View Full Version here: : Talk me into something..
xelasnave
05-12-2008, 05:00 PM
I want to replace my current imaging scope...a 6 inch MEade sn..
I have thought about a decent refractor but find my leaning is to a celestron 8 inch ota or their 9.25 inch ota...approx $1700 and $2700 respectively.
Firstly should those scopes be disregaurded in favour of a good 4 inch refractor or bigger...if so which one.
AND is it better to go for the 9.25 at an extra $1000
I do like the mak idea but I gather a good one of similar app is pretty expensive.
ANY ideas greatly appreciated.
AND not interested in planetary work just deep sky.
I have an eq6 by the way so it should manage most scopes offered for consideration so please dont tell me get a paramount mount before I get a better scope.
alex:):):)
AlexN
05-12-2008, 05:35 PM
I'd go the C9.25.
The EQ6 will have no troubles with it. They are light for their size, generally speaking they are held in high regard among imagers for having good/great even the occasional "extraordinary" optics..
They will allow you a good focal length for hunting galaxies, the option of the 6.3 reducer for wider field work... To be honest, I think either the C8 or C9.25 would be great.... But in my opinion, Aperture always wins out... (Check my signature :D)
AlexN
05-12-2008, 05:42 PM
Oh... Mind you, there is a C9.25 + Losmandy GM8 on the forums for sale for $2900ish.... You could buy that, sell the GM8 and have a C9.25 for less than the price of a new C8 OTA....
Its an option...
rally
05-12-2008, 05:58 PM
Alex,
Since you used the terms "decent refactor" and "imaging" - how about going for a second hand Takahashi refractor ?
Large flat fields and capable of larger format cameras and DSLRs and able to be upgraded with all sorts of options.
Their 100, 102, FSQ106mm refractors are bearable !
The TOA130mm would ideal !!
They make an ideal imaging platform and depending on your selection would ultimately give you better imaging potential.
Focal length is OK for your intended purpose, maybe get the appropriate 1.6x extender.
They will support eyepiece projection too (with the right adapter).
2" eyepieces can be used for great viewing
Food for thought.
Rally
robgreaves
05-12-2008, 06:52 PM
Completely different animals...
What are your imaging aspirations? Wide(er) field or large image scale stuff for small planetaries and galaxies etc.?
Bear in mind that optically, it's always better to Barlow/Powermate up than to reduce down.
Refractors are great - highly versatile, no fuss. A SCT will require constant collimation.
Regards,
Rob
xelasnave
05-12-2008, 09:30 PM
Preaching to the converted eh Alex ..I do like your choice.. what is the drum on colmination?
I saw the add but felt maybe the age went against it... mirrors go off su I believe so I wondered how many light years it had left...but I did think long and hard because as you said it is an opportunity.
alex
xelasnave
05-12-2008, 09:46 PM
Rally that is not helpful:lol::lol:...
that is what I really really want you see:) and I always have thought images with same are fantastic..so crisp...it is strange I like them so much because I have never seen one up close.
Someone said recently get one and re sell it in a couple of years and the slight drop in price see as rent upon it..that makes a lot of sence to me.
Fortunately or unfortunately depending on the day I have no one to explain my actions to and the dogs dont care so long as they are fed... and happily money is not the problem it was when there is a lady in your life.
I like refractors and love my cheap 150mm but to get a great one I would need a mortgage $30k seems the area... my absolute favorite is the astro physics 150mm... I tried to put my name on their wiating list and they told me I needed to be a resident in the USA... I felt their marketing somewaht elitist ...but clever...tell someone they cant have something and they want it all the more..but not me..they put me right off.
Thanks for taking the time you could sell me one very easily.
alex:):):)
xelasnave
05-12-2008, 09:49 PM
Thank you Rob...constant collimation puts me off...I have enough fiddly things to do during set up I will address this aspect before I do anything.
alex
AlexN
05-12-2008, 09:54 PM
I am the same, I prefer to buy scopes new... for fear that the previous owner might not be as pedantic as I, and it may not be in the condition I would like it to be...
Collimation of an SCT is laughable... Its SO easy.. I avoided learning newtonian collimation when I had a newt, because it sounded hard... I attempted it finally after 3 months of owning the newt, I gave it a go, failed, tried again, failed, tried again, better but not perfect, tried again (etc etc...) When I got the SCT, I was again cautious to attempt collimation, Even though it clearly needed to be done, because I feared the same troubles I had with the newtonian. I bit the bullet in the 3rd week of owning the C11, and within 5 minutes of starting, the views were (to my eyes) as sharp as my 4" APO refractor.. It was an absolute breeze. now, 6 weeks later, the is still as well collimated as the day I first tweaked it.
It's easy, the scopes hold collimation, sometimes for years if treated with good care....
I think for imaging, an SCT is a great choice... Sure, the refractor will be easier to use, but there really is no substitue for good old fashioned inches!! :D
Actually my C8 seems to hold its collimation very very well. In fact Ive only done it once this year :-) (Then again I dont transport it around that much, so there is no reason why it would get out of alignment)
xelasnave
06-12-2008, 12:20 AM
Thats comforting...it will be housed in an observatory (a roll away garden shed) so maybe I should not worry.
It amazes me what I get out of the 6inch so even 8 should be great..
OH I forgot I have tried the 12 inch on the eq6 and have not completed that project... I do mainly unguided even with the 6 inch as I have tweeked the eq6 rather well.. I run it with no greese and only some light machine oil...polished everything somewaht fanatically which certainly made it work well... I have had 10 minute unguided thru the 6invh sn... but the magnification is not that great so I can get away with it... My lastest widefields are all unguided...even the 300 second runs of the LMC was unguided using the 70/300 lens.and widefields at 5o mm I have done 1300 seconds...yes really a dark site only starting to glow after all that time... but it was moist and if dry I could go longer.
I had forgot about the 12 inch work I had done ,,, I know why...I got guiding cameras, then the cat threw up on the lap top and everything was set back..now the shoestring program wont download on the new lap top... got everything else including the box from shoestring to auto guide ...just been doing unguided runs for so long I have overlooked sorting that out.
alex
xelasnave
06-12-2008, 12:25 AM
Thanks everyone for the input it has been very helpful...I wont do anything until after xmas just wont have the time but I need to set my goals...now that crazy woman is gone I can get back to it
alex
AlexN
06-12-2008, 12:31 AM
I wish i had the Ba... umm. Guts to pull my EQ6 apart and make some modifications... Problem is, even if I had the guts, I wouldnt know where to start to make it better... :)
xelasnave
06-12-2008, 01:01 AM
It is scary but real easy once you have done it once...
What got me was I spent ages trying to unscrew a washer..It would turn about 5 degrees and I turned and turned ...but it was not treaded ... There was a site...Spanish ... so that was interesting.
Have a go ... I cleaned it and bought a little engraving tool and polished and polished... but on the positive I was very impressed with the gear mesh from the wear marks it was a perfect mesh out of the box...
I dont know if the lite oil really works but feel that is does... and given how slow they go lack of greese wont see it seize under high revs...but I think one needs to be able to clean the gears... I found a small spider squashed in the gears which would have translated to PE and one would suspect that dust may get in so being able to clean it is useful.
I used to blue print motor bike two stroke motors and one realises that the factory can leave stuff where is should not be..A single metal filing can stuff tracking really... but internally I was more than impressed with the eq6
alex
alex
alex
xelasnave
06-12-2008, 01:04 AM
I set the gear mesh such that they make a noise at fast tracking but that setting was superior as the teeth meshed perfect.
alex
g__day
06-12-2008, 02:17 AM
I have a Carbon Fibre C9.25 in a permanent setup. Because of the large temperature differentials I have to re-focus sometimes as often as twice a year. Collimination according to CCD Inspector hasn't moved in the last 6 months.
So maybe its luck of the draw. I've never looked though a high quality refractor and am dying to do so - particularly anything in the 4" - 6" mark. But I expect stars would be smaller and tighter in it. I can't see anything focusing as sharply as a refractor.
Hard choice indeed!
xelasnave
06-12-2008, 08:41 AM
Thank you Mat ...you have to refocus as often as that must be a pain..I hope I can get to that stage...that is exciting news I wondered about the carbon fibre I am glad you raised it...frankly with this weather one wonders why bother.
I find it encouraging that you have made such a choice as I respect your opinions very much.
Rigel003
06-12-2008, 09:16 AM
AP sells worldwide so that seems surprising - maybe that was the case long ago. Their lists are so long you'd have no chance anytime soon (10 years) but it's worth signing up online. Doesn't cost anything. There are lots of other, and cheaper, alternatives in premium refractors. I would encourage you to think refractor rather than SCT if your main interest is deep sky imaging. Wide fields and pinpoint star images. SCT images always have larger, bloated stars. I own both but the SCT is used for planetary imaging and the refractor always for deep sky.
g__day
06-12-2008, 11:05 AM
Its a trade off isn't it? A refractors costs way more, has shorter focal lengths, less light reach but gives superb focus and tighter stars (someone please check that last point is true and explain technically why!). An SCT is lower cost, so at a same price point has much longer focal length, far more light reach, but even with optimal focus I presume its stars won't be as tight as the refractors?
Is it just a function primarily of focal length that makes an SCT's stars appear more bloated at optimal focus than a refractors? Put another way if you had say a ten inch apo refractor and a ten inch SCT both shoot the same star field onto the same camera (and you didn't expose over the CCDs well depth - so no blooming) - both perfectly focused and both with say a 2.5 metres focal length - would the refractors stars be the same size as the SCTs - slightly less or significantly small and tighter?
AlexN
06-12-2008, 11:13 AM
I was just about to say that Matt... maybe its a focal length thing, the bloated stars.
On the other hand, where you said refractors have a shorter FL.. Depending on how much money you wanted to spend, a 9" F/10 TMB APO will decimate a 9.25" SCT for deep sky imaging.. although coolind down a 9" triplet APO might be harder than a 9" SCT.. who knows... (Dietmar knows!!)
If money was no object, I would go a >/= 152mm long focus APO over an SCT. However, Money is an object, and for me, $4k was the limit... So I went with the biggest, meanest SCT my mount/wallet could handle...
I always thought it was due to the larger CO of SCT's, but I am probably totally wrong. Cant even remember where I heard that from :shrug:
Rigel003
06-12-2008, 03:48 PM
I think you're totally right as to the principal cause. No central obstruction allows the diffraction patterns of stars to be as pointlike as possible. Other factors also prevent SCTs producing optimal images e.g. thermal issues, the fact that the primary is enclosed and at the bottom of the column of air rather than the top, the fact that the light rays travel 3 times through the tube in SCTs and once only in a refractor. The size of the diffraction pattern from a point source shouldn't have anything to do with focal length or focal ratio.
xelasnave
06-12-2008, 11:40 PM
Thanks for all the help everyone. I would love a good refractor and a SCT side by side when I think about it,,,,I should get rid of some stuff when I think about it...
alex
AlexN
06-12-2008, 11:49 PM
Thats the way to do it Alex. Although expensive...
I suppose, you could go for the smaller C8, and then use the $1000 difference to buy a good refractor. Something along the lines of a Meade 5000 ED80 Triplet, or a Megrez 90... Both of those are great scopes, "around" the $1k mark give or take.. Can't go wrong with the best of both worlds.. :D Thats what I always say!
xelasnave
07-12-2008, 12:10 AM
I have even been thinking about the money side...I really should get rid of my current gear and some other stuff I have been hanging onto as investment and go for it... I never play my electric guitar for example..some antique furniture you cant even sit on...I am not young ...mmm I need to get organised because I enjoy taking photos more than anything really... It is such a buzz cause it is way past anything I ever aspired to... I am lucky , we are all lucky, to have available to us the excellent gear...
You know what I wanted to set up is four scopes on the one mount so as to have each scope with a dedicated filter color plus HA and cameras for each .... I know I am getting carried away but I think that would be a great system and I would love to try it... four sbigs four astro physics 150 refractors on a paramount mount...
but at the moment just some clear sky would be nice...so bored and cabin crazy
alex
AlexN
07-12-2008, 12:28 AM
:) I hear you on that... I've not had my scopes out properly in quite a while...
I too have thought about the idea of 4 scopes/4 cams... The problem being, How would the computer handle running 4 SBIG cameras all at once? I suppose if you're going 4x150mm AP's on a paramount, the Obs PC is going to be no slouch!! :D
I do what you're doing in my head all the time... Im constantly wondering what I dont use often enough to justify having, how much I'd get for it and what marvelous astronomy gear I could afford with said money!
I even considered recently my 52" HD LCD TV, for a good triplet APO or a G11 mount :D What can I say.. Im an Astrotoy Addict!
Starkler
07-12-2008, 03:07 AM
I can think of a few contributing factors.
Most refractors have a higher optical quality commensurate with their price/aperture ratio
Most refractors are small aperture scopes and more prone to run out of light before hitting the seeing limits. At low powers they will have a smaller exit pupil than a bigger scope, so again stars will appear sharper unless the observer has zero astigmatism in his eye.
Less affected by thermal issues
Its not fair to compare a high end refractor to a chinese newtonian or sct that costs many times less per square cm of optical surface.
Now a 6" tak newtonian after proper cooling should give any 5" refractor a good run for its money.
g__day
07-12-2008, 11:46 AM
So asked a different way - will a 8" high quality apo give tighter stars than a properly colliminated, and cooled quality 8" RC given both had the same focal length?
I assume the answer should be yes - as the RC will have some thremal currents, it still has a central obstruction. But how much tighter stars will the APO have? Are we talking 5%, 20% or 30% or more?
Curious...
Matt
AlexN
07-12-2008, 01:10 PM
I dunno... I rekon the a well manufactured RC (RCOS/TAK/A&M) would compete with an equally size refractor. There would be substantial differences in the images produced, but I dare say that nobody would think those differences made one image better than the other (diffraction spikes etc) The APO might have better contrast with no center obstruction... Its really hard to say... Especially for me, someone who does not own a top quality APO, and does not own a RC scope either.
I do know that my C11 captures tightish stars, often when you take into account the different focal lenght, they seem on par with my Megrez.. Provided its been focused accurately..
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