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8" F/5 Newt palnetary and coma

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#26 Mike Spooner

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Posted 04 July 2019 - 02:25 PM

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.

Quick answer. They'll likely show approximately the same views. 

I've gotten away without a coma corrector after using Newts for 50 years.

 

Longer answer. Seeing, optical quality and the user's expertise may contribute to differences that can only be determined case by case, not as a general rule. 

 

Mike Spooner 


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#27 Jon Isaacs

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Posted 04 July 2019 - 03:30 PM

Quick answer. They'll likely show approximately the same views. 

I've gotten away without a coma corrector after using Newts for 50 years.

 

Longer answer. Seeing, optical quality and the user's expertise may contribute to differences that can only be determined case by case, not as a general rule. 

 

Mike Spooner 

 

Mike:

 

Have you ever used a Paracorr?

 

Jon


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#28 Starman1

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Posted 04 July 2019 - 05:27 PM

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. 

I can see that.

Very few eyepieces have no astigmatism to the edge of the field, and scopes of f/6 and f/8 still have coma at the edge of a field.

I'll post a revealing graph that accompanied the original Paracorr in a second.

But what you can see if coma is eliminated and there is no astigmatism in the image is the core of a globular cluster, fully-resolved into a myriad of infinitesimally small pinpoints,

stay fully resolved as it leaves the edge of the field on a 100° eyepiece.

Having seen what coma correction does to the image quality, I'd use one at f/8 these days.

Attached Thumbnails

  • Paracorr I Graphs.jpg

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#29 Starman1

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Posted 04 July 2019 - 05:31 PM

And here is the same chart for the Paracorr II:

http://televue.com/p...2_spotsizes.pdf

 


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#30 JP-Astro

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Posted 04 July 2019 - 07:57 PM

Thank you everyone for your useful advice and interesting discussion! I learn a lot through reading your replies.



#31 Jon Isaacs

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Posted 04 July 2019 - 08:12 PM

Thank you everyone for your useful advice and interesting discussion! I learn a lot through reading your replies.

 

Just to add a bit more.. today, the typical 8 inch F/5 rides on an EQ mount, most likely an AVX.   As long as the mount is reasonably well aligned and tracks reasonably well, coma in planetary images should not be an issue.  

 

My friend Jack has a Celestron 8 inch F/5 mounted on an AVX.  It seems to work to the seeing limit.  

 

To Richard's point, Tinker's versus Hands Off. 

 

The first time Jack showed up at my place, the scope was quite difficult to use.  The stock focuser is a rack and pinion and I gave it my normal rework and replaced the pads with Teflon. It was better but still not good.  Rotating the tube for comfort was difficult and risked the tube slipping and/or the mount losing alignment.  Viewing Jupiter with the eyepiece pointed straight at the ground is not optimal.  

 

Jack is a tinkerer type, a hands on guy his entire life.  Over the period of about a month, Jack had modified the scope with a hose clamp and part of 5 gallon paint bucket and a little grinding on the rings so now the scope rotated in the rings smoothly and without risking losing alignment.  He also added a sealed back fan to cool the scope.  The third thing was a better focuser which we installed together.  

 

The difference those three modifications make is remarkable.  The scope went from being.. "It's too awkward and clunky, I don't want to bother looking through it" to "Wow, this is really nice, comfortable and it provides some awesome views."  It was a total transformation.. 

 

Jack has the GSO ComaCorrector.  With my 35mm Panoptic, it provides a 2.0 degree TFoV and some beautiful wide field views of the Veil and many of the deep sky wonders of the summer skies.  The North American Nebula is quite stunning, it's enough aperture to provide a very bright image for use with a UHC filter with enough magnification to see some real detail.  

 

That won't be happening with an 8 inch SCT.

 

GSO Two Speed on Jack's Newtonian.jpg
 
Jon

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#32 Mike Spooner

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Posted 04 July 2019 - 10:50 PM

Mike:

 

Have you ever used a Paracorr?

 

Jon

Hi Jon,

 

I only tried one time in a 16" F/4 but to be fair I didn't have it adjusted for the eyepieces and the central FOV suffered at high power on Jupiter. I do have a coma free scope, the 9.8" f/4.6 L-H. The scope pretty much held its own side by side with a superb 8" APO (again on Jupiter) so it's decent quality. I'm pretty sure I'm not too sensitive to coma (but then my favorite eyepieces are 3-5mm fl) and I usually keep the object centered for the most part. I am going to get a Paracorr as I have plans for a large sub f/4 scope. Perhaps I'll be converted.smile.gif  Probably a case of not knowing what I've been missing. BTW, I have a 12.5" DK with a very fast primary and even at 400+ magnification coma limits the FOV to a few degrees.

 

Mike


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#33 izar187

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Posted 05 July 2019 - 05:15 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.

It's kinda apples and oranges IME.

My older alt-az pedestal mount Taiwanese 8" f5 is not equal an 8" SCT mounted to track, not for planets.

Mine is too heavily obstructed, comatically aberrated off axis, and hand nudge.

It is a very nice compact DSO scope for me...  which I seldom waste time on planets with.

 

I agree with others, that one that is tweeked for it, and optimally mounted, can make a good all around 8".

Coma corrected, actively cooled, optimally obstructed, well mounted.

It would best an 8" SCT on widest field in view.

 

But I have seen some dandy planets in other peoples 8" fork mounted SCT's.

Without newtonian equatorial mounted ergonomics and workarounds.

 

Both are compromised IMHO, and require different countermeasures.


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#34 N3p

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Posted 05 July 2019 - 06:43 AM

I have a 200 BKP which is similar, I don't find it capable for using 300x for the planets or the moon. I had good results once using a 7mm orthoscopic and a 2x barlow giving at 286x and a 0.7mm exit pupil on the moon. Otherwise the images are simply too dark for my taste.

 

I think a bigger Newtonian would be more suited for powers like 300x, like a 12".


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#35 JP-Astro

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Posted 05 July 2019 - 08:44 AM

I have a 200 BKP which is similar, I don't find it capable for using 300x for the planets or the moon. I had good results once using a 7mm orthoscopic and a 2x barlow giving at 286x and a 0.7mm exit pupil on the moon. Otherwise the images are simply too dark for my taste.

 

I think a bigger Newtonian would be more suited for powers like 300x, like a 12".

That's interesting in the sense that you find in limiting at high powers not because of its fast F/5 design but because of the light collection ability. In this sense won't a typical 8" SCT have exactly the same brightness limitations at 300x when compared side by side with an F/5 Newt? Of course the eyepieces will be different but an overall effect will equalize them in terms of image brightness at 300x, won't it?


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#36 Asbytec

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Posted 05 July 2019 - 08:55 AM

That's interesting in the sense that you find in limiting at high powers not because of its fast F/5 design but because of the light collection ability. In this sense won't a typical 8" SCT have exactly the same brightness limitations at 300x when compared side by side with an F/5 Newt? Of course the eyepieces will be different but an overall effect will equalize them in terms of image brightness at 300x, won't it?

There likely will be a slight difference in throughput between an SCT with a lens and three mirrors and a Newt with two mirrors and depending on the reflective and fully multi coatings of each. The Newt should be a bit brighter, if we can see the difference at all. My only comparison is between different apertures, but I believe Jupiter is a little brighter in my Newt at the same exit pupil and smaller. 

 

Would a coma corrector work on an SCT or are they specific to faster focal ratios? 


Edited by Asbytec, 05 July 2019 - 09:08 AM.

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#37 Starman1

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Posted 05 July 2019 - 10:28 AM

There likely will be a slight difference in throughput between an SCT with a lens and three mirrors and a Newt with two mirrors and depending on the reflective and fully multi coatings of each. The Newt should be a bit brighter, if we can see the difference at all. My only comparison is between different apertures, but I believe Jupiter is a little brighter in my Newt at the same exit pupil and smaller. 

 

Would a coma corrector work on an SCT or are they specific to faster focal ratios? 

There is coma in the uncorrected SCT design, but the coma correctors out there are designed for newtonians, so not appropriate for SCT use.

 

The SCT also has a curved focal plane, so stars at the edge have the additional issue of being out of focus.

The focal reducers out there for SCTs also flatten the fields, so result in better star images in the outer field.

 

One other note: though the parent f/ratio of most SCTs is f/10 or f/11, they behave more like f/5-f/6 in terms of adding to the astigmatism in some eyepieces that are

not well corrected for short f/ratios.  SCTs do not behave with eyepieces the way an f/10 newtonian would.  SCTs benefit from eyepieces corrected for shorter f/ratios.

 

Current corrected SCTs, called ACF or Edge HD have flatter fields, fully-corrected spherical aberration, and no coma.  They are closer to the ideal of the aplanatic SCT.


Edited by Starman1, 05 July 2019 - 10:29 AM.

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#38 kathyastro

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Posted 05 July 2019 - 10:51 AM

Regarding the 200mm Newt vs. 200mm SCT comparison, you are not going to notice a big difference.  The magnification limits are the same, the light-collecting power is the same, so comparing at equal magnifications, you won't see much difference.

 

At f/5, you will benefit from a coma corrector, but not as much as you would at f/4.  (I have your scope's f/4 little brother.)  More noticeable will be eyepiece selection.  With an eyepiece that is well-matched to the scope, I can view at f/4 without the coma corrector and have quite pleasing views.  With a poorly-matched eyepiece, even a coma corrector won't help. 

 

Eyepieces that are unsuitable for a fast Newt include any *70 models, such as Orion Q70 or Antares W70.  The field curvature goes the wrong way for a Newt on these models. 


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#39 JP-Astro

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Posted 05 July 2019 - 11:56 AM

Regarding the 200mm Newt vs. 200mm SCT comparison, you are not going to notice a big difference.  The magnification limits are the same, the light-collecting power is the same, so comparing at equal magnifications, you won't see much difference.

 

At f/5, you will benefit from a coma corrector, but not as much as you would at f/4.  (I have your scope's f/4 little brother.)  More noticeable will be eyepiece selection.  With an eyepiece that is well-matched to the scope, I can view at f/4 without the coma corrector and have quite pleasing views.  With a poorly-matched eyepiece, even a coma corrector won't help. 

 

Eyepieces that are unsuitable for a fast Newt include any *70 models, such as Orion Q70 or Antares W70.  The field curvature goes the wrong way for a Newt on these models. 

Tell me please then what the most desirable eyepieces yet on a budget are for the fast F/5 Newt. I'm sure this is an unrealistic request (great and inexpensive) but I have to exaggerate it so that I know of at least some decent ones that won't break the bank. I have a few SW Planetary ones (4 and 7 mm) that as far as I know previously were sold under different brands.



#40 kathyastro

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Posted 05 July 2019 - 12:10 PM

Tell me please then what the most desirable eyepieces yet on a budget are for the fast F/5 Newt. I'm sure this is an unrealistic request (great and inexpensive) but I have to exaggerate it so that I know of at least some decent ones that won't break the bank. I have a few SW Planetary ones (4 and 7 mm) that as far as I know previously were sold under different brands.

I don't have a lot of experience with different eyepieces.  (Full disclosure: 95% of what I do is AP, so I don't have a lot of experience with eyepieces at all!)  The hands-down winner in my eyepiece case is my Panoptic 27mm.  It is my wide-field eyepiece for the f/4 Newt, and the views are jaw-dropping.  Might not be considered "budget", though.  You won't go wrong with Plossls. 

 

I still have my W70s for high magnification.  The targets for them tend to be so small that I ignore the grotty outer edges anyway.


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#41 Starman1

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Posted 05 July 2019 - 01:22 PM

Tell me please then what the most desirable eyepieces yet on a budget are for the fast F/5 Newt. I'm sure this is an unrealistic request (great and inexpensive) but I have to exaggerate it so that I know of at least some decent ones that won't break the bank. I have a few SW Planetary ones (4 and 7 mm) that as far as I know previously were sold under different brands.

Well corrected at fast f/ratios; inexpensive; wide field.  Pick any 2.

In other words, stick to Plössls, and you can get good eyepieces for f/5 very inexpensively.

But, expand to widefields, ultrawides, or hyperwides, and the prices will take large leaps.

One of the best middle-ground solutions is a set of 60° eyepieces.  Most are well under $100 each, and you will find correction OK at f/5.

Examples: Astrotech Paradigm, BST Starguider ED, Meade HD60, Celestron X-Cel LX, etc.


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#42 eklf

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Posted 05 July 2019 - 01:25 PM

Tell me please then what the most desirable eyepieces yet on a budget are for the fast F/5 Newt. I'm sure this is an unrealistic request (great and inexpensive) but I have to exaggerate it so that I know of at least some decent ones that won't break the bank. I have a few SW Planetary ones (4 and 7 mm) that as far as I know previously were sold under different brands.

On the used market, an ES 68deg 28mm is a very good performer for apx $120


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#43 N3p

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Posted 05 July 2019 - 06:15 PM

I agree the 28mm is very nice with F5 and F4.7, the ES

 

That's interesting in the sense that you find in limiting at high powers not because of its fast F/5 design but because of the light collection ability. In this sense won't a typical 8" SCT have exactly the same brightness limitations at 300x when compared side by side with an F/5 Newt? Of course the eyepieces will be different but an overall effect will equalize them in terms of image brightness at 300x, won't it?

 

If your goal is to look at the planet in priority, I read a couple time that the SCT is an instrument well suited for high power planetary observation, I think it would be the logical choice over a F5 Newtonian. I also read last week that if you get a STC you should get an Edge with superior optics, it's supposed to correct a good amount of the nasty aberration visible in the regular C8. (there could be a debate.. I was convinced by the Edge team)

 

200p I like mine, I think it's a medium good quality telescope, not high and not low. I like to look at the planets and my best power to do that is with a 6mm orthoscopic eyepiece giving 166x 1.2mm exit pupil. Often, the views are great up to 212x with a ES 4.7mm 82d but 166x is my most used high power  EP for the moon and the planets, the images are bright, the contrast is good.

 

The ES68 35mm, 28mm, and 24mm are also good also.. I like all 3 of them very much with F5 Newtonian.


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#44 JP-Astro

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Posted 05 July 2019 - 07:09 PM

>> If your goal is to look at the planet in priority, I read a couple time that the SCT is an instrument well suited for high power 

>> planetary observation, I think it would be the logical choice over a F5 Newtonian.

 

I perfectly understand that most of the time seeing is the limiting factor and it's rarely that powers above 250x can be used with good results in many World locations. I've read on stargazerslounge which is predominantly a UK astro community site that 250x is a typical limit for them.

So 300x is not my ultimate goal but I wanted to know nevertheless what I was loosing in terms of the maximum theoretical power when getting a SW 8" 200P F/5 instead of a Celestron 8" SE SCT.

As far as I understand if the image is getting too dim at 300x in the 8" Newt there is absolutely no reason it would be brighter in the same 8" aperture SCT. Am I wrong?

 

Also I'm not a Newtonian planetary maniac and I perfectly understand there are better scope designs specifically for planet observations but again I wanted to know how this particular Newt would compete with a regular SCT of the same aperture size on planets. I could live with all the inconveniences offered by a Newt for the reasons I reveal below.

 

To tell you the truth I have no financial capacity for getting the 8" Edge or ACF SCT version. Locally I can get a new SW 8" 200P F/5 at the 1/3-rd of the price of a new Celestron 8" Nexstar SCT. SCTs are hugely overpriced over here.

Hence you can probably see where all my requests on comparing these 2 scopes are coming from. Honestly saying, I cannot afford myself even the regular Celestron 8" Nexstar SCT but I still can grab a new SW 8" 200P F/5 on the EQ5 right now.

Now you can probably see even better my situation. Never mind, I'm fine with that but I still wanted to know if my choice would be good and reasonable.


Edited by JP-Astro, 05 July 2019 - 07:25 PM.


#45 kathyastro

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Posted 05 July 2019 - 07:48 PM


So 300x is not my ultimate goal but I wanted to know nevertheless what I was loosing in terms of the maximum theoretical power when getting a SW 8" 200P F/5 instead of a Celestron 8" SE SCT.

As far as I understand if the image is getting too dim at 300x in the 8" Newt there is absolutely no reason it would be brighter in the same 8" aperture SCT. Am I wrong?

As noted above, there will be no significant difference between an 8" Newt and an 8" SCT.  Same light-gathering power, same maximum resolution, same magnification range.  All those things depend on aperture alone.

 

Maximum magnification ( even maximum theoretical magnification) depends on whom you talk to.  To marketers, it is always 2x the aperture in millimetres.  To observers, it is 1x the aperture in millimetres, except on very rare occasions of perfect seeing when you might be able to push it to 2x the aperture, but not more than 400x.  Everyone has their own version of these rules.  But they are all based on aperture, so there is no difference between those scopes.

 

Comparing equal magnifications, the image brightness will be the same.


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#46 N3p

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Posted 05 July 2019 - 08:10 PM

Well the 200p I think is a decent choice, I am very happy with mine and it gives me great views for the planets and DSOs. I like the fact that it's short and gives wide views of 2 degrees true field of view. That's what I like the most about the telescope, it's low power capabilities.

 

For me it's a lower power instrument but it will also provide impressive views of the planets and the moon at an affordable price. With a RA clock drive on the EQ5 you get very precise tracking which is much more then a luxury for me, especially for planetary observation.

 

I am building a Dobson base for my tube, not to replace the EQ mount but to get another type of viewing setup. Learning how to use the EQ mount is rewarding, it's an impressive and reliable instrument with tracking capabilities.


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#47 jtsenghas

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Posted 05 July 2019 - 09:24 PM

One of the best middle-ground solutions is a set of 60° eyepieces.  Most are well under $100 each, and you will find correction OK at f/5.

Examples: Astrotech Paradigm, BST Starguider ED, Meade HD60, Celestron X-Cel LX, etc.

Yes, I was about to chime in regarding the X-Cel LX series to for your criteria, and that is what my brother-in-law is starting to accumulate for his classic 8" SCT.

 

These eyepieces are all 60 degree AFOV and about 19 or 20 mm eye relief, and quite uniform in weight at about 8 ounce each. They were my "good" eyepieces for several years until I started to accumulate 82 degree ones. They cost about $75 each and are pretty darn good for the money in my opinion. I REALLY like the adjustable height firm eyecups on them when observing without glasses. If you were to start with just one at the focal length of the SCT I'd recommend the 25 mm, or an 9 mm or 12 mm for the newt. . 


Edited by jtsenghas, 06 July 2019 - 07:49 AM.

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#48 Asbytec

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Posted 05 July 2019 - 09:27 PM

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

Jon, et al, I'll look it up, but how does coma in an SCT compare? Larger diffraction limited area? Smaller? Same?

 

I agree with you, even at f/6 I can tell when the image leaves the coma sweet spot. It's simply more crisp and better resolved when the image is within the diffraction limited area. I noticed this one night observing the moon under descent seeing, rimae and small craters are more readily visible near center and much less so toward the edge. It's not entirely bothersome, but its noticeable. I just slew the scope and set up for another pass through the FOV. I'm starting to think I might enjoy a coma corrector, though. 


Edited by Asbytec, 05 July 2019 - 09:29 PM.

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#49 Stephen Kennedy

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Posted 05 July 2019 - 10:45 PM

I have a 210 mm aperture, 1,823 nm focal length (F/7.7) premium Newtonian reflector on a Pentax MS-5 GEM which is a premium motorized mount with very good tracking.  At F/7.7 I have found there is no need for a coma corrector since coma is essentially undetectable in my Newtonian.  The views I get of planets are vastly superior to those I have ever seen in an 8 inch SCT and I do not think it is all due to a slightly larger aperture.  For high magnification of the Moon and planets I use a Pentax XW 7 mm eyepiece which gives magnification of 232X which is all the seeing will normally allow.

 

  At F/5 for a Newtonian you are really right on the border of where you would need to start using a coma corrector.  Any lower is a probable yes, any higher is a probable no. The off-axis coma free area of a parabolic mirror increases as the square of the focal ratio so my Newtonian would have (7,7/5)^2 = 2.37 times less coma than an F/5.  This relationship does not hold for SCTs since their mirrors are spherical, not parabolic.   Unfortunately, at F/5 all you can do is observe with the telescope and see if there is objectionable coma.or not.

 

Newtonians provide crisper images than SCTs and while they do require a little more work to keep in optimal condition, there is really nothing that can go wrong with a Newtonian that you can not fix yourself.  Most things that go wrong with an SCT require a lengthy and costly trip back to the factory. 


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#50 Asbytec

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  • Loc: Pampanga, PI

Posted 06 July 2019 - 01:42 AM

"The standard commercial SCT has both mirrors spherical, thus K1=K2=0. Obviously, coma in such arrangement is not corrected; for the linear field, it is approximately at the level of an ƒ/6 paraboloid, with the coma increasing to the diffraction-limited level (0.80 Strehl) at about 2.5mm off-axis."

 

https://www.telescop...ions.htm#-_coma


  • JP-Astro likes this


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