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Small newt for imaging

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#1 neek

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Posted 22 April 2019 - 10:16 PM

Silly question.. why don't people image with small newtonian reflectors?

 

I'm looking at what most people do for wide-field astro-imaging and it seems the popular option is to get a 70-80mm refractor. What about something similar in a newtonian (70-80mm aperature with a FL of 300mm or something)? 

 

I guess one reason is that I've never seen one for sale for imaging, but I would think it would be a cheap way to get some comparable shots. Is the obstruction too big for its size? Is it too awkward to mount a camera to?

 


 

#2 petert913

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Posted 22 April 2019 - 10:30 PM

Not many people mount Newtonians under 6", especially for imaging.    But it's an interesting challenge !


 

#3 SilverLitz

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Posted 22 April 2019 - 10:35 PM

I am a newbie, but I can guess on a few reasons not to use a small newt: 1) most newts cannot achieve prime focus on a sensor, unless they are astrographs, you cannot rack the focuser in enough; 2) newts are physically large and can catch a lot of wind; 3) I would expect the large aluminum tube to flex more than a more compact/denser refractor.


 

#4 AhBok

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Posted 22 April 2019 - 10:38 PM

I’ve seen guys using a Starblast 4.5 for imaging. The lack of a 1.25” coma corrector is one problem. Frankly, a 6” newt is pretty small and very inexpensive. Many come with a 2” focuser that can easily handle a CC and camera. I would think a 70-80mm F4 with a 2” focuser and CC would be as or more expensive than a 300mm camera lens and maybe not as good.
 

#5 bobzeq25

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Posted 22 April 2019 - 11:27 PM

Silly question.. why don't people image with small newtonian reflectors?

 

I'm looking at what most people do for wide-field astro-imaging and it seems the popular option is to get a 70-80mm refractor. What about something similar in a newtonian (70-80mm aperature with a FL of 300mm or something)? 

 

I guess one reason is that I've never seen one for sale for imaging, but I would think it would be a cheap way to get some comparable shots. Is the obstruction too big for its size? Is it too awkward to mount a camera to?

Not silly at all.

 

It's just hard to manage.  Big, relatively heavy, somewhat unbalanced.  The cheap ones suffer from tube and focuser flexure.

 

The cheap ones may not even come to focus with a DSLR.  "Inadequate backfocus", the focal point is too close to the tube.  They're made that way because it makes a small secondary work.

 

Experienced imagers sometimes image with Newts for the speed.  But they don't get cheap ones (the ones they get are labeled as astrographs), and... they're experienced.  A good Newt costs close to what a refractor would, and those who use them can deal with the difficulties.  They usually have very good mounts. 

 

Here's a good one for imaging, specified as such.  As small as Newt astrographs come.  No experienced imager who wants to image with a Newt would want anything smaller than a 6 inch, so there's no market for them.

 

https://www.teleskop...in-Germany.html

 

For a beginner, it's just easier to start out with a small refractor.  No one ever says becoming a good imager is too easy.  <smile>  Even for experienced imagers, the ease of use of the small refractor is attractive.

 

Here's a good thread about the complexities of imaging with a Newt. 

 

https://www.cloudyni...bill newtonian

 

Note that his mount cost several thousand dollars, and that he has years of experience.


Edited by bobzeq25, 22 April 2019 - 11:50 PM.

 

#6 Ron359

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Posted 22 April 2019 - 11:50 PM

Who says they don't?  Maybe they should if they don't.  When he says 'they don't get cheap ones',  bobzeq doesn't speak for me for sure.  I see them pretty frequently for about $200 here on CNs classifieds.  

 

A 6" f/5 newt. is equivalent to a highly color corrected $10K telephoto camera lens or Astro-Physics refractor (if they even make one?) and much easier to mount and guide.  Since a newt needs no refractor color corrective lenses at all, its cheap by comparison with many much smaller aperture APO refractors.

 

 I got a Vixen essentially 'free' with a 'pre-owned' super polaris mount I was really buying for less than $200 bucks.  For ~$150 I put a 2 inch 2 speed crayford focuser, a short vixen top rail (for guide scope) and new spider on it.  Put an old newt coma corrector on APS-C Canon, mounted on my Atlas, autoguided with a 50mm finder.  First uncropped image attached, just 9 frames @ 3 min. each, if I remember right.  A 6" f/5 Newt is certainly no wind sail and might even be much lighter and probably a bit smaller than the equivalent $10K telephoto camera lens.  

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  • M8stack9 copy 2.jpg

Edited by Ron359, 23 April 2019 - 12:16 AM.

 

#7 DivisionByZero

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Posted 23 April 2019 - 12:30 AM

I haven't been out in a while because of a recent move, but I was using a 4.5" Newt. It was modified to reach focus by shifting the primary up several inches. I've been eyeing an upgrade path to a 6" OTA to get to a 2" focuser since there are no 1.25" coma correctors. I was teaching myself a lot of things, so I didn't mind the extra hassles. One issue is that balancing the thing was pretty difficult with a DSLR hanging off a standard 1.25" focuser.

I'm sure I'm not the only weirdo out there who has done this...
 

#8 StevenBellavia

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Posted 23 April 2019 - 07:19 AM

I've been waiting for someone to produce a quality, affordable, small Newtonian astrograph.

The closest thing I know of is the 130mm Takahashi Epsilon. And though it is a reflector it is not technically a Newtonian.

I got tired of waiting, so I made my own:  "The Bellavia Mini".

 

I took the parabolic, diffraction limited mirror, from a $179 Orion Star Blast II, and made a much stiffer OTA for it, and replaced the small secondary mirror with a larger one to fully illuminate a 16mm circle. (And then found another 4.5 inch mirror, and gave that entire scope, still new in the box, to a 13 year old girl in our club)

 

It is 114mm aperture, f/4, 450mm focal length. What would that cost in a refractor? The only f/4 refractors I know of are Vixen, Takahashi and Borg (I also have an f/4 Borg).

It is also excellent for EAA.

 

The build process and some imaging examples here (note, all without a coma corrector):

 

https://www.flickr.c...157692997616651

 

I could try to make more formal "plans" to give out for free, but it is essentially just a wooden box.  The primary support is a little "fancy" in that I used a very stiff and light aluminum-plastic-composite and glued the mirror at an appropriate radius determined using PLOP (no clips - clips cause diffraction flare on stars). I reused  the secondary support assembly (but had to extend the screws to reach the corners of the OTA box).

 

I also made my own custom non-rotating helical focuser (I love helical focusers), but any standard focuser will work. (if attempting this, I suggest to use a rack-and-pinion, not a Crayford, for imaging).   I also can swap in a Borg helical focuser, but that makes it more expensive, if cost is an issue.

 

And like all Newtonians, you should collimate just before every imaging session, and focus at f/4 has to be precise. You will need a good quality coma corrector. I just started using a Baader MPCC, and waiting for an Explore Scientific HRCC02-00 to arrive to see if I like that better.

 

I also own the 6-inch f/4 astrograph that is branded with many names now, but it is longer and heavier. And every version I see has as issue with the camera placement.  It is way too far back from the secondary, causing vignetting for larger sensors.  They should have made the tube longer and moved the camera as close in as possible, like a "normal" astrograph.

 

With the 4.5-inch f/4 and a 2.4 micron pixel camera (like those with the Sony IMX183 or 178 sensors), it is 1.1 arc-sec/pixel.  That is fairly ideal for many/most objects.

 

I may produce a mass-produced version one day, since nobody else seems to want to.

 

Steve

Attached Thumbnails

  • The_Bellavia_Mini_1-2-3.JPG

Edited by StevenBellavia, 23 April 2019 - 10:18 AM.

 

#9 james7ca

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Posted 23 April 2019 - 09:21 AM

Several rebranders offer six inch Newtonians for imaging (probably made by GSO). For example, OPT offers an f/6 version for just $200. I purchased one of these several years ago and have used it for both DSO and planetary imaging. Here is my first light report on that scope, with several sample images:

 

  https://www.cloudyni...n/#entry7166212

 

-- and -- here is another thread with some additional images:

 

  https://www.cloudyni...s/#entry7181406

 

 -- and -- lastly a PixInsight AberrationInspector image after I had tweaked the collimation for the second or third time (eventully using a laser collimator):

 

  https://www.cloudyni...-3#entry7210215

 

One reason beginners may shy away from Newtonians is that they generally need to be collimated after the initial purchase (and rechecked and redone periodically). However, most refractors work pretty well right out of the "box."


 

#10 StevenBellavia

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Posted 23 April 2019 - 03:29 PM

Several rebranders offer six inch Newtonians for imaging (probably made by GSO). For example, OPT offers an f/6 version for just $200. I purchased one of these several years ago and have used it for both DSO and planetary imaging. Here is my first light report on that scope, with several sample images:

 

  https://www.cloudyni...n/#entry7166212

 

-- and -- here is another thread with some additional images:

 

  https://www.cloudyni...s/#entry7181406

 

 -- and -- lastly a PixInsight AberrationInspector image after I had tweaked the collimation for the second or third time (eventully using a laser collimator):

 

  https://www.cloudyni...-3#entry7210215

 

One reason beginners may shy away from Newtonians is that they generally need to be collimated after the initial purchase (and rechecked and redone periodically). However, most refractors work pretty well right out of the "box."

James,

 

If I could have gotten my Astrotech 6-inch f/4 Newt to work like that, I would have never considered building my own Newt astrograph.

I too have the Baader MPC III, and my stars are not even close to that round off-axis. :/

 

Sometimes I get perfect round little stars.  And sometimes, I don't.  :/

 

Steve

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Edited by StevenBellavia, 23 April 2019 - 03:50 PM.

 

#11 John Tucker

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Posted 23 April 2019 - 04:43 PM

Who says they don't?  Maybe they should if they don't.  When he says 'they don't get cheap ones',  bobzeq doesn't speak for me for sure.  I see them pretty frequently for about $200 here on CNs classifieds.  

 

A 6" f/5 newt. is equivalent to a highly color corrected $10K telephoto camera lens or Astro-Physics refractor (if they even make one?) and much easier to mount and guide.  Since a newt needs no refractor color corrective lenses at all, its cheap by comparison with many much smaller aperture APO refractors.

 

 I got a Vixen essentially 'free' with a 'pre-owned' super polaris mount I was really buying for less than $200 bucks.  For ~$150 I put a 2 inch 2 speed crayford focuser, a short vixen top rail (for guide scope) and new spider on it.  Put an old newt coma corrector on APS-C Canon, mounted on my Atlas, autoguided with a 50mm finder.  First uncropped image attached, just 9 frames @ 3 min. each, if I remember right.  A 6" f/5 Newt is certainly no wind sail and might even be much lighter and probably a bit smaller than the equivalent $10K telephoto camera lens.  

The cult of the small refractor,  This site is full of enthusiastic fans of low cost newtonian astrographs such as the Orion 8", and Astrobin is chock full of great photos taken with them.

 

And if you have to drive 3 h each way just to get to a Bortle 3 site, an F6 refractor for 2x the price of a F4 newtonianis hard to get excited about,


 

#12 Ron359

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Posted 23 April 2019 - 04:43 PM

James,

 

If I could have gotten my Astrotech 6-inch f/4 Newt to work like that, I would have never considered building my own Newt astrograph.

I too have the Baader MPC III, and my stars are not even close to that round off-axis. :/

 

Sometimes I get perfect round little stars.  And sometimes, I don't.  :/

 

Steve

I doubt your coma corrector was the problem.  That lower image looks like guiding errors from chasing seeing or bouncing back an forth very slightly.  The 'not round' stars are all across the image, not just comets in the corners.   I used the Celestron-Baader 'Mark 1' (original) MPCC in my image posted above and granted f/4 is tougher than f/5,  but the image scale of f/5 is larger too, so would easily show coma.  And if you get them round sometimes, then it can't be the CC behaving badly on some nights and not others. 


 

#13 Ron359

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Posted 23 April 2019 - 04:58 PM

The cult of the small refractor,  This site is full of enthusiastic fans of low cost newtonian astrographs such as the Orion 8", and Astrobin is chock full of great photos taken with them.

 

And if you have to drive 3 h each way just to get to a Bortle 3 site, an F6 refractor for 2x the price of a F4 newtonianis hard to get excited about,

yeah.  What amazes me are some of the 'experts' here who push B&I Imagers to use PixInsight (highly complex, expensive sw).  But then, because "collimating a newt is hard", they recommend they use much smaller aperture, very much more expensive APO refractors.  I scratch my head in wonder.   


Edited by Ron359, 23 April 2019 - 06:23 PM.

 

#14 AhBok

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Posted 23 April 2019 - 07:07 PM

Well, since I am proponent of Newts for imaging, I must say learning with a small refractor is much easier. I struggled with learning to image with a C8 and then a 150mm Newt. I got results, but had misshapen stars and had difficulty discerning coma from field curvature from poor guiding from bad focus from bad processing--not to mention collimation. I switched to an 80mm ED and in weeks was producing decent images consistently. After a year, I got an 8F4 newt and it was like starting over again. it took about 6 months for me to get to the level of where I was with the 80mm. it took me another 6 months to get to the point where I felt my images with the newt were better than my 80mm and a good bit of this was just more experience in general (and a better mount).

 

I'm confident in my personal preference for imaging Newtonians, but I'm equally confident that the advice for beginner to learn with a small refractor is good advice. If I differ from that advice, it's only that I advise beginners who have yet to purchase a scope to go with the small refractor. However, I would not tell a beginner that his already purchased Newtonian or even SCT is useless for imaging. That is horrible advice and so discouraging for those with limited resources. We should help beginners do the best with what they have and can afford. But, small Apos are demonstrably easier to learn imaging.


 

#15 betelgeuse91

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Posted 23 April 2019 - 07:25 PM

I am getting a 6" Newtonian (relatively small) and noticed that the light blocked by the secondary mirror is 18% by area. It seems that regardless of the size of the aperture, the secondary mirror needs to keep up certain size in order to well-illuminate the camera sensor. My observation is that, as you lower the aperture size, the relative blocked area becomes larger, which decreases the merit of the Newtonian (economical for its aperture). Also small lenses are relatively easy to manufacture, so APO refractors of small aperture become competitive, which are in general less hassle to use compared to the Newtonians. 

 

Just my thoughts... 


 

#16 jpbutler

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Posted 23 April 2019 - 07:55 PM

I have a ts optics UNC 6" F/5 Newt that I plan on using as my main scope as soon as galaxy season ends.

I have been having problems with the cem60ec that it rides on, so that has taken most of my time.

but I think that I am really going to like it a lot.

 

john


 

#17 StevenBellavia

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Posted 23 April 2019 - 08:01 PM

I doubt your coma corrector was the problem.  That lower image looks like guiding errors from chasing seeing or bouncing back an forth very slightly.  The 'not round' stars are all across the image, not just comets in the corners.   I used the Celestron-Baader 'Mark 1' (original) MPCC in my image posted above and granted f/4 is tougher than f/5,  but the image scale of f/5 is larger too, so would easily show coma.  And if you get them round sometimes, then it can't be the CC behaving badly on some nights and not others. 

Sorry, I wasn't saying that the coma corrector was the cause of the issue being shown.  I was poorly describing two separate issues, with the bigger issue being the inconsistency of the scope.

 

The images posted were both center-cropped, so no coma there.

 

The plus-sign or cross-shaped stars is not from guiding.

That is a classic astigmatism. But I don't know from what.  It could be the primary or the secondary.

 

Look near the bottom of this article:

 

http://www.deepskywa...nal-mirror.html

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  • astigmatism_2.JPG

Edited by StevenBellavia, 23 April 2019 - 08:04 PM.

 

#18 neek

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Posted 23 April 2019 - 09:54 PM

Wow, gotta say I'm surprised at the large number of responses I got to my question since I posted it last night. Thanks everyone laugh.gif . I learned a lot from many of your posts. I should mention that I do have a 6" newt that I use for imaging already. I've had my best success with the 6" newt telescope compared to my 8" edgeHD due to its lower focal-ratio and FL. My scope has a bit of coma for sure and wasn't an 'astrograph', but it is relatively easy to take pictures with.. I should put on a signature with my equipment (will do soon).

 

I really want to try an even 'wider' field of view which got me to thinking about a smaller newtonian. But it really does sound like for practical purposes the small refractor is a great choice for that - it just gets expensive really quickly even within the small aperture range and makes me wonder what the magic price-point is.

 

Hats off to StevenBellavia though, that is a really awesome astrograph!!! That's what I'm talking about :) If they sold that in the store, I'd buy it for sure.


 

#19 17.5Dob

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Posted 23 April 2019 - 10:42 PM

 

A 6" f/5 newt. is equivalent to a highly color corrected $10K telephoto camera lens or Astro-Physics refractor (if they even make one?) and much easier to mount and guide.  Since a newt needs no refractor color corrective lenses at all, its cheap by comparison with many much smaller aperture APO refractors.

 

 

A 130mm APO has the same light collection area as a 6" f4 Newt after you subtract the massive CO, as well as having no diffraction/ light scattering secondary, yielding a much better final image. Cost has jumped $100 for those who hesitated, they now sell for $1,799.

No collimation tools needed, ready to shoot as quick as you can set up.

"The cult of the small refractor...."

This is just 32 minutes using a $550, 65mm quad APO/f6.5, no flattener required.

40586502053_0e10d37d89_b.jpg

This is just a single 60" sub using the same f6.5,  65mm,  ISO 200
32795493278_92b4cc2817_b.jpg


Edited by 17.5Dob, 23 April 2019 - 11:20 PM.

 

#20 bobzeq25

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Posted 23 April 2019 - 11:15 PM

Wow, gotta say I'm surprised at the large number of responses I got to my question since I posted it last night. Thanks everyone laugh.gif . I learned a lot from many of your posts. I should mention that I do have a 6" newt that I use for imaging already. I've had my best success with the 6" newt telescope compared to my 8" edgeHD due to its lower focal-ratio and FL. My scope has a bit of coma for sure and wasn't an 'astrograph', but it is relatively easy to take pictures with.. I should put on a signature with my equipment (will do soon).

 

I really want to try an even 'wider' field of view which got me to thinking about a smaller newtonian. But it really does sound like for practical purposes the small refractor is a great choice for that - it just gets expensive really quickly even within the small aperture range and makes me wonder what the magic price-point is.

 

Hats off to StevenBellavia though, that is a really awesome astrograph!!! That's what I'm talking about smile.gif If they sold that in the store, I'd buy it for sure.

Here are three economical choices.  $369, $469, and $618 (+about 200 for a flattener to cure funny looking stars at the edges).  They're doublets, but they're _good_ doublets, with good glass.

 

https://www.astronom...t.html?___SID=U

 

https://www.astronom...fpl-53-f-6.html

 

https://www.highpoin...or-blue-a-z73bu

 

The used market is a possibility.


 

#21 DivisionByZero

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Posted 23 April 2019 - 11:17 PM

If you're going for wide field, then the easiest thing might be one of those 50mm lenses for a DSLR (the "nifty fifty").  Just pop it on a camera tracker mount and you're good to go!  Wide field really does require short focal length and that gets harder and harder to do with a Newt below 4.5".  If you work through some of the math involved in achieving focus, you'll find that at 6" and above, you don't "lose" a lot of length due to standard-sized focusers relative to the focal length.  That helps with balance and avoiding large secondaries and potential vignetting.  This gets really handy if you have a lot of stuff in your image train (consider a coma corrector + DSLR or coma corrector + filter wheel + CCD).  The small size refractors just end up winning for shorter focal lengths.


 

#22 Ron359

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Posted 23 April 2019 - 11:42 PM

A 130mm APO has the same light collection area as a 6" f4 Newt after you subtract the massive CO, as well as having no diffraction/ light scattering secondary, yielding a much better final image. Cost has jumped $100 for those who hesitated, they now sell for $1,799.

No collimation tools needed, ready to shoot as quick as you can set up.

"The cult of the small refractor...."

This is just 32 minutes using a $550, 65mm quad APO/f6.5, no flattener required.



This is just a single 60" sub using the same f6.5,  65mm,  ISO 200

65mm finder is still a finder, no matter if it does cost over $500 bucks to correct its color refraction.  It has half the angular resolution (never resolve anything less than 2 arc sec. details)  and 2 mag. fainter mag. limit -thats 5.4x less light gathering, than a 150mm for several times the money.  No cult opinion, just the laws of physics, that Issac Newton demonstrated hundreds of years ago when he built the first small "Newtoinan."  The relative bucks per inch cost is what makes some believe 65mm is a cult classic.  


 

#23 Stephen Kennedy

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Posted 23 April 2019 - 11:47 PM

Silly question.. why don't people image with small newtonian reflectors?
 
I'm looking at what most people do for wide-field astro-imaging and it seems the popular option is to get a 70-80mm refractor. What about something similar in a newtonian (70-80mm aperature with a FL of 300mm or something)? 
 
I guess one reason is that I've never seen one for sale for imaging, but I would think it would be a cheap way to get some comparable shots. Is the obstruction too big for its size? Is it too awkward to mount a camera to?


As someone who makes mirrors for reflectors, my feeling is that grinding, polishing and figuring a 70-80 mm paraboloidal mirror would probably be at least as difficult as making a 152 mm or 203 mm mirror without the many advantages that you get from a larger aperture. If you are looking for something with a wide field you can still have a large aperture Newtonian but you need a very low focal ratio like F/3. That would be a little more expensive because it is harder to make an F/3 mirror than say a F/7 mirror and the F/3 would probably need a coma corrector. It would still be cheaper than a refractor and have greater resolution and light gathering capabilities.

As for diffraction, the biggest source of it in any telescope is the circular aperture of a telescope. This diffraction is an inverse function of aperture. A 100 mm aperture will have twice as much diffraction as a 200 mm telescope.
 

#24 17.5Dob

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Posted 24 April 2019 - 12:33 AM

65mm finder is still a finder, no matter if it does cost over $500 bucks to correct its color refraction.  It has half the angular resolution (never resolve anything less than 2 arc sec. details)  and 2 mag. fainter mag. limit -thats 5.4x less light gathering, than a 150mm for several times the money.  No cult opinion, just the laws of physics, that Issac Newton demonstrated hundreds of years ago when he built the first small "Newtoinan."  The relative bucks per inch cost is what makes some believe 65mm is a cult classic.  

My seeing and 90% of the USA is limited to 2" resolution for DSO imaging. Unless you're shooting high speed video, you'll never be able to exploit the 'theoretical resolution" of a larger OTA. You also are leaving out the fact you also need to use a coma corrector with a fast newt, that will cost as much, or more, as a used "$200" Newt,  which then eliminates "The Perfect Color Correction".......as well as the cost of collimation tools. In the end, the "$200" Newt is going to cost more than a 65mm Quad APO that is usable straight out of the box.....

In the short 32 minutes of shooting with a f6.5 , 65mm OTA, I had no problem reaching several dozen mag 18-20 background galaxies in the M13 photo. Don't equate visual specs to photographic capabilities !

I can fit my 65mm, in it's case, under the seat of my truck, drive 1 hr away from my white zone backyard to a blue/ grey site and be shooting in 45 minutes

 

45824316154_7f8afb7343_b.jpg
 

 

This was taken with my "$259.00", f7 80mm "spotting scope" and a dSLR. I've seen a lot worse from 8"- 11" CAT's and multi $1,000 dollar cameras/filters if cost is the absolute bottom line.....

32511228237_c6a173999a_b.jpg


Edited by 17.5Dob, 24 April 2019 - 12:53 AM.

 

#25 Gary.McK

Gary.McK

    Ranger 4

  • -----
  • Posts: 399
  • Joined: 29 Jan 2009
  • Loc: Geelong, Australia

Posted 24 April 2019 - 04:44 AM

Here is a 104 page thread regarding imaging with a Skywatcher 130mm newt. It has ample examples of fabulous images that rank with really good stuff.

 

https://stargazerslo...ith-the-130pds/

 

I own one - not used due to illness. F5 needs a cheap coma corrector for an APSC chip. Why would you spend 20 times as much for a refractor (even though I own a an Esprit 100..... :-)

 

cheers

Gary


Edited by Gary.McK, 24 April 2019 - 06:50 AM.

 


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