Refractor vs Reflector

[spaceboy]

Could a 4" f/9 ED refractor compete with a 10" f/5 Newtonian under any circumstances?

Neither one is better than the other and it's only down to personal preference at the end of the day that a choice can be made. Either choice would amount to some sort of compromise having to be made so in all honesty if astronomy is your thing it's better to try and get both. You will see from most members signatures that this is often the route taken anyway. If budget simply will not allow this the best way to think of it rather than Refractor 'vs' Reflector is DSO 'vs' Planets. If you want to get the best from faint fuzzy galaxies then a 10" dob will give you this. If you want to get the best from Planet detail and brighter DSO's then a 4" ED refractor will be the one to go for. Others mentioned the portability of a scope but if you want to see those faint galaxies and nebula this is one of those compromises that has to be made as smaller scopes rarely offer you the same wow factor.

SPACEBOY

[rfdesigner]

Newts get a bad name for planetary viewing.. but this need not be so

http://www.pha.jhu.edu/~atolea/WAS/thermal_management_newtonians.pdf

It would seem that a thermally controlled newt (or one permanently housed in an observatory so it doesn't need to cool down) ought to perform extremely well.

Derek

[toml42]

Hmm, i would definitely disagree about newts on the planetary front. The image through a smaller scope might be more aesthetically pleasing, as the image may be a more comfortable brightness and cleaner looking due to lack of central obstruction / less impact from seeing.

But i don't see how a 4" scope could ever give the same amount of detail as a 10", given good conditions.

[Moonshane]

I'm itching to have a shoot out between my 6" f11 newt and a decent quality (i.e. mortals can afford) ED refractor - maybe a 100mm. I am hoping to do this at the Peak Star Party later this year but if anyone near Manchester fancies a meet up sometime then please do get in touch.

[26Left]

That's fine if you have Hilux coatings, but 10" reflectors churned out by China and sold for about £350 don't. 85% reflectivity is more realistic. With a 2% loss due to the central ostruction (mine is 50mm diameter), you're looking at 0.85*0.85*0.98, ie. 71%. My own light measurements with my 10" confirm this calculation.

And that's not even considering the very significant difference in contrast between the two designs.

toml42: My 10" gives me better planetary views than my 4" refractor. But, my 5" refractor beats them both by a long way.

[Damo636]

And the debate rages on

[Ad Astra]

There's just one way to settle this - Bring your big Newt over to my place and we'll set it up in the back garden next to my big 'frac and compare views.

The loser buys the Guinness, the winner buys the crisps.

Game on!

Dan

[Moonshane]

if only........I'd gladly buy both!

[michael.h.f.wilkinson]

That's fine if you have Hilux coatings, but 10" reflectors churned out by China and sold for about £350 don't. 85% reflectivity is more realistic. With a 2% loss due to the central ostruction (mine is 50mm diameter), you're looking at 0.85*0.85*0.98, ie. 71%. My own light measurements with my 10" confirm this calculation.

And that's not even considering the very significant difference in contrast between the two designs.

toml42: My 10" gives me better planetary views than my 4" refractor. But, my 5" refractor beats them both by a long way.

So we are comparing a top notch APO frac with a cheap reflector? I thought we were assuming we had similar quality instruments.

Plain aluminium coatings reflect 95-96% when new, between 90-93 when older (as a rule of thumb). Silver starts at 97-98% and drops down (quite rapidly) to 85% and lower. Anyway, even assuming 85% reflectance and perfect transmission in the frac, in the case of the 6" vs the 10": 0.71*(10/6)^2 = 1.97222 so it gathers twice the light. For the 4" vs the 10" the difference becomes 4.4375.

With better mirrors and smaller CO (than an SCT), and assuming 95% transmission of the frac (which is on the high side) we have factors of 2.51 (one magnitude!) and 5.76 (1.9 magnitudes) respectively.

Lightbucket-wise, there is no contest.

[26Left]

So we are comparing a top notch APO frac with a cheap reflector? I thought we were assuming we had similar quality instruments.

No. The person who created this thread asked for a comparison of an F/9 4" ED frac and a 10" F/5 reflector. He could well be looking at the SW ED100 and the SW 10". They're similarly priced; similar build quality etc.

I own both and I replied that "My 10" SW reflector beats my 4" ED100 refractor on pretty much all targets. But, that doesn't means the views are more pleasant. Open Clusters are far more enjoyable using the 4"refractor, because of the pin-point stars."

I went on to mention my 5" to make the point that aperture is not everything. Once light has been "grasped", it needs to be transferred to your retina. Different designs do that in different ways with differing results.

Different scopes suite different situations and targets. If they make our hobby more enjoyable, then they are a worthwhile purchase

[LDUNN1]

......hands up, it was probably me that muddied the water - I brought up the 6" Apo's I think in discussion for imaging.......& that would not be a fair contest vs a more 'budget' reflector.....fair comment.

[Adz]

There's just one way to settle this - Bring your big Newt over to my place

Dan I will be over in 5, just got to book some flights. Put the kettle on!

Seriously though, I am just considering both. What is the worst that can happen? I come to a blindingly obvious conclusion and sell one. Or I realise that they both serve a unique purpose and keep em. It is win win.

[26Left]

Exactly right. And, it's what a lot of people do.

I was looking at Saturn a couple of nights ago and reflecting on what Galileo might have throught of the view. When he saw it 400 years ago through his telescope, he was confused by the slight elongations to the planet that any modern telescope would resolve as the familiar rings.

Really, every telescope is amazing

[spaceboy]

I was looking at Saturn a couple of nights ago and reflecting on what Galileo might have throught of the view. When he saw it 400 years ago through his telescope, he was confused by the slight elongations to the planet that any modern telescope would resolve as the familiar rings.

Really, every telescope is amazing

Just think Galileo would be turning in his grave listening to us lot rambling on about what's better, "frak or flek" when he would have probably chopped off an arm and a leg to have had anything near the quality we now take for granted.

[26Left]

Exactly.

And, when I said that I was "reflecting on what Galileo might have thought", I could just as easily have been refracting on it.

[coopman827]

More aperture always wins due to increased light-gathering and resolution capabilities, if the quality of the optics is comparable. However, for me it also boils down to my personal situation to a large degree. I can take out my SW120ED and be observing in 5 minutes. If I take out my 8" Dob, I might need to give it 15-30 minutes to cool down, so that's 15-30 minutes of observing time that I've lost by choosing the larger scope. My observing sessions might only last 90 minutes on average, so I want to spend every minute at the eyepiece. For my particular situation, this means that the smaller scope is the best choice. I also prefer the refractor due to the fact that I don't have to worry about collimation issues.