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4" ED vs. 5" achro vs. 8" SCT

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#1 rustynpp

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Posted 24 June 2019 - 10:11 AM

Hey all,

 

I've been rethinking my scope lineup for awhile now, and I'd love to hear some input from others on my thoughts.

 

I currently use a very nice 4" ED doublet refractor (WO Zenithstar 103) as my main scope, with which I observe from a Bortle 3/4 dark sky location about 15 minutes from my vacation home in Montauk, NY. Out at the dark site I spend the vast majority of my time on DSOs and clusters, with the occasional double star thrown in. From the deck of my home, which is decently dark but plagued by some local light sources (Milky Way faint but obvious), I use a C80ED refractor for solar system and some double star observing, as well as some wide-field viewing. Because I have the C80ED at home, I don't spend much time on the planets with my 4" ED - I'd rather spend my time taking advantage of the darker skies by observing DSOs and clusters.

 

Because DSOs are one of my primary interests (along with widefield clusters/sweeping) and a 4" refractor is a bit limited on aperture, I've been thinking about whether a 5" 600-700mm focal length achro or an 8" SCT might better suit my needs. Unfortunately I don't have the storage space or budget to get another scope without selling the 4" ED, so if I do opt for a 5" achro or 8" SCT, it would be supplant the 4" ED (I'll keep the C80ED regardless). Dobs are out as well, both for storage space purposes and transportation/setup hassle.

 

I've compiled a pro/con list below with +'s and -'s roughly indicating the relative degree of the particular strength or weakness. From this list it's clear to me that the 4" ED is a bit of a jack of all trades, but falls short in maybe the most important aspect - DSOs. The 5" seems to be a bit more specialized, with improvements in both object types that are important to me (DSOs and wide-field), but also would preclude the occasional satisfying double star observation, and perhaps isn't a big enough improvement on DSOs to justify the loss of the ED glass. The 8" SCT is specialized in a different way, with a big improvement on DSOs, but at the cost of a complete lack of wide-field capability as well as pretty heavily decreased convenience (cool-down, collimation, dew management).

 

Cost is not really a factor here, as any setup would be within a few hundred dollars of any other. Everything would be solidly mounted on a Stellarvue M2 and heavy duty surveyors tripod. And remember, I'm able to view the solar system from the deck with my C80ED, so I don't spend much time on those at the dark site.

 

A dark horse candidate would be to get an SCT along with a mount that is able to carry 2 scopes, which would allow me to mount the C80ED next to the SCT for the best of both worlds. But the inconvenience of an SCT combined with the additional weight of the mount and hassle of lugging two scopes instead of one makes this less appealing.

 

Anyway, thank you for reading and I would love to hear your thoughts!

 

4” ED:
Pros:

  • Convenience++
  • Solar system++
  • Double stars++
  • Build quality++
  • Wide-field+
  • Refractor views

Cons:

  • Deep sky-

 

5” achro:
Pros:

  • Convenience++
  • Wide-field++
  • Deep sky+
  • Refractor views

Cons:

  • Solar system--
  • Double stars--
  • Build quality--

 

8” SCT:
Pros:

  • Deep sky++
  • Solar system++
  • Double stars+

Cons:

  • Convenience--
  • Wide-field--
  • Build quality-
  • SCT views

Edited by rustynpp, 24 June 2019 - 10:29 AM.


#2 db2005

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Posted 24 June 2019 - 10:39 AM

I believe there's a large probability that the 5" short achromat's aperture advantage will be at least partly offset by its lack of sharpness caused by its fast focal ratio. 4" ED/APOs can be surprisingly powerful on DSOs because of their excellent contrast.

 

So personally I'd keep the ED for its general-purpose strengths and compliment it with an SCT. That's what I did with my FC-100... I got a C8 for superior light grasp and the FC-100 for sharp, pristine views, giving me the best of two worlds. I will admit that an SCT is slightly more hazzle to use: dew control (dew shield and dew heater strip, requiring separate power), and collimation, but in some ways I find the SCT more comfortable to use than a refractor. Particularly the observing position is much more comfortable than observing through a refractor. As for the "SCT views" you list as a con, I always use a 0.63x reducer/corrector visually. Not only does that reduce the C8's focal length and increases the field-of-view, but it also improves off-axis sharpness significantly.


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#3 markb

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Posted 24 June 2019 - 11:29 AM

I have them all, these are my own experieces  based on up to now moderate use, just retired and moving into higher usage.

 

As to a 5" achro, my first class 5 and 6" Jaegers achromats are great scopes, but that color, well corrected as the Jaegers are, still hurts contrast, and even f5s are bulky and the cells heavy.

 

Light grasp is not hugely improved over an apo 4. 

 

But an 8 SCT is a huge, immediately obvious, jump in both light grasp and image scale (as is the oww, so heavy 11 over the 8)

 

I have 8 and 11 SCTs. Given correct assembly at the factory (personal experience knocks Meade from consideration, please no haters on that personal opinion), the visual image scale and size are big plusses. 

 

Add a focal reducer and you are right around f6, wide views are available. 

 

Tube insulation is a popular CN topic. Everyone seems to love insulating's effects, I plan to use 1/8 polyethylene closed cell foam in roll form and flexible velcro dewshield on the 11, but the 24" wide roll will allow me to use the foam as a removable, integrated insulation and dewshield on the 8. Aluminized insulation from home Depot or lowes is most popular.

 

Build quality: Celestron's Edge HD tubes fixed a lot of the sloppy sct design 'features' from the 60s to allow integration of the baffle tube corrector, as well as tightening optical/mechanical axis coincidence, reducing the chance of getting a no so great one, a problem with buying an untested SCT used. If it is not perfect, do not hesitate to get it replaced or tuned.

 

With a tight tolerance and properly built SCT collimation is infrequent if ever. And it is easy using Bob's Knobs. After I sleeked a mak corrector I refuse to use tools at night.

 

One tip though. I also use a frictionless alt az 70% of the time. I increased my eyepiece time enormously by switching to a green pistol laser, microadjustable by Allen key, and a momentary switch, on a vixen shoe to Weaver adapter.

 

Once set up I hit planets first time without changing from a 225x eyepiece. No bending or twisting. Heavenly to enjoy a scope instead of grumbling at it.

 

Another option you may not have considered: a set of APM APO 100 binos. Sharp, comfortable (add a $30 Orion L bracket to your mount, and a simple sliding weight to a long dovetail extrusion on the other side of the mount), and super pleasing to the eye and brain on clusters etc. Mine will also pull high powers. I got a photo lighting carry case for about $80. LOVED it, as well as my limited to 10x german flak binos, 80mm, 70degree, massive 52mm oversize prisms, that can travel with me. IIRC it looks like a 6" to the brain.


Edited by markb, 24 June 2019 - 11:32 AM.

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#4 treadmarks

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Posted 24 June 2019 - 12:32 PM

If I'm reading this correctly, you already have a 4" apochromat and you're trying to decide between a 5" achromat and an 8" SCT mainly for DSO. I just wanted to make sure, because I don't see how this is even a choice. Get the SCT.


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#5 ELDavis

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Posted 24 June 2019 - 12:45 PM

I have both- an 5" AP EDT on a Losmandy G11, and a 25 year old 8" Celestron Ultima PEC. The SCT is much easier to set up and transport than the G11, and is far superior for DSO. With a 42 mm widefield eyepiece I have a field of view of almost 1.5 degrees. With your 4" apo I think the 5" would be redundant. Having said this I dearly love my 5" AP refractor.


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#6 Stelios

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Posted 24 June 2019 - 01:12 PM

There is no contest, really. The SCT is far and away the best option. SCT's have got a bad rap primarily by those who refuse to collimate them. In particular, Edge HD scopes are the best SCT's made, and I've been using SCT's since 1978. They hold collimation extremely well, and I guarantee they will beat both your ED scopes on double stars and planetary views unless your seeing is awful.

 

For me, the choice would be between 3" ED + 8" SCT or 4" ED + 8" SCT (yes, bring the 4" home and sell the 3" --why isn't the 4" better?)

 

The 5" achro is not even worth discussing as an alternative. Inferior in every way to either the SCT or the ED, it's only redeeming feature is that it "looks like a real scope."

 

Let me address a bit the "convenience" factor, as I think you've got it the other way around. The SCT is the more convenient scope by far. Unless you are a contortionist, you would hate the observing positions that a long 5" achro would put you in (and a short 5" achro will  have so much color that it would be unusable with more than 50x power). Collimation has to be done maybe once a year, cooldown just requires taking the scope out before dark (or, if you get an Edge, using the Temp-Est fans--but I've never seen a need) and dew management means putting a dew shield on (you could add a dew heater if you really need to observe through a very humid night, the combination keeps my Edge able to photograph through the night even when everything else in the morning seems like it's been dunked into a lake).


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#7 aa6ww

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Posted 24 June 2019 - 01:39 PM

An 8" SCT is an excellent choice to compliment a smaller 4" or 80ED refractor. You don't need an Edge since that's more hype then anything else and they have their issues also. They are pretty and if price doesn't so much mater, that's always the SCT of choice, even for just the "feel good" feelings you'd get from owning one. I've  had both 8" SCT's. There are currently some excellent 8" SCT's for sale used right now on CN. A nice modern orange tube Celestron Nexstar 8SE for $425 is up for sale now in excellent condition. That is not an uncommon price for something so potent as a very nice modern C8 OTA.

 

A 5" Achromat will out perform a 4" ED on deep space. Its still about aperture. I had a 6" Meade AR-152 Achro years back and sold it for a TOA-130. The TOA was absolutely perfect for a 5" but it always fell short of the 6" Achromat when observing galaxies and dimmer deep space objects.

 

Why not keep the 4" ED and sell off the 80mm? A 4" APO and a 8" SCT seems to be a nice winning combination.

 

...Ralph


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#8 rustynpp

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Posted 24 June 2019 - 01:55 PM

Thanks all for your responses. The arguments in favor of the SCT are persuasive, and I'm convinced that a 5" achro would be a poor choice. I'm willing to accept that I may have had an overly pessimistic view of the maintenance associated with a C8.

 

I do have a couple question, though.

  1. Should I expect a C8 to stay reasonably collimated even after a car ride? My dark site is a 15 minute drive, so there is some transportation involved each time I observe.
  2. Are there any 2" reducers that are compatible with the C8? I would really like to use 100* 2" EPs with this scope.

 

One tip though. I also use a frictionless alt az 70% of the time. I increased my eyepiece time enormously by switching to a green pistol laser, microadjustable by Allen key, and a momentary switch, on a vixen shoe to Weaver adapter.

...

Another option you may not have considered: a set of APM APO 100 binos.

I love using green lasers on a manual alt-az mount - it's such an effective, effortless finder! And I would love a pair of 100mm binos, but unfortunately it's not in the budget right now.

 

For me, the choice would be between 3" ED + 8" SCT or 4" ED + 8" SCT (yes, bring the 4" home and sell the 3" --why isn't the 4" better?)

Why not keep the 4" ED and sell off the 80mm? A 4" APO and a 8" SCT seems to be a nice winning combination.

The 3" is much more convenient for grab and go at home. As compared with the 4", it's a shorter, lighter scope on a smaller mount and a lighter tripod that I can leave by the deck door and take out with one hand. In order for the 4" to be similarly convenient, it would have to be so undermounted as to be unpleasant to use.

 

Thanks again!


Edited by rustynpp, 24 June 2019 - 01:55 PM.


#9 ELDavis

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Posted 24 June 2019 - 02:52 PM

The only time I needed to collimate my C8 was after a cross country journey. Local trips aren't a problem. With a F6.3 focal reducer you are limited to 1.25" eyepieces because of the internal baffle. . A 32 mm plossl will give you the widest possible field.


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#10 treadmarks

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Posted 24 June 2019 - 03:08 PM

Thanks all for your responses. The arguments in favor of the SCT are persuasive, and I'm convinced that a 5" achro would be a poor choice. I'm willing to accept that I may have had an overly pessimistic view of the maintenance associated with a C8.

 

I do have a couple question, though.

  1. Should I expect a C8 to stay reasonably collimated even after a car ride? My dark site is a 15 minute drive, so there is some transportation involved each time I observe.
  2. Are there any 2" reducers that are compatible with the C8? I would really like to use 100* 2" EPs with this scope.

I've owned my C8 for about 2 years now. In that time I've had to collimate it 2 or 3 times. So it holds collimation pretty well.

 

Regarding reducers and 2" eyepieces, you can do it with the Celestron R/C but there is some aperture reduction due to the extended light path. See this post by Ed Zarenski. The loss is 13mm so 190mm effective aperture. Whether or not the vignetting (darkening at the edges) bothers you seems to be a matter of opinion.


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#11 aa6ww

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Posted 24 June 2019 - 03:12 PM

I've never had any SCT I've ever own loose collimation once it was mine. Anything from the factory should never loose its alignment unless your using to transporting it in the back of a truck our something with a rough ride. If you do find yourself having to adjust the secondary mirror for better alignment, just ask first on how its done. Its simple but there are ways to do it so you are not guessing which screw to adjust and what direction to turn the screws.  Celestron SCT manuals go in depth on doing this.

Having said all of this regarding collimation, I know people who are constantly trying to adjust their scopes because they just don't do it right the first time, and because they don't wait till the scope is fully acclimated also.

 

My only rule on transporting is I always lie my scope down in the back seat of my car, whether its my C8, C11 or formerly C14 I  had for nearly 20 years. Turn the focus knob completely clockwise until it stops, which places the primary mirror against the back of the scope so it has less chance to wobble while its being transported. Always keep a closed SCT out of direct sunlight also, because tube currents will kill your sharpness. Even a small C8 could take an hour or more to fully acclimate to the night sky, so be patient. Focus on deep space more then planets or double stars in the first hour so the scope after set up so it has time to acclimate. Normally it wont take that long, but its still a good rule of thumb before you start blaming the scope on something that is nothing more then understanding your scope and the outside conditions better.

 

The 0.63 sct reducer works well with 1.25" eyepieces. That's how I use my C8. The big 100 deg eyepieces will work with a C8 but not really with the reducer / Flattener in place. One of my friends uses 2" eypieces in his C8 and loves them, but personally, I'm happy using my entire collection of ES 1.25" eyepieces in my C8 to keep everything compact and lightweight.

 

...Ralph

 

 

 

 

 

Ive never had a C8 which was a bad performer, and I've had plenty of all vintages. Most bad reports on mediocre optics is because the scope was either not properly acclimated to its surrounding conditions or the seeing conditions were not spectacular.


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#12 Stelios

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Posted 25 June 2019 - 02:14 AM

Two things on SCT collimation:

 

1) Ed's guide to collimation. Best and clearest way to collimate an SCT I've ever found. 

2) Do NOT get Bob's Knobs. They do make collimation a bit easier--but the collimation doesn't hold. 


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#13 carolinaskies

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Posted 25 June 2019 - 12:10 PM

If you look at my signature you'll see I have a plethora of instruments all the way to a 16" SCT.

You'll note in that list of instruments is an F/6.3 8" LX200 classic.  I don't have to use a reducer for the wider FOV.  While it is a trade-off for planetary vs the F/10 it's quite nice.  With a 32mm standard eyepiece it has 1.3degree FOV.   I used it for imaging and visual and wasn't disappointed with it's performance except when I got aperture fever for which I got larger scopes.    SCTs aren't ever truly wide-field except when used like a RASA or Hyperstar for imaging.  A 8" XLT or ACF is more than adequate for visual.  While some swear by their Edge HD, the cost of the .7x reducer leaves me flat as well as the cost of adding tempest fans simply to achieve some minor visual improvement at the outer 10% of your FOV of an eyepiece.  

You'll find if you check the weights the 8" OTA compares quite favorably to the 5" refractor with the benefit of very short tube making balancing easier.  Dual mounting with your C80ED or even your 103 would give you the best bang for observing time.  Did you know many amateurs piggyback not only ST80s but the 80EDs on their SCTs for this very reason of having the versatility?    

Ergonomicly a basic dewshield and/or reflectix wrap handle the average humidity conditions, with a basic dew-strap for the worst case conditions.  

SCTs are the all-around-scope for a reason, because most targets aren't widefield, but fall well under 1* in size.  Your other instruments handle the wider field and my recommendation isn't to get rid of any current equipment but rather find a used 8" SCT.  Price-wise a used OTA is $300-400.  8" is such a popular size that you might even find a used fork mounted version at that price.  Something like an old LX10 or LX50 or Ultima/Nexstar.  

Your 103 is a VERY NICE scope, and I think you'd miss it pretty quick so I'd look into scraping together the money for the used 8".   



 

 

Hey all,

 

I've been rethinking my scope lineup for awhile now, and I'd love to hear some input from others on my thoughts.

 

I currently use a very nice 4" ED doublet refractor (WO Zenithstar 103) as my main scope, with which I observe from a Bortle 3/4 dark sky location about 15 minutes from my vacation home in Montauk, NY. Out at the dark site I spend the vast majority of my time on DSOs and clusters, with the occasional double star thrown in. From the deck of my home, which is decently dark but plagued by some local light sources (Milky Way faint but obvious), I use a C80ED refractor for solar system and some double star observing, as well as some wide-field viewing. Because I have the C80ED at home, I don't spend much time on the planets with my 4" ED - I'd rather spend my time taking advantage of the darker skies by observing DSOs and clusters.

 

Because DSOs are one of my primary interests (along with widefield clusters/sweeping) and a 4" refractor is a bit limited on aperture, I've been thinking about whether a 5" 600-700mm focal length achro or an 8" SCT might better suit my needs. Unfortunately I don't have the storage space or budget to get another scope without selling the 4" ED, so if I do opt for a 5" achro or 8" SCT, it would be supplant the 4" ED (I'll keep the C80ED regardless). Dobs are out as well, both for storage space purposes and transportation/setup hassle.

 

I've compiled a pro/con list below with +'s and -'s roughly indicating the relative degree of the particular strength or weakness. From this list it's clear to me that the 4" ED is a bit of a jack of all trades, but falls short in maybe the most important aspect - DSOs. The 5" seems to be a bit more specialized, with improvements in both object types that are important to me (DSOs and wide-field), but also would preclude the occasional satisfying double star observation, and perhaps isn't a big enough improvement on DSOs to justify the loss of the ED glass. The 8" SCT is specialized in a different way, with a big improvement on DSOs, but at the cost of a complete lack of wide-field capability as well as pretty heavily decreased convenience (cool-down, collimation, dew management).

 

Cost is not really a factor here, as any setup would be within a few hundred dollars of any other. Everything would be solidly mounted on a Stellarvue M2 and heavy duty surveyors tripod. And remember, I'm able to view the solar system from the deck with my C80ED, so I don't spend much time on those at the dark site.

 

A dark horse candidate would be to get an SCT along with a mount that is able to carry 2 scopes, which would allow me to mount the C80ED next to the SCT for the best of both worlds. But the inconvenience of an SCT combined with the additional weight of the mount and hassle of lugging two scopes instead of one makes this less appealing.

 

Anyway, thank you for reading and I would love to hear your thoughts!

 

4” ED:
Pros:

  • Convenience++
  • Solar system++
  • Double stars++
  • Build quality++
  • Wide-field+
  • Refractor views

Cons:

  • Deep sky-

 

5” achro:
Pros:

  • Convenience++
  • Wide-field++
  • Deep sky+
  • Refractor views

Cons:

  • Solar system--
  • Double stars--
  • Build quality--

 

8” SCT:
Pros:

  • Deep sky++
  • Solar system++
  • Double stars+

Cons:

  • Convenience--
  • Wide-field--
  • Build quality-
  • SCT views

 


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#14 Jon Isaacs

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Posted 25 June 2019 - 12:39 PM

Dobs are out as well, both for storage space purposes and transportation/setup hassle.

 

 

Of the scopes you mention, the 8 inch SCT and one of the ED Refractors make the most sense. One offers wide field views, one goes deep. Replacing the 4 inch ED with a 5 inch achro would be disappointing I think.  Trading quality for a small increase in aperture is not often satisfying.  

 

As a Dob guy, I think a Dob is worth considering. Setup is quick. Dobs are robust and sit in a corner nicely. They're effective for DSOs offering both high power and wide field... An 8 inch F/6 does 2.2 degrees. Collimation is a concern for some people. 

 

If the Dob is out, then the SCT with a RACI finder and a Telrad. An 8  inch lights up the sky when compared to a 4 or 5 inch.

 

Jon


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#15 markb

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Posted 25 June 2019 - 12:51 PM

There are many opinions on focal reducers on CN. One opinion I have seen repeatedly is that the vignetting from using a focal reducer is generally not obvious to most visual users.

 

A C11 would cure the problem, but they are monsters, friendly monsters, but not for the faint of heart or back. Back to the old conundrum of offsetting advantages and disadvantages.

 

Most focal reducers have a clear aperture around 37-38mm from what I have seen in postings, but I have not measured mine. I believe some are larger, and some make an effort to push farther into the main tube, I believe the old Meade WFAS took this path.

 

Interestingly, the new Edge HD reducer appears to use a longer body rather than shorter. 

 

While my f.r. assortment has been around for several years, my SCT use is much more recent, so no opinions here. I am looking forward to getting that old, big clear aperture Lumicon mounted though...

 

Just like SCT and collimation opinions, there are many divergent opinions on focal reducers on SCTs, so I,  personally discount much of what I read on forums and fall back to (hopefully) more fact based reviews.

 

'Uncle Rod' usually carries some weight, so perhaps start here (he uses a 100 degree 16mm in the part I skimmed), https://uncle-rods.b...on-edge-f7.html

and then search for more hands-on reviews.

 

After resolving 2 SCT dogs with optical and/or mechanical alignment issues, I still recommend buying new, getting the EdgeHD not for the added lenses but for the long-overdue mechanical-optical alignment design improvements, and being prepared to return to the dealer or to the manufacturer any disappointing (compared to the expected standard) scopes for repair or replacement.

 

Most SCTs perform beautifully and hold collimation long-term and are worthy purchases for those reasons, but there are no shortage of problem scopes out there, either unrecognized by owners that never had a comparison scope, or waiting for the next victim in the ownership chain.

 

And a C8 may be the easiest and fastest to mount scope I have ever loaded onto my alt-az!


Edited by markb, 25 June 2019 - 12:54 PM.

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#16 Eddgie

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Posted 25 June 2019 - 01:49 PM

While people have said that you don't need the EdgeHD, you have also said that you listed  "SCT Views" in the Cons for the SCT.

 

And this is exactly what the EdgeHD fixes.   The standard 8" SCT design was penned at at time when 1.25" eyepeces were the standard offerering.  They were never really intended to provide a sharp view with a field stop bigger than 27mm. 

 

The standard C8 field has a negative radius of curvature of about 280mm.  For comparison, this would be about as much as a  900mm focal lenght refractor. Now, if you are using a 1.25" eyepiece, you really can't see far enough off axis to be able to see the field curvature.  The EdgeHD 8" by comparison will have a field that is more like negative 800mm, and even in the widest of wide field eyepieces, the field will be much flatter out to the very edge.

 

The standard C8 has coma that is about a strong as one would see in an 8" f/5 reflector.  Again, if using a 1.25" eyepiece, you can never really see far enough away from the center of the field for this to be an issue, but with modern wide field eyepieces, you do get far enough away to see it.

 

When the defocus is coupled with the coma, you get the kind of "SCT Views" you often read about.  The field is really only fairly sharp over a field no bigger than you can get in a 24mm Panoptic. 

 

The EdgeHD fixes all of these problems.   The difference to people who care about off-axis performance is dramatic. 

 

If you only care about the object at the center of a low power view, then maybe the standard SCT is OK, but if you do a lot of sweeping or like viewing objects that fill the field, and you like using modern 82 and 100 degree apparent field eyepieces, I personally would say that the "SCT View" will has to stay in the Con list.  If though you go to the EdgeHD, that con goes away and only bad seeing will bloat stars.  The EdgeHD will give the same sharp stars across the field than you can get with refactors, and better than some small refractors where field curvature is a serious problem.

 

Treat any comments that suggest that the EdgeHD is going to be better than a standard C8 with skepticism.   Here is a test of an EdgeHD 11" with a Strehl of .825.  That is right at the edge of the diffraction limit.

 

http://fidgor.ru/Obs...2/test_367.html

 

Another EdgeHD 11. Strehl of .882.  Not bad at all, but well short of being better than the non-EdgeHD scopes.  Pretty typical quality really.

 

http://fidgor.ru/Obs...st/test_57.html

 

 

An EdgeHD 8 with about the same barely diffraction limited quality, with a Strehl of .815.

 

http://fidgor.ru/Observers/test.html

 

Here is a test of the EdgeHD 8". This one is better than the C11, but kind of right in the middle of normal SCT optical quality which is to say good quality, but no better than the average C8 out there.

 

http://fidgor.ru/Obs...2/test_369.html

 

I still recommend the EdgeHD though.  It has hugely improved off axis sharpness. 

 

Take subjective reviews on CN with a grain of salt.   Everybody thinks their telescope is special, but few really are. 


Edited by Eddgie, 25 June 2019 - 02:11 PM.

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#17 Eddgie

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Posted 25 June 2019 - 01:57 PM

With respect to my last post, there is a perennial question on the Reflector forum that goes something like this: "At what focal ratio do you find it necessary to use a coma corrector?"

The responses will generally break down like this:

  1. No, I only care about the view at the center of the field
  2. I can see the coma at f/5 and it does not bother me 
  3. I can see the coma with wide field eyepieces, but I don't use those much
  4. I can see the coma at f/5 but it is not enough to make me want to buy a coma corrector
  5. I can see the coma at f/5 and can't stand using the telescope without a coma corrector.

Now the most important thing here is that one person is not really looking at it/for it and could care less, but the other four groups all see coma.  And in an 8" f/5, the field is pretty flat.  Imagine if the field were curved as badly  as in a standard C8! Not only would you see the coma, but it would be a enlarged by the defocus. 


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#18 rustynpp

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Posted 25 June 2019 - 02:12 PM

Thanks all - there seems to be a consensus emerging in favor of the SCT, with special consideration for the EdgeHD.

 

I would indeed miss the 4" ED - it's a beautiful scope that does a lot of things really well. But many of the things it does well, my C80ED does almost as well. And since I spend most of my time on DSOs, it seems as though an SCT makes the most sense for me. Unfortunately funds and storage space preclude me from keeping both. I keep my scopes in a small vacation home that I share with 5 family members, so the amount of space my personal stuff can occupy without being inconsiderate is limited.

 

Any more advice is much appreciated, but consider me swayed to team SCT :)


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#19 Ihtegla Sar

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Posted 25 June 2019 - 02:38 PM

 

 

Because DSOs are one of my primary interests (along with widefield clusters/sweeping) and a 4" refractor is a bit limited on aperture, I've been thinking about whether a 5" 600-700mm focal length achro or an 8" SCT might better suit my needs. Unfortunately I don't have the storage space or budget to get another scope without selling the 4" ED, so if I do opt for a 5" achro or 8" SCT, it would be supplant the 4" ED (I'll keep the C80ED regardless). Dobs are out as well, both for storage space purposes and transportation/setup hassle.

 

 

I am late to the party but I own all three, almost -- I have a four inch apo (Takahashi FC100DL) a four inch achro (Celestron 102mm widefield F/5) and an eight inch SCT (C-8), so here are my thoughts . . .

 

For deep sky aperture rules, so I would go with the 8" SCT.  But I also have a 12 inch dob, which blows the other three out of the water on deep sky objects with more than twice as much light gathering power as my 8" SCT -- I'd second Jon Issacs's recommendation to consider a larger (10" to 12") dob if deep sky is what you are after -- the views of deep sky objects (galaxies, planetary nebula, emission nubluea and globular clusters)  in my 12" Skywatcher collapsible dob are stunningly better than my C8.  But if you can't get a dob, then the 8" SCT will be the next best thing because it has four times the light gathering power of a 4" and two and a half times as much as a five inch.

 

You should also consider set up and cool down.  An 8" SCT is heavier, requiring  more work to set up and a beefier mount than a 4" Apo and it cools down slower and may have issues with tub currents and may be more sensitive to good seeing than a 4" apo.  The best scope is the one that gets used.  I use my 4" apo more often than anything else because set up takes five minutes and cool down time is very fast.  But if I am making a trip to dark skies I am taking the largest aperture I can muster.


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#20 carolinaskies

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Posted 25 June 2019 - 02:48 PM

While people have said that you don't need the EdgeHD, you have also said that you listed  "SCT Views" in the Cons for the SCT.

 

And this is exactly what the EdgeHD fixes.   The standard 8" SCT design was penned at at time when 1.25" eyepeces were the standard offerering.  They were never really intended to provide a sharp view with a field stop bigger than 27mm. 

 

The standard C8 field has a negative radius of curvature of about 280mm.  For comparison, this would be about as much as a  900mm focal lenght refractor. Now, if you are using a 1.25" eyepiece, you really can't see far enough off axis to be able to see the field curvature.  The EdgeHD 8" by comparison will have a field that is more like negative 800mm, and even in the widest of wide field eyepieces, the field will be much flatter out to the very edge.

 

The standard C8 has coma that is about a strong as one would see in an 8" f/5 reflector.  Again, if using a 1.25" eyepiece, you can never really see far enough away from the center of the field for this to be an issue, but with modern wide field eyepieces, you do get far enough away to see it.

 

When the defocus is coupled with the coma, you get the kind of "SCT Views" you often read about.  The field is really only fairly sharp over a field no bigger than you can get in a 24mm Panoptic. 

 

The EdgeHD fixes all of these problems.   The difference to people who care about off-axis performance is dramatic. 

 

If you only care about the object at the center of a low power view, then maybe the standard SCT is OK, but if you do a lot of sweeping or like viewing objects that fill the field, and you like using modern 82 and 100 degree apparent field eyepieces, I personally would say that the "SCT View" will has to stay in the Con list.  If though you go to the EdgeHD, that con goes away and only bad seeing will bloat stars.  The EdgeHD will give the same sharp stars across the field than you can get with refactors, and better than some small refractors where field curvature is a serious problem.

Any comments that suggest that the EdgeHD is going to be better than a standard C8.   Here is a test of an EdgeHD 11" with a Strehl of .825.  That is right at the edge of the diffraction limit. 

http://fidgor.ru/Obs...2/test_367.html

Here is a test of the EdgeHD 8". This one is better than the C11, but kind of right in the middle of normal SCT optical quality which is to say good quality, but no better than the average C8 out there.

 

http://fidgor.ru/Obs...2/test_369.html

 

I still recommend the EdgeHD though.  It has hugely improved off axis sharpness. 

The extra cost of an Edge and accessories aren't equitable with the OP proposition in my opinion, nor are 100* eyepieces simply to think you're utilizing the 10% edge advantage. 

99.9% visual observers eyes are focused on the center 3/4 of any field in the eyepiece.  They aren't drawn to the edge of the eyepiece and in fact the way the mind works it porthole's the view limiting the peripherial advantage of the edge most of the observing time.   Further, most of us spend our time cruising at 80x-150x on DSOs. 

Now lets consider DSO targets themselves.   M1 - bright - eye drawn to the center of the field where the attraction is.  Beyond 100x there is little benefit for an 8" to try to discern more detail.   M11- Wild duck cluster - several bright magnitude stars drawing eye to center 3/4 of field, so bright the edge of the FOV is lost to the concentration on the center.  M13 - similar to M11 except moreso as on globs the general view is to see what stars you can resolve as you move to the bright core. 
M17 - Swan - a good example of diffuse nebula - concentration of the eye is to gain contrast of the field.   I could cite many other objects, but it's well known that though some people may experience some curvature on their instruments' edge of field, the vast majority don't notice it to a degree which draws the eye from the target.  

Use of large multi-element eyepieces has something to do with the difference in viewing, causing the tail to wag the dog and resulted in the need to increase the flatness of the field.  Thus both the Edge and ACF designs were introduced.  Neither a significant advantage or advancement optically for the traditional 50-68* eyepieces, but helpful if you're investing in the expensive wider-field eyepieces to achieve the porthole effect.  Yet thousands of existing SCTs haven't become useless at all, simply because most amateurs are happily cruising years with instruments that have performed reasonably night after night. 

I'll reiterate what I said in my last post.  I don't believe selling the 103 would be a good choice in the long run.  And for what $$ might be recovered with it's sale I don't think the OP will be able to afford an Edge anyways if his budget is that tight.  Thus is make economical and performance sense to consider the other SCTs.   


 


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#21 rustynpp

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Posted 25 June 2019 - 03:05 PM

A large dob is not in the cards right now, unfortunately. The good news is that I have an SV M2 mounted on a steel column and a heavy duty surveyor's tripod, so a C8 should be no trouble. I don't think setup will be any more difficult with an 8" SCT than a 4" ED.

 

Regarding acclimation, I've been reading about the benefits of insulating the OTA with Reflectix. It seems as though folks have figured out a way to severely mitigate the effects of large temperature changes, which IMO was one of the major downsides of SCTs.

 

I am late to the party but I own all three, almost -- I have a four inch apo (Takahashi FC100DL) a four inch achro (Celestron 102mm widefield F/5) and an eight inch SCT (C-8), so here are my thoughts . . .

 

For deep sky aperture rules, so I would go with the 8" SCT.  But I also have a 12 inch dob, which blows the other three out of the water on deep sky objects with more than twice as much light gathering power as my 8" SCT -- I'd second Jon Issacs's recommendation to consider a larger (10" to 12") dob if deep sky is what you are after -- the views of deep sky objects (galaxies, planetary nebula, emission nubluea and globular clusters)  in my 12" Skywatcher collapsible dob are stunningly better than my C8.  But if you can't get a dob, then the 8" SCT will be the next best thing because it has four times the light gathering power of a 4" and two and a half times as much as a five inch.

 

You should also consider set up and cool down.  An 8" SCT is heavier, requiring  more work to set up and a beefier mount than a 4" Apo and it cools down slower and may have issues with tub currents and may be more sensitive to good seeing than a 4" apo.  The best scope is the one that gets used.  I use my 4" apo more often than anything else because set up takes five minutes and cool down time is very fast.  But if I am making a trip to dark skies I am taking the largest aperture I can muster.


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#22 Jon Isaacs

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Posted 25 June 2019 - 03:27 PM

I could cite many other objects, but it's well known that though some people may experience some curvature on their instruments' edge of field, the vast majority don't notice it to a degree which draws the eye from the target.

 

 

I think it's a matter of expectations. People talk about "refractor-like views."

 

That's because people have low expectations for other designs. They just accept the SCT view with it's coma and field curvature or the Newtonian view with it's coma and sensitivity to eyepiece quality.

 

Newtonians and SCTs can provide those stunning "no apologies necessary "refractor-like views"..  With a Newtonian, attention to collimation and thermal equilibrium, a coma corrector, the good eyepieces and the stars will be sharp and clean across the field of view.  An Edge SCT with the good eyepieces is very similar.

 

Look at M11 so it fills most of the field or M7. These can be near perfect or they can be messy.  

 

Expectations.. 

 

Jon



#23 WadeH237

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Posted 25 June 2019 - 05:13 PM

The extra cost of an Edge and accessories aren't equitable with the OP proposition in my opinion

This is 100% subjective and we can only speak for ourselves.

 

In my case, I notice coma at the edges and my attention is drawn to it.  On my F/4.6 dob, I need to use a Paracorr with longer focal length eyepieces.  The difference for me between with and without the Paracorr is quite dramatic.

 

For me the value proposition of an EdgeHD vs the XLT version is a no brainer for visual use.  It greatly increases the value of the scope to me.  If you don't notice, or notice but don't care about, the coma at the edges, then your answer would be quite different from mine.  The only way for any individual to know for sure, is to look through some scopes.

 

I would love to replace my 16 year old C14 XLT with an EdgeHD 14.  But alas, the value proposition for selling my current scope, and buying a new one, is not the same as if I had no scope and was deciding which one to buy.  My current C14 is a pretty good performer, and meets my needs (if not my ideal wants).


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#24 carolinaskies

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Posted 25 June 2019 - 07:35 PM

This is 100% subjective and we can only speak for ourselves.

 

In my case, I notice coma at the edges and my attention is drawn to it.  On my F/4.6 dob, I need to use a Paracorr with longer focal length eyepieces.  The difference for me between with and without the Paracorr is quite dramatic.

 

For me the value proposition of an EdgeHD vs the XLT version is a no brainer for visual use.  It greatly increases the value of the scope to me.  If you don't notice, or notice but don't care about, the coma at the edges, then your answer would be quite different from mine.  The only way for any individual to know for sure, is to look through some scopes.

 

I would love to replace my 16 year old C14 XLT with an EdgeHD 14.  But alas, the value proposition for selling my current scope, and buying a new one, is not the same as if I had no scope and was deciding which one to buy.  My current C14 is a pretty good performer, and meets my needs (if not my ideal wants).

Comparing the coma on a fast newtonian (Dob or otherwise) to an SCT isn't even close because there is no correction in a Newt system until you add a Parracor or coma corrector.  SCTs utilize corrected optics to help with coma straight from the get go.  And lest we forget, try to buy an 7.5" or larger refractor to equate to the light grasp of an SCT and there are many more problems and expenses to achieve correct color not to mention extreme weight/size issues for a Goto mount.  I've been looking through scopes since the 80's including SCTs and can honestly say trying to compare coma in an SCT to the coma in my old F/4.5 13.1" Coulter or my old 16" Meade dobsonian the SCT is much nicer in it's correction without bother.  And the aperture vs OTA length vs performance doesn't seem to find me lacking in very nice views.  

There is a small but noisy contingent who think 10% gain at the edge of the field is worth the extra money including all the other extras you end up having to buy to extend it's usefullness.  Yet it seems that design and it's price hasn't steamrolled the industry to that trend and cost factor.  Meade hasn't suffered and their ACF line F/8 and F/10 are more than up to the challenge without requiring the expensive extras.  Somehow there is again the false-info that even Celestron's own standard XLT is somehow plagued by severe coma when in reality it far from the truth.  The fall off in performance is far more due to poor alignment and owner desire to stock their cases with expensive eyepieces that require the Nth degree of precision to eek out the last bit of performance thinking their OTA is a 25,000 instrument instead of a 2,500 instrument.     I used the analogy in another thread... a supercar capable of 200mph is no better than a Hyundai when used where 45mph traffic is the norm.  



#25 WadeH237

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Posted 25 June 2019 - 07:47 PM

Comparing the coma on a fast newtonian (Dob or otherwise) to an SCT isn't even close because there is no correction in a Newt system until you add a Parracor or coma corrector.  SCTs utilize corrected optics to help with coma straight from the get go.  And lest we forget, try to buy an 7.5" or larger refractor to equate to the light grasp of an SCT and there are many more problems and expenses to achieve correct color not to mention extreme weight/size issues for a Goto mount.  I've been looking through scopes since the 80's including SCTs and can honestly say trying to compare coma in an SCT to the coma in my old F/4.5 13.1" Coulter or my old 16" Meade dobsonian the SCT is much nicer in it's correction without bother.  And the aperture vs OTA length vs performance doesn't seem to find me lacking in very nice views.  

There is a small but noisy contingent who think 10% gain at the edge of the field is worth the extra money including all the other extras you end up having to buy to extend it's usefullness.  Yet it seems that design and it's price hasn't steamrolled the industry to that trend and cost factor.  Meade hasn't suffered and their ACF line F/8 and F/10 are more than up to the challenge without requiring the expensive extras.  Somehow there is again the false-info that even Celestron's own standard XLT is somehow plagued by severe coma when in reality it far from the truth.  The fall off in performance is far more due to poor alignment and owner desire to stock their cases with expensive eyepieces that require the Nth degree of precision to eek out the last bit of performance thinking their OTA is a 25,000 instrument instead of a 2,500 instrument.     I used the analogy in another thread... a supercar capable of 200mph is no better than a Hyundai when used where 45mph traffic is the norm.  

Perhaps you missed the point where I mentioned that I own a C14 XLT (and an EdgeHD 8, and previously had an XLT 8, and a Meade 8" ACF, and a 6" SCT as well).  The coma with a long focal length eyepiece is quite obvious.

 

I get that you don't find the coma objectionable, but some of us do.  The Meade was very  noticeably better for visual than the 8" XLT, due to the coma correction.  I'll confess at this point that I've never actually looked through my EdgeHD 8, since I've always had a camera on it.  I have many, many sessions doing visual observing through all of the others.

 

I'll reiterate that for those of us bothered by coma at the edges, the price premium for an EdgeHD vs an XLT for visual is a huge bargain, for the value - at least to me.

 

Again, it's 100% subjective.  It's really clear that you either don't notice the coma, or don't care.  But kindly stop telling those of us who do care, that it's not important.

 

On edit, I have two more comments:

 

Regarding the Meade ACF scope, I find it to be a better visual instrument than a classic SCT because of the coma correction.  It's worth noting that the ACF scopes still have the typical field curvature that you'd see in a regular SCT.  Personally, I don't see the field curvature visually at all (I can measure it in images, though).  There are some people, though, who are sensitive to field curvature.  For those people, the EdgeHD is a noticeably better scope.  And for those people, I would not presume to deny their preference.

 

And second, my suggestion to the OP is to take everything that any of us say with a grain of salt.  We can do a pretty good job of describing the differences between scopes.  But when it comes to the question of which one you should buy, the absolute best thing that you can do is to look through some of the ones that you are interested in.  If you can get to a star party, you are almost certain to find at least one 4" ED and 8" SCT.  The 5" achros are a bit rarer, but you would learn a lot about your own preferences by looking through a variety of scopes.

 

-Wade


Edited by WadeH237, 25 June 2019 - 08:12 PM.



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