Refractor "equivalent" for a 6" reflector

[Xiionn]

I like the Omni XLT range, the newt is pretty decent. I got a chance to look through the 120 xlt refractor and its also pretty decent. There is some CA though. Whats your budget? Decent Apos around 120mm start at £900 odd. less than a third of that gets you the 120 XLT.

[Roy Challen]

What about long focal length achros? F/10 or more, and the CA all but disappears. Not that good for imaging but great for visual use. They're also not too heavy on the wallet and they're forgiving when it comes to eyepieces and they look nice:)

[John]

Wow: Thanks for all the replies. Here are few more details that folks inquired about.

My reflector is an 6", F/5 Celestron Omni XLT riding on an Advanced VX mount. I've started using it for AP and don't have an immediate plans to change that: right now my equipment is more capable than I am. :-)

I expect to be doing most of my observing under our light-polluted skies though for types of targets I mentioned (clusters, planetaries, planets) I haven't found LP to be a huge impediment.

Mostly what I'm looking for are crisper views of pinpoints of light in clusters especially, primarily for visual observing.

Okay ... now off to read your many responses. May not be able to respond further until tonight but I really appreciate all the insights.

Thanks! -- Joel.

You might find a 6" F/8 newtonian compliments the F/5 well and gives you those sharper pinpoints that you are looking for. The Skywatcher 150PL tubes can often be picked up for around £100. An ED100 or ED120 will cost you between 3x and 5x as much on the used market.

[Joel Shepherd]

Thanks again! Forget to mention budget. I'd probably like to stay at or below $1500-$2000 US. Appreciate all the suggestions so far: definitely gives me some options to dig in to.

Thanks -- Joel.

[mikeDnight]

Astronomics in the US has an excellent write-up regarding light loss etc, its worth reading the whole thing at www.astronomical.com/why-buy-a-refactor-t.asps

Here's an excerpt:

"........refractors also have the highest light transmission - the percentage of light gathered by the scope that actually reaches your eye. Refractors typically transmit 90% or more of the light they collect compared with 77% to80% transmission of reflectors and 64% to 75% of catadioptrics. (The reflector / catadioptrics percentages only concern the reflectivity of standard aluminium mirror coatings. They do not take into account the light blocked by the secondary mirror, which can reach a hefty 15% to 20% additional light loss in some scopes)."

It goes on to state ".....reflectors and catadioptrics can lose up to 1% to 1.5% reflectivity per year."

The 90% transmission rating for refractors can further be improved upon by choosing a ED or better still a true fluorite refractor.

The graph showing light transmission in percentage terms provided by Takahashi shows standard BK7 to give approximately 90 to 92% light throughput, where as a lens made of calcium fluorite CAF2 allows close to 99% light transmission. A higher transmission rate than any ED glass.

Mike :-)

[michael.h.f.wilkinson]

That information seems outdated. Light transmission using modern dielectric coatings give an SCT like the C8 a total transmission including 34% CO by diameter (responsible for 10% light loss) of some 85%. The other 5% is lost in 97-99% reflective mirrors. People keep trotting out information based on simple aluminium mirrors. This is simply misleading.

[laser_jock99]

".....reflectors and catadioptrics can lose up to 1% to 1.5% reflectivity per year."

Mike :-)

So my 1990's is Cape Newise Reflector is only 70% reflecting now..........still looks pretty good to me!

[Dude_with_the_tube]

as you are in USA,i would strongly suggest to find the old good Meade 127ED F9 refractor. its a 5" frac with ED glass and long focal length of F9 will give you practically CA free views and also gathers plenty of light for planetary and bight DSO`s.Its a beautiful instrument,very high quality lens`s and you can pick them up for about 1000 usd what i think is an absolute bargain.

[mikeDnight]

That information seems outdated. Light transmission using modern dielectric coatings give an SCT like the C8 a total transmission including 34% CO by diameter (responsible for 10% light loss) of some 85%. The other 5% is lost in 97-99% reflective mirrors. People keep trotting out information based on simple aluminium mirrors. This is simply misleading.

How can it be outdated or misleading when the data is accurate and many reflectors today still use basic aluminium coatings?

The original post mentions the desire for a telescope that gives the clean, crisp, contrasty images of a refractor while retaining the bright views found in a 6" reflector. A ED of around 5" will do this but no matter what the aperture or coatings of an 8" SCT, it can never give the refractor like star images desired. Neither can it give the superb wide field vistas that only a refractor can deliver. It is physically impossible for it to do either!

The obvious advantage of light grasp that a larger aperture scope such as a SCT offers is something to be considered, but it would be at the cost of image quality in terms of definition, contrast and tack sharp images across the entire field.

A good 5" refractor coupled with modern top end eyepieces can reach 14th mag and split binaries as close as 0.9 seconds of arc. Giving low power tack sharp rich fields of well over 3°, while also capable of extreme high magnification of X100 per inch on some top models, the refractor is the perfect all round instrument. The only disadvantages are weight, for models of 6" and over, and cost.

Mike.

[Xiionn]

That information seems outdated. Light transmission using modern dielectric coatings give an SCT like the C8 a total transmission including 34% CO by diameter (responsible for 10% light loss) of some 85%. The other 5% is lost in 97-99% reflective mirrors. People keep trotting out information based on simple aluminium mirrors. This is simply misleading.

No this information is not outdated, or misleading.

For your example you quoted 85% (Including CO) for a Celestron C8. This is not correct.  Starbright XLT coatings are quoted in their bumf at 83.5% total system element light transmission (When spanking brand new of course). That is just the transmission figures for the three  elements. This does not take into consideration the effect of the CO AT ALL!!. And i quote how they calculate system transmission:

"System transmission is the percentage of light that arrives at the focal plane compared to the light that enters the telescope. It is calculated by taking the product of the corrector lens transmission, the primary mirror reflectance, and the secondary mirror reflectance. Here is an example: if the corrector lens transmits 92% of the light, and the primary and secondary each reflect 89% of the light, then total system transmission is .92*.89*.89 = .73, or 73%."

They are not making any allowance in those figures for the 10% (By your own figures) light loss caused by the CO, simply for the losses involved in the optical elements. For an 8" SCT you are as quoted above talking in the range of 70% of the light that hits that 8 inch aperture makes its way to the eyepeice. And this is a fairly high end reflecting telescope. compared to the typical 93-97% transmission values of a refractor which has no CO to cause further diffraction abberations.

Further, Whilst the high end scopes such as the C8  may have sophisticated coatings, the vast majority of reflecting telescopes will come with standard aluminium coatings. A speciality coated reflector loses one of the reflectors big advantages over a refractor, its price. The C8 is £800.

[mikeDnight]

So my 1990's is Cape Newise Reflector is only 70% reflecting now..........still looks pretty good to me!

I've only ever used two Cape Newise reflectors, both had been loaned to my local astro club and both were optical disasters. Both models were said to be prototype but neither could exceed X100 before the images broke down. The optics on both were sleeked on both primaries and one primary was still pitted from the grinding process. The corrector lenses in both focusers had several scratches. The optical windows appeared uncoated. Both scopes were on loan during the early 2000s. I'm not sure what the light transmission would be on a perfect model but I'd be amazed if the models I used briefly could deliver anywhere near that of a modern SCT.

Mike.

[jetstream]

I would assume that I'm not alone in using a mirror diagonal with my refractors? Possibly most do? If this is the case this must be factored into the transmission figures and of course any distortion or aberration that follows them. I'm curious if there are any refractor telescope transmission figures available, test reports etc.

In my experience refractors dislike cold weather, while my newt copes very well. My diagonals can also act up under these conditions which really messes up the view. Very large refractors need a VG lens cell - which is a challenge- no sense in having a large scope with a fussy cell.

I hope some frac trans test's come along to help the OP in his decision.

[michael.h.f.wilkinson]

I simply do not see how they can arrive at those figures. They are bland statements, not backed by calculations. Let me therefore do the maths.

If I take aluminium coatings with the standard silica overlay that even my old Newt had, the reflectance is 92-95%. Assuming 92% we arrive at 84.64% total system transmission, 95% gives 90.25% transmission. Simple maths (0.92x0.92 and 0.95x0.95). Here I do not use any outlandish mirrors. The figure of 77-80% holds if the reflector has mirrors with less than 90% reflectance (if we do not take CO into account). The figure of 64% for catadioptrics suggests either mirrors of about 80% reflectance (and why should they be worse than the figure for reflectors?) or if they are given the same (conservative) 90% reflectance of reflectors, a 10% loss at each of the glass/air interfaces of the corrector plate. This is outlandishly high, as even uncoated glass has only 5% reflectance. In Maks, you might say the thick mensicus lens causes significant loss, but even then a 20% loss for a single glass element is far too high, especially given the claim of 90% or better transmission of a multiple element refractor. A single element should be closer to 5% loss.

Thus, simple maths suggests the information is outdated even if I work with standard mirrors already available in the late 70s. Mirrors do age, but a 1-1.5% loss per annum is a worst-case figure, I would suggest. The protective silica coating helps prefent aging, more modern dielectrics do a better job. I am not at all sure my 19 year old C8 has lost 20% of its transmission (possible, haven't measured it), given the faint objects I have been able to see with it recently.

I also don't see why I cannot use higher end mirrors in my comparison, when the refractor analysis uses ED glass and fluorite in theirs. With a 97% Hi-lux coating I can get 94.1% transmission. Replace the secondary with a 99% transmission type, and I get 96% transmission. Well in the same ball-park as refractors.

Unless I see a mathematical proof that the above reasoning is wrong, I will call those 64-70% figures for catadioptrics and 77-80% for reflectors wrong.

[Tim]

Interesting question.

I've had a 6" newt, and I now use a 6" 'frac. On clusters and indeed anything star wise, the 'frac is beautiful, snappy focus, bright clear images.

The closest results to the refractor I have seen were through a MN190 Mak-Newt, but the stars are still sharper and smaller through the glass, rather than the mirror.

HTH

Tim

[michael.h.f.wilkinson]

Just done some more calculations. With respect to a 5mm maximum dilated pupil, a 5" ED gives 7 magnitudes gain (assuming no light loss in the optical path, at 90% transmission we have just 0.1 mag loss). To reach mag 14, you need mag 7 skies. If your pupils dilate to 7mm we arrive at 6.3 magnitudes gain, requiring NELM 7.7 skies. I personally have pushed my C8 to mag 13.9 under mag NELM 5.5 skies (quasar OJ-287), but never managed that in a smaller scope.

[Pig]

LoL.... It sounds to me like everybody is happy with what they have.... thus the answer is quite simple ..... all refractors do a pretty sound job, even my little peashooters are better than my 6" SCT apart from the magnification achievable on the Lunar surface.(which isn't a requirement for you)  I'm not sure of the exchange rate but it sounds like you have the funds to support a pretty good set up.

There is a rather splendid Astro Tech 106 EDT for sale in the classifieds that will be pretty hard to better  !!!

[michael.h.f.wilkinson]

I have both a fraction and SCT. The fraction gives beautiful tight stars and wide fields (never said otherwise). The light grasp of the SCT is much greater. Two scopes are always better than one.

[Pig]

Very true Michael.... One is good two are better  three are simply great

LoL @ Fraction

[ollypenrice]

I'm with Tim.  

It ain't what you see, it's the way that you see it so far as I'm concerned in any refractor-reflector comparison. More aperture will always take you deeper but really crisp refractor optics can, I find, take you 'nicer.' However, they are never going to be cheap. Over the Atalantic I dare say there would some of the older premium doublets available on the used market. Tak annd TeleVue, for instance. An older Genesis would be within price range and, with its short focal length, can deliver remarkable views of very wide fields. You can frame Stock 2 and the Double Cluster together, for instance, or the Pleiades with an expanse of dark sky. Yet the Genesis can also handle 200x on the planets and probably more.

Olly

[Xiionn]

I simply do not see how they can arrive at those figures. They are bland statements, not backed by calculations. Let me therefore do the maths.

Unless I see a mathematical proof that the above reasoning is wrong, I will call those 64-70% figures for catadioptrics and 77-80% for reflectors wrong.

The figures were backed up in my previous posts by celestrons very own test results! Are you saying that Celestron is incorrect? You can find this information here: http://www.celestron.com/university/astronomy/starbright-xlt-optical-coating-system

The original poster quoted around 64-70% for an SCT. If you re-read the post, it does specifically mention STANDARD aluminium coatings. Not Starbight or Dielectric coatings.

However what i did was look up Celestrons own analysis, which arrived at 83.5% for just the optics, and then you factor in 10% on top of that for the central observation blockage, which gives you a total of 73.5%. I should think that celestrons own data with the added allowance for the CO would be enough proof?

The maths: Given a value of 97.4% transmission for the C8 Corrector lens. A mirror coated with StarBright XLT reflects 95% (Taken from Celestrons Data). Therefore to calculate the peak total system transmission value for just the Corrector element (97.4), primary Mirror (95) and Secondary Mirror (95) we get a peak value of.

.95%*.95*.974 =.879 or 87.9% (They Quote a peak value of 89%) That is the mathematical upper limit. In reality nothing ever reaches its mathematical peak value, not ever. However we take the graph below, taken from celestrons own starbright data.

The overall system transmission is 83.5% because you have to average over the entire spectrum from 400 to 750 nm. Which to be fair is exactly what celestron have done in thier own data. So the figure of 83.5% transmission for JUST the starbright coated elements is correct. Now we have to allow for the central obstruction, which is not factored in to the above data. If we are going to be correct in the allowance for this we should take the CO as 10% by area loss. If 100% hits the Aperture, and 90% therefore hits the corrector lens and makes its way into the scope to be transmitted, with an 83.5% transmission value. At the eyepeice we will therefore see:

.90x.83.5 = 75.15% of the light that hits the 8 inch aperture gets to the eyepeice.

I'm not saying the C8 is a bad scope, its obviously not, its a top notch scope but the facts about light transmission don't lie. You are dealing with corrector lens and mirror losses resulting in 16.5% light loss before you even take into consideration the 10% total blockage due to the central obstruction and spider. You just cant expect any SCT no matter how good to collect anywhere near the 8 inches of light that hits the aperture, the physics just don't allow for it.

[Pig]

Sooooo just how much will my gas bill be this quarter

[ollypenrice]

I don't know about light but plenty of heat seems to be generated by any mention of reflectors and refractors in the same sentence...

Olly

[Stu]

Gentlemen, I applaud the obvious passion for the various different scope types but honestly do think we are getting further away from answering the OPs question.

The original request was for tighter star images on open clusters whilst also being able observe PN and planets without too big a compromise on contrast and brightness.

The OP's Newt is a fairly fast one at f5 and 750mm f/l. This will give a 2.1 ish fov with a 24mm 68 degree afov eyepiece.

One of my suggestions was a 120ED. One 'issue' with this lovely scope is the 900mm focal length will obviously give a smaller fov which won't help for the very large OC's.

I suppose my view is that it is best to get a frac which complements the newt as much as possible. With a decent ED apo, contrast should not be an issue in comparison with the newt, and say a high quality 4" scope at f5 or f6 will give wider fields of view with the tight stars the OP is seeking. Planetary views can be lovely too given the contrast but I suspect PNs may still be brighter through the Newt.

Everyone is different, and this game is about enjoyment in my book. I happen to love refractor views for their purity, and my simple little heart sings when I look through one. I've said in another thread that the best planetary views I've had have been with a 16" Newt and an 8" Mak, but if I was stuck with a 4" apo I would still be a very happy man.

[Xiionn]

I love both, and SCT's. Don't get me wrong, this isn't heated at all. In fact i find the maths fascinating, the data and testing enjoyable. There are some great articles on light transmission, comparisons with newts and fracs, diffraction...

However, looking through the eyepeice is what really counts, not what scope you have, or the aperture, right!!

Not to mention all the figures don't take into account eyepeices (How many elements are there in an EP), glasses (If you wear them) seeing conditions, dust on elements, collimation, scratches due to cleaning, light polution. There are so many factors its not even funny.

[Xiionn]

The Esprit ED is marginally faster at 840mm, but a big step up in price.