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8" F/5 Newt palnetary and coma

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#1 JP-Astro

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Posted 02 July 2019 - 09:37 AM

I have a quick (hope so) question on a SW Explorer 200P which is an F/5 1000mm reflector with a parabolic mirror.

First - how will it compare to an avearge 8" NexStar SCT orange tube in terms of planetary performance on higher mags, say at up to 300x when seeing allows it? Will they approximately match in the level of contrast and detail and which of them would show a sharper image for visual?

Second question - can I get away without using a coma corrector such as Paracorr on this F/5 Newt for visual usage? I don't know what parabolic mirror by itself offers in terms of coma corection since I never owned a Newt of any kind.


Edited by JP-Astro, 02 July 2019 - 10:45 AM.


#2 happylimpet

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Posted 02 July 2019 - 09:49 AM

I suspect it'll deliver a fine image, assuming well collimated and at thermal equilibrium (probably using a fan behind the mirror). I have a 300p and its a peach. No need for a paracorr for planetary as you'll just put the planet in the central sweet spot, unless youre using it on a dob mount when you'll want it to stay sharp while drifting.

 

It should deliver slightly higher contrast than an SCT due to the central obstruction being smaller, all else being equal(!).


Edited by happylimpet, 02 July 2019 - 09:49 AM.

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#3 nirvanix

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Posted 02 July 2019 - 10:37 AM

I don't use a Paracorr with my f/5 dob and neither do quite a few others. Perhaps some folks are more sensitive to coma and choose to use one.

 

My 10" delivers very high contrast views as would any quality optic.


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#4 Jon Isaacs

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Posted 02 July 2019 - 10:56 AM

I don't use a Paracorr with my f/5 dob and neither do quite a few others. Perhaps some folks are more sensitive to coma and choose to use one.

 

My 10" delivers very high contrast views as would any quality optic.

 

From my point of view, it's not that I'm sensitive to coma. Rather the image is sensitive to coma.  

 

At F/5, the coma free region is 2.75 mm in diameter.  That's the region where a perfect mirror has degraded because of coma to being  diffraction limited. In an 8 inch F/5, that's an AFoV of 31 degrees at 200x. In a 10 inch F/5, that's 25 degrees AFoV at 200x.

 

Without a coma corrector, the loss of contrast and crispness is apparent. With a driven mount, the planet can be kept centered, with a manual Dob mount, the coma corrector provides a much wider "sweet spot."

 

Jon


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#5 JP-Astro

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Posted 02 July 2019 - 12:33 PM

Adda a few more thoughts on 8" F/5 Newt vs regular 8" SCT, please - in terms of hight power planetary performance.



#6 Vic Menard

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Posted 02 July 2019 - 12:48 PM

As Jon noted above, the coma "free" field diameter in an f/5 Newt is about 3mm. If you're using a 4mm Plossl with a 50-degree afov, the field stop diameter will be approximately 3mm, so the entire field will be essentially coma "free". This is about 250X for your 8-inch Newt, which should be a good planetary magnification.

 

Compared to a "regular" 8-inch SCT, if the Newtonian optics are top notch and you're using good eyepieces, I would expect the Newtonian to deliver slightly better planetary image performance. That said, each scope will have its own pros and cons that may, or may not, be a deal breaker--and planetary image performance may not be high on that list (of pros and cons).


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#7 macdonjh

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Posted 02 July 2019 - 12:50 PM

Adda a few more thoughts on 8" F/5 Newt vs regular 8" SCT, please - in terms of hight power planetary performance.

First, +1 to Jon Isaacs.  Coma correctors aren't needed if you're happy viewing the center of the field of view.

 

As far as comparing a Newtonian to an SCT...  All things being equal I doubt you'd see a difference.  By all things being equal I mean:

  • Are both scopes properly collimated? (kind of a draw, both designs are easy to collimate)
  • Are both scopes properly cooled/ acclimated? (I think the Newtonian wins here)
  • Are both scopes dew-free? (the Newtonian wins here)
  • Are you in a comfortable position to observe? (the NexStar SCT may win unless the Newtonian is mounted with rotating rings or on a Dobsonian/ alt-az mount)

I'm sure there are considerations I've not listed but you get the idea.  Each scope design has aspects you need to take into account before and while you observe.  If used properly, each will provide good views.


Edited by macdonjh, 02 July 2019 - 12:53 PM.

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#8 JP-Astro

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Posted 02 July 2019 - 03:11 PM

Thank you for your additional comments. I was worried a little about the Newt F/5 performance at higher powers because I often encountered comments that an SCT with its longer F/10 focal ratio would be a sharper scope compared to a short F/5 Newt. According to the comments in this thread I understand they should be at least on par which is good news for me.



#9 CHASLX200

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Posted 02 July 2019 - 06:10 PM

A Paracorr lives in any Newt faster than F/5.6 that i use. Coma drives me nuts like FC does with fast APO's.


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#10 CHASLX200

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Posted 02 July 2019 - 06:12 PM

Thank you for your additional comments. I was worried a little about the Newt F/5 performance at higher powers because I often encountered comments that an SCT with its longer F/10 focal ratio would be a sharper scope compared to a short F/5 Newt. According to the comments in this thread I understand they should be at least on par which is good news for me.

9 out of 10 times a super well made 8" Newt, at any speed from F/8 to even a super fast F/3.5 will beat out the same size SCT time and time again.


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#11 Starman1

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Posted 02 July 2019 - 06:31 PM

9 out of 10 times a super well made 8" Newt, at any speed from F/8 to even a super fast F/3.5 will beat out the same size SCT time and time again.

Yes. waytogo.gif


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#12 doug mc

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Posted 02 July 2019 - 07:35 PM

For planetary viewing a good barlow should expand the viewing area. Planetary eyepieces with a barlow lens as part of the design will do the same.
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#13 Richard Whalen

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Posted 02 July 2019 - 09:54 PM

All of this assumes a well made newtonian. Is the scope the OP mentioned fall into this catagory? I have not used one, cant say. But I bet many here would not consider it a premium newtonian with premim optic? I agree a premium newtonian should do well against your average SCT, but not sure about a mass produced one doing any better than a new 8" SCT. Jump up to a edge and you might find it a better scope for the planets for several reasons.

 

1. Longer focal length, use of more comfortable eyepieces with minimal optical elements.

2. Comfort observing

3. Slightly less affected by body heat when using a dew shield. Also higher off the ground.

4. No spider/diffraction spikes.

5. No paracor required 

6. Should not have to be collimated as often.

 

I find most newtonian owners who are avid observers are tinkerers and need to be to get the best out of them. Many are ATM's and have wonderful scopes which they built or have modified. At the same time I have seen many beginners buy a mass produced dob and after struggling with collimation and other issues give up and leave the hobby. 

 

I have had several newtonians over the years, both dobs and gem mounted. Both custom and massed produced. While they can have great views, it usually takes some work and experience to achive them even when they are optical good or better. Thats why I rarely recommend a mass produced newtoinian as a first scope. And with SCTs getting more consistent, and the reasons from 1-6, a SCT might be a better first scope for planetary observing. 


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#14 JP-Astro

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Posted 03 July 2019 - 10:43 AM

Oh, of course we are not talking about a 'premium' Newt as I'm not going to pay a premium for it. Just an average SW 8" F/5 Newt provided it's made up to its specs . Neither do we talk about a premiun SCT. Just an average mass produced 8" F/10 SCT.

I think the comparison should be fair then.



#15 Richard Whalen

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Posted 03 July 2019 - 12:25 PM

Oh, of course we are not talking about a 'premium' Newt as I'm not going to pay a premium for it. Just an average SW 8" F/5 Newt provided it's made up to its specs . Neither do we talk about a premiun SCT. Just an average mass produced 8" F/10 SCT.

I think the comparison should be fair then.

I understand. My point being that many here use premium newtonians or modified ones or have the experience to get the most out of a stock one and come to their perspective from that experience which would be different from a first time newtonian user. Experience counts for a lot and makes everything easier. If your new to the hobby I would recommend the SCT unless you like to tinker and have a lot of patience. If this is not your first decent scope by all means go for the newtonian if that is what you think will float your boat. At the price point you are looking at optical quality is somewhat the luck of the draw.


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#16 CHASLX200

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Posted 03 July 2019 - 05:58 PM

Most mass produced Newts the last few years don't come close to the old school F/6's and 8's and or the Zambuto Newts i have owned.


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#17 JP-Astro

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Posted 03 July 2019 - 06:47 PM

Most mass produced Newts the last few years don't come close to the old school F/6's and 8's and or the Zambuto Newts i have owned.

Often comments regarding the recent mass produced SCTs go in a different direction: many suggest that their quality has improved dramatically.



#18 CHASLX200

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Posted 03 July 2019 - 06:59 PM

Often comments regarding the recent mass produced SCTs go in a different direction: many suggest that their quality has improved dramatically.

I don't think they have that much. I just had the worst C8 i ever had made in 2005.  


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#19 Mel M

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Posted 03 July 2019 - 08:49 PM

Some support for the basic newt.

The average person does need somebody to help them when they start. Collimation and scope set-up is much easier if somebody helps the beginner. Yep, some that refuse to get help could struggle so much they would get discouraged.

I use eyepieces with more elements and think they compare very well with those using fewer elements.

Scopes need the wind or a fan to insure that air moves into our body. At times I thought heat from my head was easy to see.

No spikes is nice though a newt can have less obstruction.

The paracor is always in and collimation checked every use!

Is it easier to have a newt of higher quality than it is to have a cat of higher quality? Maybe not.


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#20 macdonjh

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Posted 03 July 2019 - 10:23 PM

Oh, of course we are not talking about a 'premium' Newt as I'm not going to pay a premium for it. Just an average SW 8" F/5 Newt provided it's made up to its specs . Neither do we talk about a premiun SCT. Just an average mass produced 8" F/10 SCT.

I think the comparison should be fair then.

For a bit of perspective, my son and I used an Orion XT6i 6" f/8 Newtonian for a couple of years.  It was sharp enough when well collimated (which it always was, my ten year old son thought it a badge of honor he could do it himself) to give good views at 300x when the atmosphere cooperated.  I'm sure a Zambuto primary and Antares secondary would have been sharper, but I didn't feel the need to try it with the stock mirrors.


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#21 Jon Isaacs

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Posted 04 July 2019 - 07:05 AM

I understand. My point being that many here use premium newtonians or modified ones or have the experience to get the most out of a stock one and come to their perspective from that experience which would be different from a first time newtonian user. Experience counts for a lot and makes everything easier. If your new to the hobby I would recommend the SCT unless you like to tinker and have a lot of patience. If this is not your first decent scope by all means go for the newtonian if that is what you think will float your boat. At the price point you are looking at optical quality is somewhat the luck of the draw.

 

I think of it somewhat differently.. When I recommend a telescope, it's to try to match the individual to the scope.  

 

- If you don't like to tinker, if you prefer a scope that is what it is, that can't be improved, then the SCT is probably a better choice.  SCTs are essentially black boxes, they are what they are.  

 

- If you are a hands on type that enjoys being engaged with your equipment, like the fact that you can make a difference in it's performance, then the Newtonian is probably a better bet.  

 

And regardless of the type of telescope someone chooses, patience is probably the number one personal trait necessary for an amateur astronomer.  One must be patient, willing to wait, it might be waiting for the clouds to clear or the SCT to finally reach thermal equilibrium or any number of things..

 

Jon


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#22 Auburn80

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Posted 04 July 2019 - 11:28 AM

Jon, I disagree a bit with one of your points. IMHO,the most "black box" and simplest to use is a refractor. Typically no collimation and very little acclimation needed under many circumstances. Of course they don't scale well and can get very expensive.
I'm in the camp of suggesting a Newt as a first scope. Very good aperture for lower $ and simple tools available to collimate. One doesnt have to upgrade mirrors or spiders or focusers until experience is gained. And if that's not a user's cup o tea, that's OK.

Edited by Auburn80, 04 July 2019 - 11:30 AM.

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#23 CrazyPanda

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Posted 04 July 2019 - 11:39 AM



Oh, of course we are not talking about a 'premium' Newt as I'm not going to pay a premium for it. Just an average SW 8" F/5 Newt provided it's made up to its specs . Neither do we talk about a premiun SCT. Just an average mass produced 8" F/10 SCT.

I think the comparison should be fair then.

The difference in views between two non-premium scopes (one a newt, one an SCT) that are both:

 

* Diffraction limited

* Thermally acclimated

* Properly collimated

* Driven to track the sky

 

will be essentially indistinguishable. If you have superb seeing, you will notice a slight advantage in contrast of very, very subtle features in the Newt thanks to the smaller central obstruction (which will be around 25% vs the 35% in the SCT), but this difference will require some keen observing experience to see.

 

If the Newt is undriven, a driven SCT will blow it out of the water. Not because of reduced coma mind you, but because in my experience tracking adds the equivalent of about 2" of aperture. You can stare at the planet for extended periods of time, catching fleeting moments of steady air, or really being able to concentrate on seeing subtle features without breaking concentration to re-position the object.

 

IMO, a motor drive on an EQ mounted scope is a must have if you want to take true advantage of the design of the EQ mount.


Edited by CrazyPanda, 04 July 2019 - 11:40 AM.

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#24 Starman1

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Posted 04 July 2019 - 12:01 PM

If the Newt is undriven, a driven SCT will blow it out of the water. Not because of reduced coma mind you, but because in my experience tracking adds the equivalent of about 2" of aperture. You can stare at the planet for extended periods of time, catching fleeting moments of steady air, or really being able to concentrate on seeing subtle features without breaking concentration to re-position the object.

 

At 304x, I'm only nudging my scope every minute and a half, while watching the planet cross the field from one side to the other.

I haven't found that to make a difference in what I can see in terms of small details on either the moon or the planets or other targets compared to tracking scopes.

In some cases (and I've seen it), the reverse occurs because some mounts do not track 100% smoothly at 300-500x.  

Of course, that presumes that there is no coma, the eyepiece is in sharp focus from edge to edge, and that it has a very wide field, all circumstances that pertain in my case.

 

One can obtain a newtonian with premium optics commercially.  But one cannot obtain an SCT with premium optics commercially.

 

And, it depends on aperture.  I can't imagine the difficulty with cooling a 12" SCT, whereas cooling my 12.5" newt is easy.

That, probably as much as optical quality, is why I've never seen really pinpoint star images in any SCT above 8".


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#25 Richard Whalen

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Posted 04 July 2019 - 02:19 PM

At 304x, I'm only nudging my scope every minute and a half, while watching the planet cross the field from one side to the other.

I haven't found that to make a difference in what I can see in terms of small details on either the moon or the planets or other targets compared to tracking scopes.

In some cases (and I've seen it), the reverse occurs because some mounts do not track 100% smoothly at 300-500x.  

Of course, that presumes that there is no coma, the eyepiece is in sharp focus from edge to edge, and that it has a very wide field, all circumstances that pertain in my case.

 

One can obtain a newtonian with premium optics commercially.  But one cannot obtain an SCT with premium optics commercially.

 

And, it depends on aperture.  I can't imagine the difficulty with cooling a 12" SCT, whereas cooling my 12.5" newt is easy.

That, probably as much as optical quality, is why I've never seen really pinpoint star images in any SCT above 8".

 

Hmmm, with all my Newtonians the best planetary view was when the planet was dead center in the fov. Was very noticable difference from the edge of the fov. Even in my f8 and f6 scopes. In my 18" f4.2  best view was also in the center without a Paracorr, with a Paracorr (both type 1 and 2) it  had a hair less contrast and sharpness in the center, though much better towards the edge than without. 

 

For planetary viewing I find smooth tracking invaluable, more so at higher magnifications. 


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