Refractor equivalent of an 8" reflector?
Posted 02 August 2008 - 06:33 PM
I agree with you that reflectivity loss is rarely mentioned. I'm looking for more aperture, and having a hard time deciding between refractor and reflector. I guess a 12"mirror will gather more light than a 6" lens for a long time, but I have difficulty getting over the idea that I'm losing performance over time.
Posted 02 August 2008 - 06:56 PM
Posted 02 August 2008 - 07:26 PM
Sorry if it is, but the point I am trying to make is that in the real world the consistent performance of the refractor means that it will equal that of a reflector of 1.5 to 2x its aperture. Most reflectors out there are not in pristine condition. Reflector users on this site will obviously tend to their equipment more regularly. The question was simple, 4" refractor vs 8" reflector. If I was given that choice I would go with the refractor every time. If it isn't quite as good deep sky as the reflector when both are new it will be in a few years time. It has crisper views with more contrast from the moment you step out of the door with no fuss or fiddle, year after year. And don't forget the wide field views that only the refractor can give. And being smaller it will be used more. I am not anti reflector, in large sizes there is nothing to compare, but to me this question is a slam dunk no brainer. The only strong argument for the reflector that I can see is its cheap cost.
The question wasn't limited to a 4" refractor vs. an 8" reflector. '
Here is the original post:
"Here is the original question:
"Compared side by side which size refractor would be similar to an 8 inch f5.9 reflector scope?
For example 80m -> 6in, 100mm -> 8in?
I'm asking this because for my next scope I plan on stepping over to the refractor camp (guiding and Goto)."
To say a 4" refractor isn't quite as good a deep sky telescope as an 8" reflector is a gross understatement. I love refractors but I also believe my eyes, at least most of the time. I'm sorry you don't have easy access to a mirror recoating facility. An 8" mirror would be pretty dirty if it was giving an image as bright as a 4" refractor.
Posted 03 August 2008 - 01:03 AM
Posted 03 August 2008 - 08:08 AM
While the loss of reflectivity is compounded by the secondary mirror in the the reflector, light loss through the refractor is compounded by a mirror as well - the diagonal. You can get a durable, high reflectivity dielectric diagonal, but you can also get a dielectric secondary mirror for the reflector.
The hassle and difficulty of collimating the 8" reflector is overblown. At f-6, the optical system is pretty tolerant of minor collimation errors. An 8" scope is most likely a solid tube scope, and if the mirror is mounted on a good cell, collimation once set will need to be re-adjusted very infrequently.
Cool down times for an 8" size mirror is pretty quick, especially with a fan installed. Also, cool down for the refractor is not exactly instantaneous, especially when you are talking about high magnification views.
Don't get me wrong. I love the aesthetic contrasty views in my refractor (SV-90T), but when I want to see something faint, I go to my 8in Royce conical reflector, especially in my light polluted back yard.
Posted 03 August 2008 - 02:15 PM
It's a simple matter to clean your mirror. I just get a gallon of distilled water and flush off the debris. You'll have a clean mirror in a few minutes and no more ribbing. (o:
Posted 03 August 2008 - 04:35 PM
My 8" f/6, with it's stunning Mike spooner primary, and an equally spectacular Anatares Optics certified INSANELY flat and reflective secondary, has been known to shame some well-known APOs in apertures into the 7's, for lunar/planetary, and into at LEAST equal aperture for DSO's...
I have FOUR primary mirrors for this scope, and they are ALL quite nice, including the original, which came from Meade. But I don't know if, in this size range, Mike Spooner can be beat... Tyrannosaurus Wrecks notwithstanding...
Equalled, perhaps... But even that hasn't happened YET...
No. The EON 120 ISN'T gonna' beat it... Haven't tested THAT yet, but I know what Spooner optics can do... And a 120mm ED refractor versus a 200mm Spooner reflector is a foregone conclusion...
Posted 03 August 2008 - 05:30 PM
I've compared views on an awesome night at Stellafane years ago between an 8" f/7 with Spooner primary and several 4-6" apos from top flight companies. This was mainly on Jupiter when it was really good. The 8" Spooner walked all over the apos. I'm a refractor lover as much as the next guy, but there are some seriously misinformed folks when it comes to comparing good refractors to good reflectors.
Posted 03 August 2008 - 06:10 PM
'Ever heard this... "There is no substitute for cubic inches?" Sadly, I expect the same is largely true of telescope aperture.
Posted 03 August 2008 - 06:12 PM
Posted 03 August 2008 - 06:30 PM
Posted 03 August 2008 - 06:58 PM
And my 6" $250 Orion 6" f/8 Dob has run totally neck and neck with my buddy's Tak 102 on more than a few nights on the moon and planets. And it's beaten the 4" apo on some nights. Same result with a TV102 and a Vixen Fluorite 102. And it beats my TV85 and his Tak 78 100% of the time. Myths die hard.
Posted 03 August 2008 - 07:00 PM
Posted 03 August 2008 - 07:17 PM
My experience with them, or side by side with them, is they lack the consistency of a fine refractor at giving high resolution, high contrast images.
For some of you with very small central obstructions, you might check to see how much light loss you are experiencing with an undersized diagonal.
I can afford a very nice Newt and have the mounts to put it on, but I love my refractors.
Posted 03 August 2008 - 07:38 PM
Posted 03 August 2008 - 07:50 PM
One would think.... but apparently not!
Posted 03 August 2008 - 09:01 PM
But I'll expand that...
Mike Spooner Newtonians are known quantities... So when you start fabricating generalities around them, chances are, you'll get burned...
Such is the case here...
Side by side, Spooner Newtonians DO NOT lack consistency... They are SUPERB as a class... Certainly up to top craftsmen in other optical venues... And folks who own Spooner optics know that...
Posted 03 August 2008 - 10:29 PM
I'm not talking about local seeing from night to night. I'm suggesting that if you set up a high end APO and a high end Newt of roughly the same size (maybe a 180mm APO and 8" Newt) side by side over several nights, the APO (refractor) will more often show better high res, high contrast, planetary detail.
Posted 04 August 2008 - 12:54 AM
Posted 04 August 2008 - 01:58 AM
IMO the most appropriate answer in this thread.However it is obtained,even if financial considerations are the bottom line to ones choice of telescope type,to me that is the bottom line answer.
8" of quality aperture is hard to beat.
Posted 04 August 2008 - 07:08 AM
coating technologies have improoved over the years.
A german stargazer measured the reflectivity of his over 20 year old
8"f/5 mirror with 88% reflectivity and it came out with 82% refectivity.
Clear skies, Karsten
Posted 04 August 2008 - 11:53 AM
Posted 04 August 2008 - 12:34 PM
Posted 04 August 2008 - 07:22 PM
I was out looking at the Saturn nebula last night. Great seeing conditions. But not an easy target. I had my well collimated 14" Dob (with 10yr old mirror coatings) next to my 6" achro. Firstly the view through the refractor was much more pleasant as expected. The Neb was also easier to find as contrast was better and no coma so anything that isn't a star is obvious. Through the reflector the image was brighter, and it had the saturn look. But through the refractor at 200x the extensions on either side of the planatary were revealed also. Why, - better contrast. The eye is a contrast detector.
As for your question about a 12"Dob or 6" refractor. Well thats pretty much what I have. And when I reach for a scope it is always the refractor these days. Just look at the number of happy posts on the celestron 6" scope. Must be the biggest topic on the site! a great scope for a great price.
As for improved coatings, that is true. But 82% of 82% is still pretty poor. And that 33% approx lost light isn't just lost, it scatters and reduces contrast. Then add lost light due to secondary plus diffraction affects on top and viola, more lost contrast, then a few spikes sticking out from every star, plus at least 50% coma around the periphery. Then some dust, grime and mis-collimation to top it off. Crapola! Unless you go BIG with a Dob you are much better off with a refractor.