Refractor "equivalent" for a 6" reflector

[Joel Shepherd]

Between my own druthers and the constraints imposed by our light-polluted skies, I've found I get the most thrill out of viewing open clusters, planetary nebula and planets, in about that order. I have a 6" reflector but am considering a refractor for cleaner, crisper views of open clusters in particular.

That said, I don't want give up much in terms of brightness and contrast (e.g. by going to a much smaller aperture), and don't want to spend a small fortune on a large refractor.

So, especially for folk with a lot of experience with both, how large of a frac would I want to get similar or better views (esp. with regard to brightness and contrast) as I enjoy with a 6" Newtonian?

Thanks -- Joel.

[ronin]

Simply a 6" refractor.

To collect 6" of light you need a 6" diameter objective.

[orley]

A 6" refractor can be a weighty beast. What mount do you sit your newtonian on?

[Zakalwe]

A 6" refractor has 6" of clear aperture. A 6" reflector (either Newtonian or SCT) will have a large central obstruction, so it's normally said that a much smaller 'frac will give similar light-grasp. you have to consider focal length and speed too. These will probably have more bearing than the aperture.

[Moonshane]

If you had very dark skies a small frac would be ok but in any situating a 6" newt is hard to beat on the targets mentioned without spending a lot more money.

[laser_jock99]

6" reflector = 4" refractor, roughly speaking.

The refractor will give better contrast due to the lack of central obstruction. The optics will not need collimation and it will be smaller (but not neccessarily lighter) than the reflector.

The reflector has bigger apperture to gather more light and also have higher resolving potential. It will likely be much cheaper than any ED120 scope- although you should add in the cost of a coma corrector and possibly collimation tools if considering imaging.

Reflectors are also available in very fast focal ratios (e.g. F4 and faster) another factor to consider if imaging faint objects or viewing faint objects is your thing (from a light polluted site or for planetary/lunar work a 'fast' scope is not so useful).

The other advantage, often overlooked, is that an all mirror reflector is a true APOCHROMATIC system - no refractor is.

So- lots of factors and compromises to consider. If I had the choice of only one reasonably priced 'all rounder' scope, it would probably be a 6" or 8" Newtonian. Large, high quality refractors are very nice, but they come with a large price tag match.

[Stu]

I guess I have a few questions first.....

What is the spec of your 6".... Focal ratio, length etc?

Do you intend to take the frac to a dark site?

How important are the tighter star images to you?

Are you bothered by CA?

What's your idea of budget?

Let me know and I'll be able to comment better

Stu

[michael.h.f.wilkinson]

6" reflector = 4" refractor, roughly speaking.

The refractor will give better contrast due to the lack of central obstruction. The optics will not need collimation and it will be smaller (but not neccessarily lighter) than the reflector.

The reflector has bigger apperture to gather more light and also have higher resolving potential. It will likely be much cheaper than any ED120 scope- altough you should add in the cost of a coma corrector and possibly collimation tools if considering imaging.

The other advantage, often overlooked, is that an all mirror reflector is a true APOCHROMATIC system - no refractor is.

Sorry, but that is a myth. The 6" F/8 Newtonian with only 23% obstruction by diameter or about 5% light loss in terms of area (turning a 152mm instrument into a 148mm instrument in terms of light gathering aperture). I have looked at planets, open clusters and planetary nebulae with a 4" refractor and my 6" Newtonian, and the 6" hit the 4" for six every time.

[Chris]

There are a few factors which add up to a smallers refractor equaling a slightly larger reflector.

- central obstruction 

- mirrors don't reflect quite as much light as lenses refract (but modern coatings are making this a close thing)

- the light is folded over twice in e reflector as it bounces off the primary then the secondary so there is more light scatter. In a frac the light is travelling in one direction even when using a diagonal.

You often hear that a 6" newt = a 4" frac, but I think you also need to factor in the chromatic aberration (CA) of an achromat degrading the image compared to mirrors which have no CA. 

Maybe a 4" ED frac would = a 6" reflector ?

The thing is, a reflector is much cheaper per unit aperture than ED glass.

[mikeDnight]

If you're leaning towards a refractor for observing clusters and nebulae mainly, I'd suggest you don't drop below 120mm. A 120 SW 120 ED is a good all round performer and will deliver superb lunar and planetary views when the mood takes you. A 120 is also easy to mount and a joy to use.

You might also consider a 6" F5 SWST or a 6" F5.9 Astara if planetary study is not your thing. Both are outstanding RFTs and the CA is no problem on DSOs. The Astara needs a strong mount!

You could of course spend a lot of money on a top end apo but for visual DSO observing I seriously doubt you'll get your money's worth.

If you were looking for a top class planetary performer then the new FC 100 D from Tak is the way to go. It will also deliver great views of clusters and brighter nebulae, and makes a nice lightweight rft.

Mike :-)

[michael.h.f.wilkinson]

There are a few factors which add up to a smallers refractor equaling a slightly larger reflector.

- central obstruction 

- mirrors don't reflect quite as much light as lenses refract (but modern coatings are making this a close thing)

- the light is folded over twice in e reflector as it bounces off the primary then the secondary so there is more light scatter. In a frac the light is travelling in one direction even when using a diagonal.

You often hear that a 6" newt = a 4" frac, but I think you also need to factor in the chromatic aberration (CA) of an achromat degrading the image compared to mirrors which have no CA. 

Maybe a 4" ED frac would = a 6" reflector ?

The thing is, a reflector is much cheaper per unit aperture than ED glass.

Light refracts off far more interfaces in a refractor. For your typical air-spaced doublet 4 surfaces, for a triplet 6. There is no reason why a refractor should have less light scatter on those grounds. Light can travel in two directions without problems (photons are fermions, after all). Transmission losses of 5 or 6 % can occur by absorption in glass as well, especially at large apertures (when glass paths are long). Newtonians with small CO are sometimes referred to as APO-killers. Having looked a TEC 140 and the high grade 6" F/8, I would say in terms of sharpness the TEC has an edge, especially on M13, but the 6" Newt would give a 120-127mm  ED a serious run for its money. In terms of light grasp, differences between a TEC-140 and 6" F/8 Newt are too close to call. You would really need to do a side-by-side test on borderline objects.

[DaveS]

Being oil-spaced the TEC apo, like the CFF ones, only has two air-glass surfaces.

[michael.h.f.wilkinson]

Being oil-spaced the TEC apo, like the CFF ones, only has two air-glass surfaces.

Indeed, excellent piece of kit, very tight stars. This is why it showed more stars in M13 than the 6" Newtonian could. The TEC-140 OTA was about 10 times the cost of the Newtonian, of course.

[ngwillym]

Being oil-spaced the TEC apo, like the CFF ones, only has two air-glass surfaces.

Doesn't matter - unless the oil has the same refractive index as the glass. Scatter and light loss happens at every interface be it air/glass, oil/glass, glue/glass and even glass/glass

[michael.h.f.wilkinson]

Doesn't matter - unless the oil has the same refractive index as the glass. Scatter and light loss happens at every interface be it air/glass, oil/glass, glue/glass and even glass/glass

That is true, but the degree of scatter is smaller as the difference in refractive index is smaller.

[Chris]

Light refracts off far more interfaces in a refractor. For your typical air-spaced doublet 4 surfaces, for a triplet 6. There is no reason why a refractor should have less light scatter on those grounds. Light can travel in two directions without problems (photons are fermions, after all). Transmission losses of 5 or 6 % can occur by absorption in glass as well, especially at large apertures (when glass paths are long). Newtonians with small CO are sometimes referred to as APO-killers. Having looked a TEC 140 and the high grade 6" F/8, I would say in terms of sharpness the TEC has an edge, especially on M13, but the 6" Newt would give a 120-127mm  ED a serious run for its money. In terms of light grasp, differences between a TEC-140 and 6" F/8 Newt are too close to call. You would really need to do a side-by-side test on borderline objects.

I hadn't considered this I must admit. In that case it's remarkable how crisp and bright the views are through my ES100 degree eyepieces with their 8 or so elements.

So that's a lot of light loss if you have say a triplet refractor with a 100 degree eyepiece! 

[Stu]

It seems modern coatings certainly allow multiple element eyepieces and objectives to maintain excellent transmission with minimal scatter.

If I may be so bold.....

....this thread is about options for a refractor to give similar views to a 6" reflector "without spending a fortune". I don't think TEC140s really come into that topic.

Personally I would suggest a 4" ED able to give nice widefield views to complement the newt, but it would help to know answers to the questions I posted earlier

[Chris]

Yes, I think we got slghtly into the old frac vs Newt debate rather than specifically answering the OP's question, sorry about that

Yeah, something like an  ED120 would be great if budget allows?

[Moonshane]

if you are used to the CA free views of a newtonian then I'd agree that an ED frac makes sense. I really liked my 120mm f5 skywatcher on the sun and clusters etc but found that the CA bugged me on the moon and planets. My 120ED is a completely different ball game.

[laser_jock99]

Doesn't matter - unless the oil has the same refractive index as the glass. Scatter and light loss happens at every interface be it air/glass, oil/glass, glue/glass and even glass/glass

Beat me to it- all interfaces will have some losses.

Newtonian scopes only have two, both of which reflect all wavelengths (UV thru IR) at equally the same angle. It all goes wrong of course when you a coma corrector in the image train- hence this needs to be a high quality optic. 

[jetstream]

My SW120ED compliments the VX10 very well, both are top notch scopes. The 10" shows a lot more of objects like M13 for example and is also great on open clusters. Off course here but I would think a bigger dob would suit your needs well. My 10" split Izar @ 96x last night with very tight stars.

Actually the 10" shows more of everything....

ps I do love my refractors though- a lot

[rowan46]

I have an 80 triplet and a 127 sct and I find lp makes a difference. Under dark skies the refractor does very well, almost as good as the sct. But back in the city that extra apparture makes the difference

[michael.h.f.wilkinson]

BTW, the concerns about few percent light loss through scatter and central obstruction pale in comparison to the gains to be had by:

1.) Moving to a dark site

2.) Experience as an observer.

More time behind the scope means you learn to pick out more detail. Various sites suggest that a really experienced observer could gain as much as a full magnitude in brightness compared to complete novices. Likewise, a site with NELM 6, or better skies gains one or two magnitudes compared to suburban NELM 4 or 5 skies. Compared to that, the .05 mag lost by a 23% CO is totally insignificant.

[Joel Shepherd]

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.

[Xiionn]

120mm Apo Refractor will give decent light gathering performance compared with a 150mm Newt and blow it completely away in terms of IQ. Apo is better, but costs more. Achros have CA, but depends on how much that bugs you. The Skywatcher range includes a £950 ED Apo 120.