69 replies to this topic

[starcruiser]

How does a modern C5 compare with these  4" refractor variants?: C5 vs Achro, C5 vs ED, C5 vs APO.

After subtracting light loses, the C5 XLT has a throughput equal to a 4" with perfect light transmission. So on non-widefield DSO views how does it compare on objects like M13, M42? Same brightness? Same contrast? And how much of that secondary interferes with planetary contrast vs the clear 4" refractor? I'm just curious... The C5 looks like a very compact alternative to a 4" frac if widefield isn't a priority.

[Hesiod]

I guess it ends on how good the C5 is. Due to its very large CO it needs to be really well corrected for SA in order to match the sharpness of a good APO.
I do not have a C5 but the IM500 has broadly the same specs (5" f/10 CAT, f/2 main, f/5 secondary) and it outmatch easily my TAL100 (a decent f/10 achro) and comes close enough to the FC100DL to make ergonomics or other "practical aspects" the deciding factor on what telescope to use.

[Sebastian_Sajaroff]

According to my experience, a good 4" refractor and a good 5" SCT/Mak give similar performances.

Focal ratio will probably influence your choice, an F/5 refractor offer wide field but needs more expensive eyepieces than an F/10 SCT.

Don't forget the other variables :

 

1. Weight and bulk (which one is easier to transport for you)

2. Ergonomics (which one is easier and more comfortable for you to use)

3. Mount

4. Budget

[NinePlanets]

For ME, If optical properties were the only consideration, I'd take the 4" refractor every time. But the reality is that I usually use a small telescope only in fast-setup situations.

 

i.e.:  I have big reflectors and refractors, but when I want a very fast setup and quick peeks, I literally lift the forked C5 and tripod by its wedge and step out the back door of the garage and plug in the extension cord for the drive. Setup takes what, 45 seconds? Plus another 30 seconds to return for the chair I sit on. The C5 has enough optical ooomph to satisfy. Barely.

 

Setting up one of my refractors takes several minutes, trips back and forth, and a fist full of bolts to assemble.

[havasman]

For ME, If optical properties were the only consideration, I'd take the 4" refractor every time. 

[elwaine]

How does a modern C5 compare with these  4" refractor variants?: C5 vs Achro, C5 vs ED, C5 vs APO.

After subtracting light loses, the C5 XLT has a throughput equal to a 4" with perfect light transmission. So on non-widefield DSO views how does it compare on objects like M13, M42? Same brightness? Same contrast? And how much of that secondary interferes with planetary contrast vs the clear 4" refractor? I'm just curious... The C5 looks like a very compact alternative to a 4" frac if widefield isn't a priority.

In my experience…

On M13: since resolution depends on aperture, the C5, if well collimated, will show more individual stars

On M42: brightness also depends on aperture, so more of the nebula will be visible in the C5

On the Moon: just about equal with maybe a very slight edge to the C5… maybe… maybe not

On Jupiter, Saturn and Mars: a 4” refractor will show the subtle low contrast regions better than the C5 will. 

 

Keep in mind that while the differences between the two scopes are visually noticeable, we are not talking about big differences, so if I were to chose between the two, factors other than optical performance would out weight the small differences in optical performance. E.g., cost, size of mount and tripod best for each scope, cool down, occasional need to tweak collimation (C5 only), possible future use for astrophotography, etc.

 

Bottom line: you can be happy with either choice. It’s more a question of where your head is at than which scope can outperform the other under different conditions. Just my $.02

[vtornado]

Unless I was maximizing the portability aspect, I would take a 4 inch frac (ED) over a 5 inch SCT.  

 

• Wider field not only for targets, but finding and framing.
• Thermally easier to manage.
• Probably more rugged
• Probably more dew resistant
• 2 inch eyepieces with no vignetting
• Due do the more efficient nature of the refractor not much less light reaches the eyepiece.
• Is the optical quality of an AstroTech AT ED statistically better than a 5 inch Cat?

For the CAT fan-boys,  5 inch frac vs 6 inch Cat the economics/ergonomics shift a lot.

Edited by vtornado, 10 May 2024 - 09:13 AM.

[12BH7]

Unless I was maximizing the portability aspect, I would take a 4 inch frac (ED) over a 5 inch SCT.  

 

• Wider field not only for targets, but finding and framing.
• Thermally easier to manage.
• Probably more rugged
• Probably more dew resistant
• 2 inch eyepieces with no vignetting
• Due do the more efficient nature of the refractor not much less light reaches the eyepiece.
• Is the optical quality of an AstroTech AT ED statistically better than a 5 inch Cat?

For the CAT fan-boys,  5 inch frac vs 6 inch Cat the economics/ergonomics shift a lot.

I've looked into that jump from a 4" to a 5" refractor.  That's the point where you go from a reasonable setup to a serious big setup. Sort of like the 8" to 9.25" SCT cutoff point. 

Edited by 12BH7, 10 May 2024 - 01:39 PM.

[Oldfracguy]

How does a modern C5 compare with these  4" refractor variants?: C5 vs Achro, C5 vs ED, C5 vs APO.

After subtracting light loses, the C5 XLT has a throughput equal to a 4" with perfect light transmission. So on non-widefield DSO views how does it compare on objects like M13, M42? Same brightness? Same contrast? And how much of that secondary interferes with planetary contrast vs the clear 4" refractor? I'm just curious... The C5 looks like a very compact alternative to a 4" frac if widefield isn't a priority.

The answer depends on what type of 4" refractor you are considering here.  You are quite correct that these SCTs are excellent alternatives to refractors when wide-field observation is not a priority.  I have had my share of 4" to 8" SCT-type scopes (Maks and CCs as well), as well as a number of 4" refractors, both basic achromats, as well as ED and SD (APO) types.  

 

First of all, a C5 would be ideal for a small, lightweight mount that is easy to set up and use.  Here is a larger C6 actually on a little SW AZ Pronto Alt-Az mount that would definitely be too small to use with a 4" refractor due to its much great length and larger moment of inertia:

 

 

Due to its larger aperture, the C5 will show a few more stars in the smaller open clusters like M37, and you will be able to see a few more stars around the outer edge of brighter globular clusters like M13 compared with a 4" refractor, but as far as sheer detail and crispness of the images, a decent ED refractor like the AT102ED will outperform the C5.  Stars will be more pinpoint, especially double stars.  For planetary and lunar detail on a night of good seeing, an ED refractor will outperform a C5.  A 4" APO-type refractor will not only show better detail on the planets and the Moon, and allow you to see small, closely spaced double stars much better, but will also share the one advantage the C5 has over those other 4" refractors, namely the lack of any Chromatic Aberration that creates a bluish fringe around bright targets, especially at higher magnifications.  That residual CA is noticeable in ED scopes, and stares you right in the face with achromats. Some people don't seem to mind a little CA, but others cannot tolerate even a hint of it.  It's really a personal preference.

 

Even though on paper the C5 might have the same equivalent light throughput as a 4" refractor, it's all about what a telescope does with that light.  The C5 will allow some of that light to scatter, causing stars to appear slightly larger and "softer" that they do in even a decent ED refractor. In fact, a 4" APO with the right diagonal and eyepiece, and provided you have good eyes (which I no longer do), might allow you to tease out the E and F stars in the Trapezium embedded in M42, but M42 in its entirety will look fine in a C5 or any of the 4" refractor variants, even better using a nebula filter. 

 

If you have a mount that has a carrying capacity of about 20 lbs. at least, even one of those 4" APO Doublet refractors will work fine. You can still get by with a 4" APO on a slightly smaller capacity mount, but it will wobble around a little.  That can be annoying to some people, but not so much with others. If you like using smaller, lighter weight mounts, then a C5 is a very good choice.

 

Good luck with your choice!

Edited by Oldfracguy, 10 May 2024 - 01:18 PM.

[MarkMittlesteadt]

They are very close in what you can see with them, so the optics are comparable, but probably sharper in the refractor.

 

So, the other aspects come into play...mounting options (with regards to weight/length, convenience, portability, etc.).

 

I've had many SCT's and refractors. I like them both for different reasons, but I've found the C5 to be so much more versatile, portable and convenient. And with the ability to add a 6.3 reducer/corrector on the C5, it will also do high power at its native f10, but also widefield at f6.3.

 

I just find the portability of the C5 makes it quite a compact, versatile powerhouse. The C5 is my most used scope now, for almost all my targets. Every choice is a compromise of some sort, so which one comes down to personal preference and priorities. 

 

So which one? 

Edited by MarkMittlesteadt, 10 May 2024 - 01:01 PM.

[PXR-5]

My C5 edges out my C102 f10 refractor, and has no CA.

But what really makes the difference is the mounting requirements, the 102 doesn't play well with my Vixen Porta Mount.

The C5 with the focal reducer is a nice Milky Way cruiser

[12BH7]

I've had friends with refractors of all sizes. If wide field is your thing than that's the scope for you.

 

If you need a "jack of all trades" then a SCT is the way to go. 

[PXR-5]

I've had friends with refractors of all sizes. If wide field is your thing than that's the scope for you.

If you need a "jack of all trades" then a SCT is the way to go.

Well, in my case I'm getting about 2° in the C5 (with the FR/C) and about 1.6° in the frac.

Of course one could go wider with a faster frac and 2" EPs that would make mounting easier also.

[Doug Culbertson]

I sold my AT102EDL shortly after I got my C5 and I can't say I miss it. It pretty much shows me everything I want to see in a small scope, and the 13" long 6# OTA is easily carried. My SkyTee mount doesn't even know the C5 is on it. 

[starcruiser]

I sold my AT102EDL shortly after I got my C5 and I can't say I miss it. It pretty much shows me everything I want to see in a small scope, and the 13" long 6# OTA is easily carried. My SkyTee mount doesn't even know the C5 is on it. 

That's interesting, the AT102EDL is well-regarded in the refractor forum. Good to hear that the C5 does well against it.

[Oldfracguy]

That's interesting, the AT102EDL is well-regarded in the refractor forum. Good to hear that the C5 does well against it.

It is well-regarded, but when you look at what it takes to mount one just for visual observation even:

 

 

 

(above: AT102EDL on SW AZ5--not a good combination  )

 

 

compared with what it would take to mount a C5:

 

 

 

(above:  SW 102mm Mak on SW AZ Pronto.  Note my hand holding up this entire Grab-and-Go setup while the other hand is holding the camera)

 

 

and you are not all that interested in wide-field viewing, so 1.25" eyepieces will do the trick:

 

 

 

 

A C5 on a little Grab-and-Go mount makes a lot of sense  .  

Edited by Oldfracguy, 10 May 2024 - 05:11 PM.

[revans]

I use both a C5 and my Tak FSQ-106N quite a bit.  I like both but the C5 is easier to setup and take down, being lighter and more compact.  I use them both on an EQ6-R Pro mount.  The refractor gives me a bigger FOV, the C5 gives me more image scale for AP.  The refractor is faster.  The refractor images are easier to calibrate with flats.  The refractor has sharper stars.

 

For imaging most DSOs I'd use the refractor,  In the case of imaging a small galaxy I might prefer the C5.  For anything deep sky visual I'd always prefer the refractor.  For the moon and planets I might prefer the C5.

 

Rick

[Oldfracguy]

One thing that hasn't been mentioned, unless I missed it, is that the C5, like any catadioptric scope, will have a "cool down" period to contend with.  These small SCTs and Maks are really not too bad in that regard compared with, say, a C8 or a 150mm Mak, but it will take some time for the views to stabilize in the C5.  A 4" refractor, especially a Doublet, will usually be ready to go in a half hour or less depending on the difference between the outside air temperature and where it had been stored.

 

I will invite the C5 owners chime in here.  How long do your C5s take to cool down sufficiently before the image becomes steady?

[Doug Culbertson]

I keep mine in the case out in my astronomy shed unless it’s brutally hot, so it’s usually not too far from ambient temperature anyway. I did wrap it in Refectix but I couldn’t tell any difference; meaning I have never noticed any heat plumes, so I took the Refectix off. That stuff is just too ugly to leave on for nothing.

[Echolight]

One thing that hasn't been mentioned, unless I missed it, is that the C5, like any catadioptric scope, will have a "cool down" period to contend with.  These small SCTs and Maks are really not too bad in that regard compared with, say, a C8 or a 150mm Mak, but it will take some time for the views to stabilize in the C5.  A 4" refractor, especially a Doublet, will usually be ready to go in a half hour or less depending on the difference between the outside air temperature and where it had been stored.

 

I will invite the C5 owners chime in here.  How long do your C5s take to cool down sufficiently before the image becomes steady?

It must have a really thin mirror. Weighs about two-thirds of a 130 f5 newtonian.

 

I've never bothered to let mine cool. Views seem to be sharp right off the bat.

But I do live in an SCT friendly environment.

[Echolight]

A 4 inch apo of similar weight is three to four grand.

 

If the weight of the scope is less of a factor than price, throw a 130 f5 newt in the fight.

Edited by Echolight, 10 May 2024 - 06:25 PM.

[CHASLX200]

I had a past insane nuts freaky sharp C5 that would do better on Jupiter than my past insane nuts freaky sharp C102.  Color in the 102 kills the view for me. But at 600x the moons of Jup are perfect and sharp balls. But that C5 at 400x showed the trap of M42 as four perfect balls.

 

Wide field the 102 would really win if i could have used 2" stuff.

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Edited by CHASLX200, 10 May 2024 - 06:26 PM.

[topomountain]

the op does not state where he lives...

 

lots of the biggest fans of sct live in easy climates i think

 

some say using refletix and insulation makes cool down a non issue....

 

if its freezin out side or colder or fast swinging temps, the sct is nothing like a 102ed to me

 

i sold my c5, maybe should have kept it, but i like my 102sd a lot better... the c5 is very powerful for its size for sure

 

i will say the at102ed vs c5 is not the same fight as a 102edl vs a c5.... not a huge diff but a clear differnece, wesp at higher mags, that better apo pulls ahead for me, and maybe im picky about wide fov, the c5 with the reducer def lost some sharpness to me over 100x

[RichA]

How does a modern C5 compare with these  4" refractor variants?: C5 vs Achro, C5 vs ED, C5 vs APO.

After subtracting light loses, the C5 XLT has a throughput equal to a 4" with perfect light transmission. So on non-widefield DSO views how does it compare on objects like M13, M42? Same brightness? Same contrast? And how much of that secondary interferes with planetary contrast vs the clear 4" refractor? I'm just curious... The C5 looks like a very compact alternative to a 4" frac if widefield isn't a priority.

The C5 might lose on planets, there are a lot of variables, but it won't lose on deep-sky.  You can't overcome a 50 percent surface area advantage, even though the 5 inch has a central obstruction.  I had a good C5 and it was beaten on planets by a 120mm f8.3 achromatic refractor in a noticeable but not major way.  

[starcruiser]

the op does not state where he lives...

 

lots of the biggest fans of sct live in easy climates i think

 

some say using refletix and insulation makes cool down a non issue....

 

if its freezin out side or colder or fast swinging temps, the sct is nothing like a 102ed to me

 

i sold my c5, maybe should have kept it, but i like my 102sd a lot better... the c5 is very powerful for its size for sure

 

i will say the at102ed vs c5 is not the same fight as a 102edl vs a c5.... not a huge diff but a clear differnece, wesp at higher mags, that better apo pulls ahead for me, and maybe im picky about wide fov, the c5 with the reducer def lost some sharpness to me over 100x

I live in "easy climates"... Yes I'm fan of SCTs and own a C8...and it never needed insulation. I'm looking at the C5 as a compact alternative to the C8.