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How much does central obstruction affect views of deep sky objects?

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#1 Simon B

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Posted 21 October 2017 - 02:35 PM

I've read many times that a large central obstruction (40~50%+) can affect viewing low contrast detail on planets.

 

I have an opportunity to purchase a GSO 8" Ritchey-Chretien for a good price, and was thinking to use it for DEEP SKY VISUAL ONLY

 

I realize that these were designed for astrophotography, but would it not be useful for this purpose, since it gives a native flat field, and wider maximum FOV than typical 8" SCTs (2000mm FL vs 1600FL)? I believe also that the field stop diameter is slightly larger in the GSO RC than most Meade/Celestron 8" SCTs, although I don't know how relevant that would be, perhaps not so much.

 

I was thinking it would offer other advantages too, such as having less cool down time and anti-dewing properties due to the open tube design.

 

I COULD get a reflector I know, but I really appreciate portability - I already have a sturdy high quality case for an 8" SCT-sized tube, which wouldn't fit a newtonian. Also I have an HEQ5 and I have read things get a little shaky with an 8" newt on them, what with them being longer and heavier... plus having to rotate the tube in the rings for eyepiece placement sounds annoying.

 

So, my original question - how much would a relatively large central obstruction (I believe 47% on the GSO 8" RC) affect DEEP SKY performance? (not in terms of light loss - I know that's minimal.) Are there any types of deep sky objects that it may negatively impact? (globulars, planetary nebulae?)



#2 Phil Barker

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Posted 21 October 2017 - 03:42 PM

Obstruction from what I've seen does negatively affect deep sky.  I recall a side by side comparison with an 8 inch f-6 newt and a c8 both of which had for their respective design well-corrected optics.

 

The 8 inch f-6 had parks mirrors the c8 was Starbright coated 1994 standard Celestron.

 

obstruction was 34% for the c8 and 22% for the Newt.

 

On lower-medium power objects like eta carina and even gobulars the views were superior in the newt.  Not a huge margin but stars sharper more structure in eta carina better detail in the newt.  Aso image was brighter in the newt.

 

Globs looked better in the newt the tighter stars were noticeable.   


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#3 leveye

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Posted 21 October 2017 - 03:44 PM

Personally if it's just for visual I'd look at a used 8-12 inch Dob/Newt instead.


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#4 Richard O'Neill

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Posted 21 October 2017 - 05:05 PM

 I've found that large central obstructions combined with low magnifications tend to deliver less than desirable images. In such systems large exit pupils sizes that equal the eye's entrance pupil exhibit a most annoying black spot in the center of the field of view. With increasing magnification this annoying factor tends to subside, but never completely. That's why minimally obstructed optics produce more pleasing wide field images and unobstructed ones the best.

 

Richard


Edited by Richard O'Neill, 21 October 2017 - 05:05 PM.


#5 Jared

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Posted 21 October 2017 - 09:10 PM

I think people often overstate the effects of central obstruction when you are talking about 30, 35, even 40 percent. Eventually, though, the effects really do start to hamper the performance, and not just for planetary. When you start looking at 45-50 percent obstructions (and I think the 8” RC’s are in this range) it really starts to matter. Not just for high power planetary views, either. Keep in mind that at lower powers what is coming out of the eyepiece is really a column of light with a hole in it—the shadow of the secondary. That means almost all of the light is being focused through the outer part of your eye which has really poor performance compared with the central part of your eye. Contrast drops off dramatically. Even diffuse objects suffer. I almost feel like I am straining to see when I look through an RC (or any scope with a large central obstruction) at low power.

My 10” RC was perfectly usable as a visual scope from, say, 90x to 150x or so, but that was about it. There are quite a few deep sky objects that look good in that power range, but by no means all. It’s not a good general purpose scope. It’s intended for astrophotography.

It may be a good deal, but Ibwould expect to be able to find a good deal on a used ACF, Edge, or traditional SCT as well, especially the traditional SCT. I’d much rather have one of those for primarily visual use.
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#6 carolinaskies

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Posted 21 October 2017 - 09:58 PM

CO becoming visible is a problem for any telescope which has a secondary mirror ONLY in specific instances where a specific eyepiece chosen makes the CO visible to the viewer based on several factors.

For deepsky objects an 8" RC will be visually 'passable' but the effective light gathering is less than other cassegraine type instruments due to the larger CO reducing the light gathering potential moreso than obscuring the views and because these aren't true 8" mirrors but more like 7.7". 

If you want an SCT with wider field consider a used F6.3 Meade whose FOV will be about 27% wider than the GSO.  



#7 Patrick

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Posted 21 October 2017 - 10:49 PM

I've read many times that a large central obstruction (40~50%+) can affect viewing low contrast detail on planets.

 

I have an opportunity to purchase a GSO 8" Ritchey-Chretien for a good price, and was thinking to use it for DEEP SKY VISUAL ONLY

 

I realize that these were designed for astrophotography, but would it not be useful for this purpose, since it gives a native flat field, and wider maximum FOV than typical 8" SCTs (2000mm FL vs 1600FL)? I believe also that the field stop diameter is slightly larger in the GSO RC than most Meade/Celestron 8" SCTs, although I don't know how relevant that would be, perhaps not so much.

 

I was thinking it would offer other advantages too, such as having less cool down time and anti-dewing properties due to the open tube design.

 

I COULD get a reflector I know, but I really appreciate portability - I already have a sturdy high quality case for an 8" SCT-sized tube, which wouldn't fit a newtonian. Also I have an HEQ5 and I have read things get a little shaky with an 8" newt on them, what with them being longer and heavier... plus having to rotate the tube in the rings for eyepiece placement sounds annoying.

 

So, my original question - how much would a relatively large central obstruction (I believe 47% on the GSO 8" RC) affect DEEP SKY performance? (not in terms of light loss - I know that's minimal.) Are there any types of deep sky objects that it may negatively impact? (globulars, planetary nebulae?)

In terms of contrast, an 8" scope with 47% obstruction is equivalent to about a 4.24" unobstructed scope (Aperture - CO).  A 4-5" f/6ish refractor is going to be a lot more satisfying than an 8" RC, while still maintaining similiar portability.  That size achromatic refractor is made for DSOs.  

 

Patrick



#8 Eddgie

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Posted 22 October 2017 - 08:39 AM

 

 

In terms of contrast, an 8" scope with 47% obstruction is equivalent to about a 4.24" unobstructed scope (Aperture - CO).  A 4-5" f/6ish refractor is going to be a lot more satisfying than an 8" RC, while still maintaining similiar portability.  That size achromatic refractor is made for DSOs.  

 

Patrick

 

Now, funny thing here.   Ask people using 4" refractors what they think of the contrast in their scopes and they will tell you that they are amazing!  Go to the Refractor forum and you will find 1000s, of post that will tell you how much people can see in a 4" refractor!

 

High contrast transfer is really only vital for detail that is quite small and fine.   As the targets and details within the target get larger, the contrast transfer matters less and less.

 

An 8" scope with a 40% obstruction can do quite satisfactory observing.

 

My 6" f/2.8 telescope has a 41% obstruction, but I can resolve a lot of structure in nebula, and I can see extensive faint nebula around brighter nebula.  Now I am using an image intensifier, but the point is that if the scope did not have enough contrast to show these, I would not be able to see them even with an image intensifier.  The contrast is either there, or it is not.

 

Here is a picture taken with a scope having a 41% obstruction.  Look at this and tell me how the big obstruction has ruined contrast?   http://www.teleskop-...on-vorschau.jpg

 

And for richest field observing, nothing beats a really fast scope with a big obstruction.   Look at the observations being done with a small scope with a very large obstruction here:

 

http://www.bbastrode...l#OtherPurposes

 

I am not saying that a big obstruction does not lower contrast, but what I am saying is that for extended targets, (galaxies, and nebula) the effects of obstruction are grossly overstated.  

 

I have provided two examples where it is clear to see that scopes with very large obstructions are still capable of showing a very high level of detail and my own scope that is 6" f/2.8 with a 41% obstruction is capable of resolving considerable detail in extended targets.

 

(Did you know that M29 actually sits on nebula?   The entire field of M29 is covered with nebula, and M29 itself sits on a small peninsula of nebula extending out from Gamma Cygni complex, which itself is nothing short of spectacular.  Now, people will say "Hey, that doesn't count because Ed is using an image intensifier" but here is the fact,  If the contrast was not sufficient to see these, It would not matter that I was using an image intensifier..  The image intensifier does not change the contrast at the focal plane.  Either it is there, or it is not.  The image intensifier can't make it appear where it does not exist.   Also, look at Mel's drawings and eyepiece impressions.  He is seeing things that most people here are never going to see, and he is doing it with fast scopes with very large obstructions, and I can absolutely confirm that he is seeing the structures he is drawing.   

 

Now for planets, I would not be satisfied with the detail I can see even in a 5" Apo, so of course I would not choose this for seeing festoons on Jupiter, but for seeing shapes of Galaxies, or structure in nebula, these big obstructions really don't matter that much.  Once the detail gets bigger than about 2 or 3 Airy Disk diameters, you are past most of the serious contrast loss and even at 2 or 3 diameters and 50% contrast loss, you can still see a lot of detail.  It all depends on how much contrast is there to start with.


Edited by Eddgie, 22 October 2017 - 08:42 AM.

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#9 Eddgie

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Posted 22 October 2017 - 08:54 AM

And as for light transmission.   My Boren Simon has a 41% obstruction and that sounds huge.   Here is the thing though.  The vast majority of the light gathering in a mirror is coming from the outer 50% of the mirrors surface.   

 

Lets to the math on an 8" scope with 40% obstruction.

 

The surface area of the primary is a bit more than 50 square inches.  If you have a 40% obstruction (3.2" secondary) this will shade only 8 square inches of primary.   This means that you are still using 42 inches of light collection.

 

In other words, you are now working with the same light gathering of a completely unobstructed aperture of about 7.3 inches.   This is soooo small a difference that most people would struggle to see it in side by side comparison. 

 

Now a C8 would have an unshaded area of only 44.5 square inches and absolutely no one could see the difference between 44.5 and 42 square inches of light collection area.

 

People grossly overstatement the impact of a large obstruction for general observing.  I have used obstructions as large as 50% and can report that obstructions this size do not make the instrument horrible to use.  Again, I would not use it for planets, but for DSOs, it does little damage except when the structure is very fine scale and in fact in nebula, the unaided eye is usually incapable of really resolving this fine scale structure because it just isn't bright enough..  It is staggering how much more detail I can see in Orion using an image intensifier than when using a glass eyepiece.  It is like looking at a photograph vs glass eyepiece, where only the very large scale features are easily visible.    So, having a smaller obstruction does not make the image substantially brighter, so you really don't get as much extra detail as one would believe by having a smaller secondary.   

 

If you were looking for the finest scale structure in the Orion Nebula, I guess you have to pull out your 8" Apo.. lol.gif

 

Again, look at Mel's drawings and you can clearly see that he is getting very good views.  How many of you have seen the Horse Head Nebula with the detail he shows in his sketch?  The scope used for that has about a 33% obstruction but the 6" scope has a considerably larger obstructions.  This was necessary because of the use of a 21mm Ethos with a very large field stop.  As reflectors get smaller, to get the same size illuminated circle, the secondary has to get bigger as a percentage of obstruction.  As the scope gets larger, you can actually keep the same size fully illuminated field with progressively smaller secondary mirrors.  The 13" scope would normally only require a 20% secondary even with very wide field eyepieces, but the 13" needs over a 30% to be able to illuminate even the center of the field.

 

Mike Lockwood is talking about building a very fast smaller aperture scope and he and I have had a brief exchange about secondary size and he is now looking to fully illuminate an 18mm field, so his obstruction will have to be considerably larger than he has been using in his bigger scopes. We though are very much into nebula these days, and for that, I have recommended that he fully illuminate an 18mm circle. 

 

Bottom line is that for DSOs, the obstruction size is not all that important.  Yes, a smaller secondary will have better contrast than a larger one, but the difference in the detail you can see is not glaring, and in fact, can be quite subtle. 


Edited by Eddgie, 22 October 2017 - 09:17 AM.

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#10 sg6

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Posted 22 October 2017 - 09:23 AM

If it is for visual then I woud not suggest it. Getting an RC collimated can be a troublesome action. They are, or can be,  difficult to do so.

 

At 6" it does not seem with the potential trouble.

 

The central obstruction will generally block light - to be expected - but the obstruction also reduces the contrast, or as some say "softens" the final result.

 

Depending on the cost a good 6" or 8" reflector may be better option, but that decision is yours to make. Especially as I much prefer refractors but a nice 5" apo or even ED is likely to out of whatever price range is under consideration.


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#11 Cotts

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Posted 22 October 2017 - 10:18 AM

The only worrying thing that I see with a 40%+ central obstruction is that at the lowest magnifications, with the largest exit pupils, there will be a very obvious 'donut' appearance.   The centre of the field of view will be noticeably darker caused by the out-of-focus image of the secondary mirror.  Since the OP states he wants the largest possible fields of view this means he will have to contend with the 'donut'.....

 

As for 'loss of contrast' this is nonsense for any viewing other than high power planetary/lunar/solar viewing where the altered structure of the diffraction pattern caused by the presence of the large central obstruction reduces contrast transfer.  This loss of contrast transfer does not affect the blackness of the sky background at all -loss of contrast only occurs at scales of 1" or less because that's how big the Airy diffraction pattern is.  

 

If any scope is well baffled, flocked and has clean optics etc., it will deliver excellent wide-field low and medium power contrast no matter how large the central obstruction is (the donut notwithstanding, of course).    

 

Dave 


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#12 Jon Isaacs

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Posted 22 October 2017 - 10:32 AM

High contrast transfer is only vital for detail that is quite small and fine.   As the targets and details within the target get larger, the contrast transfer matters less and less.

 

 

Eddgie's point is an important one, deep sky contrast, that is the object against the sky, is not affected by the CO.

 

As far as the fact that at large exit pupils, the light is using the outer portion of the eyes lens which is not as sharp as the center, this is always the case, if the CO is zero, 75% of the light is still in the portion of the exit pupil that's beyond 50% radius. And too, RCs are relatively slow, F/8 or so, achieving a large exit pupil requires long focal length eyepiece's, 7 mm exit pupil = 56 mm, no wide fields here.

 

The only time I see the donut effect is if I'm viewing during the day or viewing the moon, under these circumstances, the eye is not dilated and the diameter of the shadow of the CO approaches the diameter of my non dark adapted pupil.  

 

I suspect that if the views through particular RCs are substandard, something else is going on. Coming from the Newtonian world where a small CO is under 20% and 25% is considered the maximum acceptable, there's a certain irony in the SCT folks thinking 35%-40% is acceptable but larger.. we don't want that..

 

When faced with a question about the importance of the CO, a a qualitative feel can be gotten by just adding a paper CO to an existing CO.. I have done this for planetary.. it's surprising how little effect a 40% CO has on the planetary contrast of a refractor.. I was trying to figure out why the planetary views though a 120mm ED were so much more contrasty than a 127 Mak.

 

The 40% CO did affect the refractors contrast but it was small, something else was going on.

 

Jon



#13 AxelB

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Posted 22 October 2017 - 11:38 AM

If that form factor is a priority, for your intended purpose, get an 8" Edge hd, a 2" visual back/diagonal and a wide field eyepiece like a Nagler 31 or Pan 41. Keep the distance minimal between the back and the field stop.

There will be some vignetting but for visual it won't be troublesome.

Edited by AxelB, 22 October 2017 - 11:41 AM.


#14 Magnetic Field

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Posted 22 October 2017 - 12:08 PM

 

I've read many times that a large central obstruction (40~50%+) can affect viewing low contrast detail on planets.

 

I have an opportunity to purchase a GSO 8" Ritchey-Chretien for a good price, and was thinking to use it for DEEP SKY VISUAL ONLY

 

I realize that these were designed for astrophotography, but would it not be useful for this purpose, since it gives a native flat field, and wider maximum FOV than typical 8" SCTs (2000mm FL vs 1600FL)? I believe also that the field stop diameter is slightly larger in the GSO RC than most Meade/Celestron 8" SCTs, although I don't know how relevant that would be, perhaps not so much.

 

I was thinking it would offer other advantages too, such as having less cool down time and anti-dewing properties due to the open tube design.

 

I COULD get a reflector I know, but I really appreciate portability - I already have a sturdy high quality case for an 8" SCT-sized tube, which wouldn't fit a newtonian. Also I have an HEQ5 and I have read things get a little shaky with an 8" newt on them, what with them being longer and heavier... plus having to rotate the tube in the rings for eyepiece placement sounds annoying.

 

So, my original question - how much would a relatively large central obstruction (I believe 47% on the GSO 8" RC) affect DEEP SKY performance? (not in terms of light loss - I know that's minimal.) Are there any types of deep sky objects that it may negatively impact? (globulars, planetary nebulae?)

In terms of contrast, an 8" scope with 47% obstruction is equivalent to about a 4.24" unobstructed scope (Aperture - CO).   

 

 

 

The ,,D - CO" is a perpetuated myth:

 

https://web.archive....obstruction.htm


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#15 Richard Whalen

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Posted 22 October 2017 - 01:36 PM

More important is optical quality, collimation and conditions when viewing DSOs.



#16 WadeH237

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Posted 22 October 2017 - 04:39 PM

I don't normally observe visually through my 6" RC, but I needed to test a mount the other night.  I wanted to slew the mount around for a few minutes and it was forecast to rain the next morning.  My refractor was mounted on my imaging mount under cover.  I really wanted to keep it simple, since it was forecast to rain the next morning and I would have to haul in the new mount.  That left me the 6" RC and the 8" SCT as options.  Since it was cold and damp, the SCT would have collected dew instantly, so I used the RC.

 

The optics on the RC are pretty decent, and it is well collimated.  I looked at a few deep sky objects, but between poor transparency and some light pollution, it was not very satisfying although the images were sharp).  I ended up speding some time on carbon stars and double stars.  The main thing that I noticed with the RC is that diffraction rings are very apparent at higher magnifications.  When I split the Double-Double, for example, it was a clean split, but the diffraction rings from the close pairs intersected each other quite a bit.

 

All in all, if my primary usage would be visual, I would definitely opt for an SCT or a newt.  For imaging, the RC's are very good, as long as you can keep them collimated.


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#17 Jeffmar

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Posted 22 October 2017 - 06:16 PM

The best view I have ever had of Jupiter was through my C11 edge. The first thing I looked at when my scope was new was Jupiter. The detail and the contrast were amazing. For star clusters, nebulae, and galaxies it works even better. I have looked through a lot of scopes and the only ones that were better were not the expensive apochromatic refractors. It was only larger dobs and sct's that showed any improvement over my C11. The only time I have ever seen the dark circle in a Schmidt Cassegrain is with very low powers in bright skies. Central obstruction is secondary to aperture size. It just may be that the central obstruction fear is perpetuated by people who have never had a good reflector scope of any type or those who did not collimate their scopes well. The qualifiers to my opinion here is that I can't speak from experience about Ritchey-Chretien scopes and I have never looked through an 8 inch apochromatic refractor. They tend to be rare. Both my sct's have obstructions of about 35% give or take so that could be a qualifier also. 

 

Another thing to think about is there is no such thing as a large affordable apochromatic scope. It is an oxymoron. I wouldn't mind at all having a 7 inch apochromatic refractor but it still doesn't gather any more photons than a C8 or even maybe even the Ritchey-Chretien we are discussing on this thread. If seeing is very good and the site is very dark the difference between a equivalent size scopes with and without a central obstructions is not nearly as large as some might tell you. So there is my soap box statement for the daywink.gif

 

So far have have never heard of a comparison between refractors and reflectors that had impartial observers.


Edited by Jeffmar, 22 October 2017 - 06:18 PM.


#18 Richard Whalen

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Posted 23 October 2017 - 05:54 PM

I've read many times that a large central obstruction (40~50%+) can affect viewing low contrast detail on planets.

 

I have an opportunity to purchase a GSO 8" Ritchey-Chretien for a good price, and was thinking to use it for DEEP SKY VISUAL ONLY

 

I realize that these were designed for astrophotography, but would it not be useful for this purpose, since it gives a native flat field, and wider maximum FOV than typical 8" SCTs (2000mm FL vs 1600FL)? I believe also that the field stop diameter is slightly larger in the GSO RC than most Meade/Celestron 8" SCTs, although I don't know how relevant that would be, perhaps not so much.

 

I was thinking it would offer other advantages too, such as having less cool down time and anti-dewing properties due to the open tube design.

 

I COULD get a reflector I know, but I really appreciate portability - I already have a sturdy high quality case for an 8" SCT-sized tube, which wouldn't fit a newtonian. Also I have an HEQ5 and I have read things get a little shaky with an 8" newt on them, what with them being longer and heavier... plus having to rotate the tube in the rings for eyepiece placement sounds annoying.

 

So, my original question - how much would a relatively large central obstruction (I believe 47% on the GSO 8" RC) affect DEEP SKY performance? (not in terms of light loss - I know that's minimal.) Are there any types of deep sky objects that it may negatively impact? (globulars, planetary nebulae?)

Simon, if it is a great deal give it a try. If it does not work out you can always flip it.



#19 Patrick

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Posted 25 October 2017 - 09:46 AM

 

 

I've read many times that a large central obstruction (40~50%+) can affect viewing low contrast detail on planets.

 

I have an opportunity to purchase a GSO 8" Ritchey-Chretien for a good price, and was thinking to use it for DEEP SKY VISUAL ONLY

 

I realize that these were designed for astrophotography, but would it not be useful for this purpose, since it gives a native flat field, and wider maximum FOV than typical 8" SCTs (2000mm FL vs 1600FL)? I believe also that the field stop diameter is slightly larger in the GSO RC than most Meade/Celestron 8" SCTs, although I don't know how relevant that would be, perhaps not so much.

 

I was thinking it would offer other advantages too, such as having less cool down time and anti-dewing properties due to the open tube design.

 

I COULD get a reflector I know, but I really appreciate portability - I already have a sturdy high quality case for an 8" SCT-sized tube, which wouldn't fit a newtonian. Also I have an HEQ5 and I have read things get a little shaky with an 8" newt on them, what with them being longer and heavier... plus having to rotate the tube in the rings for eyepiece placement sounds annoying.

 

So, my original question - how much would a relatively large central obstruction (I believe 47% on the GSO 8" RC) affect DEEP SKY performance? (not in terms of light loss - I know that's minimal.) Are there any types of deep sky objects that it may negatively impact? (globulars, planetary nebulae?)

In terms of contrast, an 8" scope with 47% obstruction is equivalent to about a 4.24" unobstructed scope (Aperture - CO).   

 

 

 

The ,,D - CO" is a perpetuated myth:

 

https://web.archive....obstruction.htm

 

 Is THIS ARTICLE no longer relevant then?

 

Patrick


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#20 Jared

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Posted 25 October 2017 - 11:25 AM

Let's back up a little bit.  Yes, of course, RC's large central obstructions can be used visually.  They can even be quite effective for deep sky objects--anything where the power doesn't need to be too high (contrast transfer issues) or too low (shadow of the central obstruction).  Let's take that as a given.

 

Would any of you actually CHOOSE an 8" RC as a deep sky visual instrument? I wouldn't, unless the price was so good it allowed me, for example, to get an 8" RC over a 6" SCT for the same money.  It just wouldn't be my first choice.  If that's what I had access to, of course I would use it.  But deals on good used 8" SCT's are just too easy to find.  For this use, I'd rather have that.  And this is coming from someone who has owned a 10" RC, used it very effectively, and doesn't mind difficulties in collimation.



#21 MrJones

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Posted 25 October 2017 - 12:08 PM

So, my original question - how much would a relatively large central obstruction (I believe 47% on the GSO 8" RC) affect DEEP SKY performance? (not in terms of light loss - I know that's minimal.) Are there any types of deep sky objects that it may negatively impact? (globulars, planetary nebulae?)

I've been thinking the same for many of the same reasons. Kind of tired of correctors in dew and frost situations.

 

The big miss with most CN and other statements on this is context and comparison.

 

"Central obstructions decrease contrast". What you should always ask is, "as compared to what?" For the most part what you see is still related to area and not the % obstruction, especially for DSOs. And with SCT comparisons to anything else you always need to question the optical quality as well since SCTs are generally mediocre quality as compared to refractors and better Newts.

 

Anyway some C8 vs. RC8 with 6" refractor and 6" SCT comparisons to ponder. I did these as I am thinking of replacing my 6" SCT with a RC8 mostly for visual to start and wondering how the 6" refractor compares. Again, mostly for DSOs.

 

C8 - 203mm/70mm obstruction - 28500 mm^2 and 85% optical transmission

RC8 - 203mm/95mm obstruction - 26700 mm^2 and 98% optical transmission

6" achro - 152mm - 18100 mm^2

6" SCT - 152mm/60mm obstruction - 15300 mm^2

 

I found it VERY interesting that the 99% coated RC8 might actually make up in transmission what it loses in area to a C8. Also from these numbers the C8 and RC8 have enough area increase over a 6" refractor to maybe complement it rather than just compete.

 

A problem with the calculations is that I've seen one post somewhere where the person said the RC8 mirror is 195mm and it also seems unlikely that the C8 primary is a full 8".



#22 Jared

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Posted 25 October 2017 - 03:21 PM

Based on my experiences, for visual use on deep sky objects and looking at viewing experience only (not counting portability, viewing position/location of eyepiece, time to equilibrate, tracking and slewing capabilities, cost, dew susceptibility, etc.)  I would rank some typical scopes as follows:

 

* Typical high quality 8" Newt

* Typical high quality 6" apochromatic refractor

* Typical moderate quality 8" SCT (preferably ACF or Edge for low power views / flatness of field)

* Typical moderate quality 8" RC

 

All four scopes are going to be quite similar in terms of light grasp (all within about 40% of each other from the 8" Newt to the 6" refractor once one accounts for scatter, central obstructions, reflectivity coating efficiency, diagonals, etc..) But light grasp isn't the only factor here.  Nor is aperture/resolution.  This is just from the perspective of what views do I like the best and what shows me the most.  

 

A 40% difference in light grasp seems really big on the surface, but I think we all know that perceptually it's a logarithmic scale, so it will seem more like a 10-12% difference.  It might matter, but for me other factors would dominate.  I can't tell you for certain WHY an 8" RC is not as good a choice as an 8" SCT for deep sky.  Is it the larger central obstruction?  Typical optical quality being lower perhaps?  How they handle scattered light? Some subtleties in how tube currents manifest in closed tubes vs. open tubes?  I assume it is a combination of factors.  But I've spent enough time viewing through the classes of instruments above (except 8" RC's--I owned a 10" not an 8") and seen enough samples to be pretty confident about my own preferences.  

 

Now, which would I rather own?  That's an entirely different question.  To answer that, I'd need to incorporate all the factors I discarded at the beginning--cost, portability, tracking / mounting, etc..  



#23 Jon Isaacs

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Posted 25 October 2017 - 04:10 PM

 

 

 

I've read many times that a large central obstruction (40~50%+) can affect viewing low contrast detail on planets.

 

I have an opportunity to purchase a GSO 8" Ritchey-Chretien for a good price, and was thinking to use it for DEEP SKY VISUAL ONLY

 

I realize that these were designed for astrophotography, but would it not be useful for this purpose, since it gives a native flat field, and wider maximum FOV than typical 8" SCTs (2000mm FL vs 1600FL)? I believe also that the field stop diameter is slightly larger in the GSO RC than most Meade/Celestron 8" SCTs, although I don't know how relevant that would be, perhaps not so much.

 

I was thinking it would offer other advantages too, such as having less cool down time and anti-dewing properties due to the open tube design.

 

I COULD get a reflector I know, but I really appreciate portability - I already have a sturdy high quality case for an 8" SCT-sized tube, which wouldn't fit a newtonian. Also I have an HEQ5 and I have read things get a little shaky with an 8" newt on them, what with them being longer and heavier... plus having to rotate the tube in the rings for eyepiece placement sounds annoying.

 

So, my original question - how much would a relatively large central obstruction (I believe 47% on the GSO 8" RC) affect DEEP SKY performance? (not in terms of light loss - I know that's minimal.) Are there any types of deep sky objects that it may negatively impact? (globulars, planetary nebulae?)

In terms of contrast, an 8" scope with 47% obstruction is equivalent to about a 4.24" unobstructed scope (Aperture - CO).   

 

 

 

The ,,D - CO" is a perpetuated myth:

 

https://web.archive....obstruction.htm

 

 Is THIS ARTICLE no longer relevant then?

 

Patrick

 

Thierry's article is a good one.  However, he is discussing planetary contrast.. 

 

Jon



#24 freestar8n

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Posted 25 October 2017 - 04:40 PM

These discussions often focus on MTF - but MTF does not take into consideration - at all - the key things that determine what I think people really mean when they compare "contrast" - and that is scattering and baffling.

 

The overall background glow from imperfect baffling and scatter off spider vanes and the secondary are not included - at all - in MTF discussions - yet I believe they would be the primary factors when looking in a refractor compared to another scope - especially when looking at the moon or bright planet against a dark background.

 

A larger secondary in a cassegrain might be easier to baffle well and could actually improve what people tend to mean by "contrast."  Or it might end up worse.  But little of what matters here pertains to diffraction and MTF - unless you are looking at high frequency detail within Jupiter's belt.  That's where MTF and diffraction plays a role.

 

The separate issue of ergonomics and visually seeing a big hole in the middle of the field at low power is also a significant factor - and isn't part of what MTF talks about.

 

If you want wide field views with bright objects standing out against a black background - I would get a good refractor.  If you want high power views to show detail in jupiter's belt - I would get a large aperture cassegrain - as well baffled as possible - with good cooling and ventilation - etc. - so the optics can perform as well as MTF predicts.

 

Frank



#25 Redbetter

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Posted 25 October 2017 - 05:28 PM

The part that seems to apply to DSO's is that larger obstructions reduce contrast across the extended surface.  This is an inevitable result of the "encircled energy" being reduced several airy disk diameters away,  spreading over a wider angular area as a result of the obstruction.  On a point source it doesn't matter that much, but on an extended object you essentially have a large number of point sources spreading an increased fraction of their light over the other point sources.  This is the "soft" image effect for planetary views.  The details are there, but they are airbrushed. 

 

How would this impact DSO's?  Bright planetary nebula will obviously suffer in a way not unlike planets.   Higher surface brightness galaxies will have some of the same problems where dark lanes, gaps between arms, etc. are somewhat washed out.  What about moderate surface brightness arms and such?  These features' diameters are a small fraction of the size of the object.  A feature that has adjacent  5-10 arc second brighter and darker areas is losing considerable contrast on both sides at this scale from what I can see.  Detecting small scale spiral structure or dust lanes at high power is already threshold and is impacted by seeing.  While folks claim that seeing doesn't impact DSO's, that is really only true at lower powers.  Once you get in the upper mid or higher power range required to see structure in many galaxies (or to even detect faint companions) seeing has much more impact.  And if seeing is impacting the view, then so is obstruction.

 

What happens to detection of very low contrast large DSO's where one is trying to detect subtle gradients?   Sculptor, Fornax, Ursa Minor and Draco dwarf galaxies come to mind.  I don't know.  I haven't tried viewing them with  35 or 40% obstruction in my big scope...although that might be interesting.  The scale is likely so large that it has little impact other than a slight dimming of the field. 

 

Instead what is likely to be more noticeable is the effect of increased exit pupil when comparing fast and slow scopes.  Even with only 6.2mm exit pupil on the Pleiades I see a large amount of nebulosity widely surrounding the cluster--as in Mel's sketch, but only in small chunks panning with the 20" with nominal ~16% obstruction.  This is true even in mediocre transparency.  What is missing is enough field to keep it in context.  Same with Barnard's loop, etc.  Mel's approach has been to take a max apparent field eyepiece and marry it to max exit pupil views for max true field.  This combines the most possible aperture with the brightest and widest field.




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