit more weight.. not just the weight of the guide scope, but the camera, the rings, the piggyback bar or side by side plate (I recommend side by side over piggy back, as it keeps the center of gravity closer to the RA axis) Also, I found guiding through same OTA to be more accurate...

I would also recommend looking into some upgrades for the EQ6.. For starters, www.admacessories.com sell a replacement head for the EQ6, that converts it from the skinny (read: flimsy) vixen style dovetail, to the beefy Losmandy D-plate. I found this made an incredible difference to the overall rigidity of the system. Without a doubt..

You'll want a focal reducer, as imaging at 2800mm with an EQ6 is going to make you lose your hair quickly.. I use the Celestron F/6.3 reducer, giving me a focal length of 1756mm.. Depending on the camera you intend to use, you could consider the F/3.3 reducer, but for any larger sensored camera (like a DSLR, QHY8 or upper end CCD) the 3.3 reducer will heavily vignett the field...

Be prepaired for lots of nights of trial and error. I've found with this much weight on the EQ6, balancing the system is CRUCIAL.. it needs to be perfect... That being said, the rewards when you finally get that last glitch ironed out are fantastic!.

As far as newts are concerned.. they have their ups and downs... a newtonian OTA is A LOT cheaper than an equal aperture SCT. They are also quite a bit heavier.. a 10" skywatcher newtonian is about 3kgs heavier than the C11..

Planetary imaging.. well... without any further modifications, the newtonian will generally cool down faster... this is a big plus. SCT's being a sealed system generally take a long time to cool down... Paul Haese has completed a fairly drastic modification to his C14 in order to quickly cool the mirror down to ambient (http://paulhaese.net/) Whilst this looks seriously overkill at first glance, after using my C11 for planetary imaging a few times, I've started to strongly consider doing something similar... It really does take quite a long time to cool down.

Overall.. I will say this... 6 months ago, I was in your shoes... tossing up between the meade/celestron SCT's or a 10~12" newtonian... It took me quite a lot of time to come to a decision, although now, 6 months later, I'm quite sure I made the right decision... I love my SCT... I dont see it going anywhere for quite some time!
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Old 10-02-2009, 10:08 AM
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ps - A lot of people in the states are having problems with the CGEM's out of the box... a few dead ones, a few with horrible tracking, some wont guide well... Celestron still aren't selling separate counter weights (which everyone would surely need..) I would steer clear of those for the time being... the EQ6 is tried and tested.. and a great work horse mount.
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Old 10-02-2009, 12:42 PM
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madwayne (Wayne)
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Gee Alex congratulations on an excellent write up. I hope Frank takes on board everything you have said. And here was me going to suggest the EQ6 with an ED80 and a guider .

Frank ignore me, listen to Alex and good luck with your decision making, Alex has taken the guess work out of the equation for you.

Clear skies.

Wayne
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Old 10-02-2009, 03:11 PM
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Thanks for the advice Alex,

I must admit that I have looked at the CGEM and originally thought it would be the answer to my questions.

Unfortunately I am not sure now that it is what I am after. For the life of me I cannot understand why they would be advertised at $3000 US dollars in the States and advertised for roughly $8000 AUD in Oz. The exchange rate is bad but surely not that bad.

The EQ6 with C11 seems infinitely sensible to me, although I am not sure about the off-axis guider. I have read that a small ED80 can be guided by webcam and K3CCD (there is a guiding capacity in the software). There is also the Orion Scope Guider which is not overly expensive.

I will be using a Canon EOS 350d that I already have for imaging and the weight of the Camera must also be considered. As you have said, balancing will be crucial and I am happy to learn and also to do a lot of crying and hair pulling on the way.

The other option would be a LX200 10" out of the box but I don't particularly want to go down the Wedge track and I think I could get the C11 and EQ6 for considerably less money. (I am talking myself into it)

The idea of upgrading the mount is also a good one so once again thanks for the advice.

All other advice greatly appreciated.

Regards
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Old 10-02-2009, 04:16 PM
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Yes, Off axis guiding can be a pain, as finding a suitable guide star at 1700mm can be tricky, however, the accuracy of guiding through a the imaging scope is much better than guiding with a separate tube when it comes to SCT's. As the primary mirror in an SCT is not fixed, it has an element of "flop" if it shifts/flops half way through a 10 minute exposure, your image is buggered. where as if you're guiding through an off axis guider, your guiding will adjust for the image shift, and all will be ok. Not to mention that guiding 1750mm focal length with a 400mm focal length (like the Orion ST80 Guide scope), whilst possible, is by no means "easy" and its all extra weight on the mount. a second scope, mounting gear etc... will easily take up another 2~3 kgs overall. (I know my guidescope/rings and dovetail is 3.3kgs... where as the off axis guiding setup weighs a total of 760g.. Better for everyone!

Cheers.
Alex.
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Old 10-02-2009, 04:40 PM
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darrellx (Darrell)
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Alex

Interesting and timely comments about the off-axis guider. I have been toying around for a few weekends now trying to get my first attempt at guiding to work, but as you say, weight is an issue.

I have just had a quick look around for an oag. There doesn't seem to be a big market for them - or I am just looking in the wrong places.

What brand guider do you have , and where did you get it?

Thanks
Darrell
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Old 10-02-2009, 05:06 PM
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renormalised (Carl)
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Post

Quote:
Originally Posted by AlexN View Post
ps - A lot of people in the states are having problems with the CGEM's out of the box... a few dead ones, a few with horrible tracking, some wont guide well... Celestron still aren't selling separate counter weights (which everyone would surely need..) I would steer clear of those for the time being... the EQ6 is tried and tested.. and a great work horse mount.
Didn't know that....it's what happens when you move your production over to a country with very few quality controls in their industry...you get lemons. So much for saving money on making them, instead you lose reputation!!!!
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Old 10-02-2009, 06:44 PM
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Newtonians have really made a comeback in the last few years as superb imaging machines since the advent of the Wynne corrector like that used in ASA scopes.

Some of the finest widefield images have been with ASA Newtonian scopes (when they worked) as per example Wolfgang Promper.

SCTs strength I understand is planetary imaging. Apart from Fred's great images I generally do not see too many high quality SCT images. That's for a few reasons.

Long focal length imaging is hardest of all as all the imperfections of balance, tracking, polar alignment etc etc gets magnified along with the image.

SCT alt az mounts have long been known to generally speaking (I am sure there are some good ones around) been great for visual but simply inadequate for long focal length imaging. That is if you want to do image exposures longer than a minute. So unless you get yourself a very expensive mount with an SCT or Meade ACF (they seem the best) OTA only you probably will be disappointed/frustrated.

So you need to work out what type of imaging you want to pursue. Short focal length wide field imaging will be easiest. It will be most forgiving and most likely to end with a good result even with some imperfections of balance, tracking and guiding.

Then there is the question of F ratio. SCTs are usually F10. That is not a good imaging F ratio. Whilst CCD cameras are linear in response meaning F5 and F10 will give the same signal to noise ratio, to get shorter exposures 2 scopes of the same F ratio means the one with the larger aperture will give the shorter exposure times for the same brightness of image.

So ideally for an imaging machine (given setup time or travel,cloud, work, moon limit our available imaging time) then a widerfield, fast (F5 or less) Newtonian of largest affordable aperture will outperform other machines easily.

I have seen lots of fast (often less than F5), large (bigger than 8 inches) Newtonian images over the last few years that are simply sensational. Newts give very sharp images due mainly to the fact that a simple Newt mirror is easy to make very perfectly compared to other odd shapes.

So I suggest if you want to go the imaging route you:

1. Buy the best mount you can afford. A good mount with a crap scope will be better than a good scope on a crap mount.

2. Go Newtonian and go shorter focal length with low F ratio and get a nice corrector like a Wynne 3 inch corrector from Phillip Keller or even a Televue parracor.

3. Setup to do autoguiding well. The QHY is a popular choice with free PHD guiding software.

4. There are also upgrades available for the EQ6 including a gear/worm coating that reduces periodic error tremendously. if you can afford it you would be better of with a smaller Tak mount rather than a G11. I have had Gemini system and it is a pain and time consuming and not particularly user friendly. Tak leaves it for dead. Any Tak model. A lighter one would be an EM200 and they can be gotten 2nd hand on Astromart for around US$3300 or so perhaps less. It would last you a long time and you will get results instead of the inevitable constant upgrades which is where the real money is often spent in this game trying to make something work better than it does out of the box.

A nice Newt on a 2nd handTak EM200 with a AT66ED or similar guidescope and a QHY guider would probably cost the same as a largish SCT with Alt Az
that is fabulous for visual but a pain for imaging.

The difference in results speaks for itself. I can guide you to numerous examples of superb images with similar setups to the above.

My 2C.

Greg.
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