what size refractor comparable to 8" SCT?

[Stu]

Some good points and I would not overly dispute many of them, but many SCTs owners criticise their scopes without ever really understanding their requirements in terms of cool down and collimation. Personally I've used most kinds of scope - but always happy to get back to my couple of Cats. Given a choice between a five inch refractor and say a C8 Edge I'd take the latter every single time as an excellent all rounder. Visually when it does come together the planets can be stunning.

Dixie, I think we share similar views but coming from opposite directions

I've been pleasantly surprised by my Edge 8" and recognise that once the issues are understood and dealt with they can give excellent views. It often comes down to circumstances ie if rushed or with a small viewing window, the frac is just ready to go.

[Dixie]

Yes I agree with that. Unanimity!

[Jon_Doh]

I have a Skywatcher 120 Pro APO and although it is a doublet, it is not an achromat as someone commented.  It has high contrast FPL-53 ED glass matched with BK7 Schott.  When I purchased it I also owned an 8" SCT.  One evening i took both scopes out for comparison.  In my analysis, the 120 was equivilent to the 8 SCT.  Some things the 120 did better.  For example, I could see more of the Great Andromeda in the refractor.  The Dumbbell was as clear in the refractor without a filter as it was in the SCT using a filter.  Same with the Great Orion Nebula.  Saturn looked a little better in the SCT and globular clusters much better.  One caveat.  I was viewing from my home with fairly light polluted skies.  The results of my test might be entirely different at a dark site.  But I think in general from what I have experienced an APO refractor in the 120-130mm range will be roughly equivalent to an 8" SCT.

[mikeDnight]

Much would depend on the kind, and quality of the refractor that you're comparing to the SCT. Refractors can be as different as chalk and cheese, even among the high end fracs, so really you'd need to state a particular refractor type, or even brand, as they are not all equal. The SCT has the advantage of greater light grasp, so DSO's will be brighter than a much smaller aperture frac. An SCT also has greater theoretical resolution but delivers softer images than a refractor, so although there may be more detail in the image of The SCT, it may not be as readily seen, especially if affected by thermal instability. The beauty of a quality refractor lies in its ability to deliver sharp, well defined and contrasty views of everything within its light grasp, within a few minutes of setting up. From wide, rich field , low power vistas of the milkyway like nothing else can, to amazingly detailed, laser etched high power views of the Moon and planets, they are a joy to use. For observing lunar and planetary, id prefer a 4" refractor to an 8" SCT, but that's because I like the image produced by the refractor. I just see more in a quality refractor!

Mike

[Peter Drew]

Depends a lot on your eyesight as well. These days I need a 8" to see what others can manage with a 4".  

[acey]

A scope with aperture A and central obstruction diameter D has effective clear aperture sqrt(A^2-D^2). I don't know D for an 8" SCT but that's how you'd work out the aperture of a refractor with the same light grasp. Though of course there's far more to a scope's performance than light-grasp, as the comments in this thread illustrate.

For example, if an 8" SCT had central obstruction 3" then the effective clear aperture would be sqrt(64-9) = 7.4 inches. A 7.4 inch refractor with the same optical quality as an 8" SCT would be expected to have the same limiting magnitude for stars, but slightly better contrast (e.g. on planets) because of the diffraction (light scattering) introduced by the central obstruction.

No scope is perfect, and every scope deviates from perfection in its own way, so the question has no precise answer. But if the main aim is to view DSOs then light grasp is the most important factor, while contrast and resolution are important for planets, in which case a refractor of higher optical quality and significantly smaller aperture may well out-perform the SCT.

[ronin]

Easy answer: Another 8" in whatever design you decide on.

A newtonian may have a secondary but a refractor has either 4 air/glass or glass/air interface to lose light from, an apo has 6 of them. In an apo with 6 interfaces if you lose 2% at each face then by the time the light gets through the objective you are down to 88%, just about the same as the sceondary blocks.

Usually on plain glass it is a 4% loss but AR coatings will reduce this to 2% and usually not less then 2%, just gets too costly.

[Peter Drew]

It also depends on where you are observing. My C8 in Tenerife gives at least as good planetary images as my 8.5" refractor does in the UK. 

[jetstream]

On 2011-04-17 at 00:39, dweller25 said:

For the planets a good 4" refractor would challenge an 8" SCT and a good 5" refractor would beat it.

Except for the Intes 715? which offers Takahashi 128mm like views

I realize the design differs from the C8 but the Intes performance illustrates how capable an obstructed scope can be. Actually if you are going to consider selling that Intes please keep me in mind David!

[Chris]

On 4/1/2017 at 07:45, ronin said:

Easy answer: Another 8" in whatever design you decide on.

A newtonian may have a secondary but a refractor has either 4 air/glass or glass/air interface to lose light from, an apo has 6 of them. In an apo with 6 interfaces if you lose 2% at each face then by the time the light gets through the objective you are down to 88%, just about the same as the sceondary blocks.

Usually on plain glass it is a 4% loss but AR coatings will reduce this to 2% and usually not less then 2%, just gets too costly.

I might have missed something but what about the Newt losing about 8% light from each reflected surface ontop of the central obstruction? Not dissing Newts as I have one and I'm grinding the optics for one, just saying  

[John]

Overall system strehl ?

How does that compare between newts, cats and fracs ?

(I'm another who has a foot happily in both camps by the way)

[Peter Drew]

1 hour ago, John said:

Overall system strehl ?

How does that compare between newts, cats and fracs ?

(I'm another who has a foot happily in both camps by the way)

I think the optical surface quality of a reflecting surface has to be twice  that of a refracting surface to provide the same overall wavefront error.  

[dweller25]

3 hours ago, jetstream said:

Except for the Intes 715? which offers Takahashi 128mm like views

I realize the design differs from the C8 but the Intes performance illustrates how capable an obstructed scope can be. Actually if you are going to consider selling that Intes please keep me in mind David!

Hi Gerry, I don't think I had the IM715 when i posted in this thread in 2011, however your right the IM715 and FS128 give very similar planetary views - the IM being easier to set up but slower to cool down. I will be taking the IM on vacation to France in the summer. I don't think I will ever sell it - sorry !

[jetstream]

Can't say as I blame you! Excellent choice in scopes David

[newbie alert]

On 17/04/2011 at 06:39, dweller25 said:

For deepsky you would need at least an 6" refractor to match an 8" SCT.

For the planets a good 4" refractor would challenge an 8" SCT and a good 5" refractor would beat it.

An 8" Newtonian would beat an 8" SCT as it has better cooling properties and a smaller secondary obstruction.

If you want a good all rounder then the Newtonian design cannot be beaten - cheap too.

Not so sure I'd agree.. I own a 8 inch sct  and a 80mm frac...yes the image is sharp on the frac but I'd rather look at planets with my sct 

Also looked thou a 6inch newt and again rather disappointed,rather look throu my sct.. .

Horses for courses...were not all the same.

[Chris]

18 minutes ago, newbie alert said:

Not so sure I'd agree.. I own a 8 inch sct  and a 80mm frac...yes the image is sharp on the frac but I'd rather look at planets with my sct 

Also looked thou a 6inch newt and again rather disappointed,rather look throu my sct.. .

Horses for courses...were not all the same.

Just out of curiosity which 80mm frac do you have? Big difference between say an ST80 and an ED80 for example.

[newbie alert]

 Its the 80ed..600mm fl native but I use a .80 reducer in it

[John]

Not really a fair fight though, is it ?. 80mm vs 204mm ?

I spent several months a couple of years back comparing the performance of an ED120 refractor with an excellent 6" F/5.8 Intes maksutov-newtonian and found that their planetary / lunar double star performance was very much the same (excellent in both cases !) and the larger aperture scope showing deep sky objects slightly better (no surprises there).

The central obstruction of the SCT is proportionately quite a bit larger than the mak-newtonian but if well collimated and cooled (crucial for an SCT to perform) I would think that a good 8" SCT would be able to match a good 5" refractor on the moon / planets / double stars and would show the deep sky better.

Refractors tend to be able to cut through mediocre seeing conditions better than other, larger aperture, designs I think so appear to "punch above their weight" more often.

Our personal experiences are all going to have varied on this though so perhaps concensus is going to be elusive here ?

 

 

 

[Rob]

I did this test some years ago. C8 is fantastic piece of kit IF you have a good one. I've owned two, real ying & yang. That said ED100 frc really gave it a run for its money I have to say. Not much in it!.