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Imaging newtonians - why so little love?

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#1 Dan_I

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Posted 18 October 2021 - 07:49 AM

  Hi,

 

I am always puzzled that, while there are plenty of entry level imaging Newtonians, there are very few mid-range or high-range options on the market, compared to other type of mirror-based astrographs (RC, CDK, SC Edge,...).

 

Most popular imaging camera nowadays have sensors around 3.6µm pixel pitch, which is, for most of us with average seeing in the 1.5" to 2" range, a perfect match for focal lengths in the 1000mm-1400mm range, in other words for 10" f/4 to 14" f/4 Newtonians.

 

RC or CDK work typically at f/8, so are often used at bin2 to avoid significant oversampling. Of course it is not without disadvantages, as either one covers one fourth of the sky area compared to the corresponding f/4 Newtonian working at bin1 (for a given camera), or one needs to buy a larger sensor camera ($$$) to cover the same sky area.

 

Naturally Newtonian OTA's are quite bulky above 10" but is it the only reason? Is it also because Newtonian telescopes are often perceived as budget telescopes hence are more difficult to sell at premium prices? Are they perceived as not keeping collimation well (as most people experience is  based on cheaply build models)? Are there other reasons?

 

Thanks for your insights,

 

Dan


Edited by Dan_I, 18 October 2021 - 07:50 AM.

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#2 happylimpet

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Posted 18 October 2021 - 08:00 AM

I wonder the same thing. As an f4 and f5 Newtonian user I love them to bits. And so does my wallet.


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#3 iantaylor2uk

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Posted 18 October 2021 - 08:01 AM

Glad you asked this because I have also wondered this. I was using a 12" f/4 Orion Optics DX300 for a while, and I got some good results with a ZWO 071 OSC. I managed some half-decent shots of M51 using just 10 second exposures. I didn't have a coma corrector at the time so had to crop quite a bit but the middle of the image was OK. 

Anyway, the scope was a bit too heavy and unmanageable, and if there was any kind of wind the autoguiding would not be good (although it was on a Losmandy G11 mount) since the scope acted like a sail. 

 

I managed to sell that telescope recently and have just got an Orion Optics CT-8 telescope (8" f4.5 newtonian) which I intend to use over the winter (and I also now have a coma corrector too). This is quite a bit lighter, with better optics and a good focal length (900 mm).

 

I know OO have a bit of a mixed reputation on here, but I personally have had some good telescopes from them (originally a Europa 8" f/6, then the 12" f/4, which i bought second hand, and I also had at one time a second hand OMC140). Also, I live quite close to their premises, so I picked the scope up in person rather than risk it being delivered. 

Some of the images from the 12" f/4 from earlier in the year (without the coma corrector) can be found at: 

M13: https://drive.google...iew?usp=sharing (30 sec exposures x 50)

M51: https://drive.google...iew?usp=sharing (10 sec exposures x 180)

M101: https://drive.google...iew?usp=sharing (1 min exposures x 120)

(this Newtonian had a straight spider rather than a crossed spider which is why the diffraction pattern is why it is)


Edited by iantaylor2uk, 18 October 2021 - 08:21 AM.

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#4 Ron (Lubbock)

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Posted 18 October 2021 - 08:28 AM

There are some truly incredible images being produced with imaging Newtonian telescopes.  Where else can you get 8 to 12 inches of aperture at f/4?  RASA gets you to f/2 with the well-known limitations, but there are few other options.  I'd love to see a 10" f/4 triplet APO hit the market for $1000 or so.  lol.gif

 

The problems with Newts are mostly mechanical, not optical.  If that tube flexes just a little bit under the weight of your heavy, new camera and filter wheel, then.... ouch.  If you hate collimation, too bad.  If you live in a dusty, windy climate like parts of Texas where I lived, then forget the open tube design.  If you're going to knock the optics, then it's the diffraction spikes that break the deal for some people, but not others.


Edited by Ron (Lubbock), 18 October 2021 - 08:29 AM.

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#5 ks__observer

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Posted 18 October 2021 - 09:29 AM

I love Newts as well.

For several years I used an 200mm (8in) Orion f/3.9.

Something happened to the Orion that I am trying to fix, and I have been using a 8in Apertura f/4.

I am trying to cure some corner star issues.

That is my biggest issues with my Newts -- trying to get low eccentricity stars across the field.  

That could be peculiar to me, as I have seen many good images with Newts with good stars across the field.

I really don't know why Newts don't get more love -- IMO they are the best scopes on the market.

(Side note: I use a ES-HRCC.)


Edited by ks__observer, 18 October 2021 - 09:44 AM.


#6 andysea

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Posted 18 October 2021 - 09:44 AM

I agree Newtonians are excellent. My current favorite imaging scope is the epsilon 180.

#7 rockstarbill

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Posted 18 October 2021 - 11:40 AM

At the high end options are very limited. The ONTC scopes from TS Optics get good reviews from folks here. There are others but they are lacking the critical mass of ownership to know much about them. There's the Epsilon line for wide field use, which gets mixed reviews related to Collimation challenges, which may or may not actually be Collimation at all.

Edited by rockstarbill, 18 October 2021 - 11:41 AM.

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#8 Dan_I

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Posted 18 October 2021 - 11:54 AM

 

 

The problems with Newts are mostly mechanical, not optical.  If that tube flexes just a little bit under the weight of your heavy, new camera and filter wheel, then.... ouch.  If you hate collimation, too bad. 

 

The point I'd like to make is that if a newtonian is made to a similar price point as high-end astrograph solutions on the market - or even to half or one third of it - the mechanics will be up to the task. I never owned an Epsilon, but the most recent models reputedly hold collimation very well.

 

 

At the high end options are very limited.

 

Exactly, the question is why?



#9 rockstarbill

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Posted 18 October 2021 - 12:03 PM

The point I'd like to make is that if a newtonian is made to a similar price point as high-end astrograph solutions on the market - or even to half or one third of it - the mechanics will be up to the task. I never owned an Epsilon, but the most recent models reputedly hold collimation very well.



Exactly, the question is why?


Yeah the recent designs of the Epsilon seem to have very good mechanical components and the collimation adjustments are easier than the older models. The focuser however leaves much to be desired. My hunch is that folks encounter issues caused by the focuser on the current models and try to collimate the telescope, don't see a change in results, and give up. It's really a shame since the scope and optics are so nice.

As for why the high end is void of options, ASA and AGO have both stopped making Newts suitable for amateurs. Dreamscope exists but the prices are pretty high. Orion UK exists but doesn't have a very good reputation.

#10 Morefield

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Posted 18 October 2021 - 12:34 PM

  Hi,

 

I am always puzzled that, while there are plenty of entry level imaging Newtonians, there are very few mid-range or high-range options on the market, compared to other type of mirror-based astrographs (RC, CDK, SC Edge,...).

 

Most popular imaging camera nowadays have sensors around 3.6µm pixel pitch, which is, for most of us with average seeing in the 1.5" to 2" range, a perfect match for focal lengths in the 1000mm-1400mm range, in other words for 10" f/4 to 14" f/4 Newtonians.

 

RC or CDK work typically at f/8, so are often used at bin2 to avoid significant oversampling. Of course it is not without disadvantages, as either one covers one fourth of the sky area compared to the corresponding f/4 Newtonian working at bin1 (for a given camera), or one needs to buy a larger sensor camera ($$$) to cover the same sky area.

 

Naturally Newtonian OTA's are quite bulky above 10" but is it the only reason? Is it also because Newtonian telescopes are often perceived as budget telescopes hence are more difficult to sell at premium prices? Are they perceived as not keeping collimation well (as most people experience is  based on cheaply build models)? Are there other reasons?

 

Thanks for your insights,

 

Dan

 

For me the first answer is limited back-focus.  But also more flexure, specifically in the focuser mounting.  Your observation is about higher end scopes and these things become more important as you ask more of your equipment.  

 

The back-focus limitation means there is often no room for rotator, OAG, and tilt corrector.  All of those are a must for me personally.  I could do without the tilt corrector at F7 but I think one of the points of the newts is a fast F-ratio.  I require the rotator to properly frame my images.  The advantages of a fast, wide Newtonian scopes are best matched to nebulae imaging and I don't want to just stick the object(s) in the center and hope for the best.  Also, with high end equipment, I expect to get optimal resolution and an OAG is required in most cases for that. 

 

I looked into a large high end Newtonian once and talked to the manufacturer.  When I asked about a rotator he told me that, even if the back-focus were there, no rotator made would be able to maintain a flat field while rotating.  I don't know if he was referring to the precision of the rotator itself or focuser attachment to the tube.  I feel like the Planewave focuser/rotator could handle the tolerances well but that thing is several inches thick and weighs 14 pounds!  Add that to a larger full frame camera, massive filter wheel to hold seven 2" filters, etc and you've got >25 pounds hanging off the end of the tube.  

 

You just don't want to make compromises when spending a ton of money to get the best equipment.

 

All that said, leaving aside the rotator, a fast Newtonian can do amazing things in the hands of a real professional.  Here's my best example: https://www.astrobin.com/users/BieDie/

 

Kevin



#11 TelescopeGreg

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Posted 18 October 2021 - 02:23 PM

As an entry-level Newtonian survivor, my experience is that the Newtonian design has its challenges for both imaging and viewing.  It's chief advantage is cost per inch of aperture, but it kind of ends there.

 

Issues for viewing is that the focuser is invariably aimed in an odd direction, requiring either a ladder or a yoga mat and knee pads.  One can loosen the rings and rotate it around, but then you've surely upset the mount balance and negating every other mechanical refinement that you spent hours working on.

 

For imaging, one needs to rotate the OTA so that the focuser is in-line with the counterweight bar, otherwise you'll never get the thing to balance properly.  That means you'll have even more trouble with a dual-use (visual and imaging), per the "yoga-mat-as-accessory requirement" above. 

 

Beyond the focuser (which on my scope was truly horrible, and needed upgrading), consider the need for a coma corrector, a "required option" which can cost as much as the OTA, and that adds a bunch of weight to an already dumbbell-weighted assembly.  That negatively affects guiding.

 

Add to this the need for collimation, which involves purchasing yet more accessory equipment.  A laser collimation tool is required, since there is no way to look through a collimation cap while reaching for the alignment screws on the other end.  My arms aren't long enough.  And as others have noted, the flex in the OTA means that collimation is orientation-dependent.  Simply going from one side of the meridian to the other is enough to throw the collimation off.  Likely specific to the Celestron model that I used, the back cover plate that covers the collimation screws is fastened with impossibly little screws that were very hard to get back into their little tiny holes.  Very annoying. 

 

Finally, the shape of the stars.  There are the diffraction spikes, which I truly didn't mind.  But the "fur" around stars caused by other stuff in the optical path (e.g. mirror clips) was something that just pushed me over the edge.  When my dream scope appeared on the Stellarvue CPO site a few years ago, I put my Newt in the spare bedroom closet, and out of my misery.


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#12 DeanS

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Posted 18 October 2021 - 02:30 PM

I have a 12.5" F/4 AG Optical newt.  the first of about 5 he made.  When I ordered it I said if you can make it as rock solid as my Epsilon 160 I will buy it.  He did, well actually better in the way of mechanics and optics.

 

Too make a high quality newt that has zero flex and holds colliimation well is expensive.  Which is probably why he no longer offers them for sale.

 

I might be interested in selling mine, but it will not be cheap wink.gif

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Edited by DeanS, 18 October 2021 - 02:33 PM.

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#13 rockstarbill

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Posted 18 October 2021 - 02:32 PM

 

For imaging, one needs to rotate the OTA so that the focuser is in-line with the counterweight bar, otherwise you'll never get the thing to balance properly.  That means you'll have even more trouble with a dual-use (visual and imaging), per the "yoga-mat-as-accessory requirement" above. 

 

For the larger Newts, this is largely true. For the Epsilon it is not. I can balance it just fine with the camera pointed up. 



#14 rockstarbill

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Posted 18 October 2021 - 02:32 PM

I have a 12.5" F/4 AG Optical newt.  the first of about 5 he made.  When I ordered it I said if you can make it as rock solid as my Epsilon 160 I will buy it.  He did, well actually better in the way of mechanics and optics.

 

Too make a high quality newt that has zero flex and holds colliimation will is expensive.  Which is probably why he no longer offers them for sale.

 

I might be interested in selling mine, but it will not be cheap wink.gif

I've been trying to buy one of those AG Newts for eons. :)


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#15 Dan_I

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Posted 18 October 2021 - 02:39 PM

For me the first answer is limited back-focus.  But also more flexure, specifically in the focuser mounting.  Your observation is about higher end scopes and these things become more important as you ask more of your equipment.  

 

The back-focus limitation means there is often no room for rotator, OAG, and tilt corrector.  All of those are a must for me personally.  I could do without the tilt corrector at F7 but I think one of the points of the newts is a fast F-ratio.  I require the rotator to properly frame my images.  The advantages of a fast, wide Newtonian scopes are best matched to nebulae imaging and I don't want to just stick the object(s) in the center and hope for the best.  Also, with high end equipment, I expect to get optimal resolution and an OAG is required in most cases for that. 

 

Backfocus limitation is a very good point. At most with 3" coma corrector you get something like 60 to 75mm backfocus. Fitting a focuser + rotator + OAG can work (using for instance a focuser/rotator combo) but I agree that cats are easier to deal with in this respect.

 

Regarding flex issues, I believe that it can be solved with proper design choices (for instance a carbon tube with a flat side on which the focuser and dovetail bar are mounted).

 

 

As an entry-level Newtonian survivor, my experience is that the Newtonian design has its challenges for both imaging and viewing.  It's chief advantage is cost per inch of aperture, but it kind of ends there.

 

Issues for viewing is that the focuser is invariably aimed in an odd direction, requiring either a ladder or a yoga mat and knee pads.  One can loosen the rings and rotate it around, but then you've surely upset the mount balance and negating every other mechanical refinement that you spent hours working on.

 

For imaging, one needs to rotate the OTA so that the focuser is in-line with the counterweight bar, otherwise you'll never get the thing to balance properly.  That means you'll have even more trouble with a dual-use (visual and imaging), per the "yoga-mat-as-accessory requirement" above. 

 

Beyond the focuser (which on my scope was truly horrible, and needed upgrading), consider the need for a coma corrector, a "required option" which can cost as much as the OTA, and that adds a bunch of weight to an already dumbbell-weighted assembly.  That negatively affects guiding.

 

Add to this the need for collimation, which involves purchasing yet more accessory equipment.  A laser collimation tool is required, since there is no way to look through a collimation cap while reaching for the alignment screws on the other end.  My arms aren't long enough.  And as others have noted, the flex in the OTA means that collimation is orientation-dependent.  Simply going from one side of the meridian to the other is enough to throw the collimation off.  Likely specific to the Celestron model that I used, the back cover plate that covers the collimation screws is fastened with impossibly little screws that were very hard to get back into their little tiny holes.  Very annoying. 

 

Finally, the shape of the stars.  There are the diffraction spikes, which I truly didn't mind.  But the "fur" around stars caused by other stuff in the optical path (e.g. mirror clips) was something that just pushed me over the edge.  When my dream scope appeared on the Stellarvue CPO site a few years ago, I put my Newt in the spare bedroom closet, and out of my misery.

I fully agree about issues for viewing, but I was considering in this discussion the use of a newtonian as a pure astrograph. What you say about the need for collimating tools and correctors are true, but the same can be said for all imaging reflectors.  The various issues that you mention are, in the main, related to entry-level Newtonians, rather than intrinsic to the design. Good mechanics avoid flex and collimation shift, and, with an aperture mask, diffraction by mirror clips can be avoided.  It's good though that you have found a solution that suits your needs.


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#16 TelescopeGreg

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Posted 18 October 2021 - 03:06 PM

For the larger Newts, this is largely true. For the Epsilon it is not. I can balance it just fine with the camera pointed up. 

The balance issue is with the focuser / camera off to one side, which is certainly only an issue for visual use.  Hanging straight down or up balances evenly, with the downward orientation preferred for guiding purposes (it has lower moment of inertia compared to the other orientations).

 

I'd like to second the other comments about back focus challenges.  I was looking at one point to adding a focal reducer to my Newt, and could not find one that would work with the Paracorr coma corrector that I have, and within the back focus that my DSLR demanded. 


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#17 rockstarbill

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Posted 18 October 2021 - 03:10 PM

The balance issue is with the focuser / camera off to one side, which is certainly only an issue for visual use.  Hanging straight down or up balances evenly, with the downward orientation preferred for guiding purposes (it has lower moment of inertia compared to the other orientations).

 

I'd like to second the other comments about back focus challenges.  I was looking at one point to adding a focal reducer to my Newt, and could not find one that would work with the Paracorr coma corrector that I have, and within the back focus that my DSLR demanded. 

Yeah I think the OP was asking from the perspective of imaging though, so visual constraints are not likely an issue for him.

 

Backfocus is certainly at a premium when it comes to fast imaging Newts. 



#18 Joe F Gafford

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Posted 18 October 2021 - 03:48 PM

I have two Newtonians at F/4.5. The 18" and the 10" and the mounts of these were modified over the years to accommodate the cameras and with the better drive modifications. My 18" took a step back recently and I'm seeing the same problems I had in my film days with the older drive was put back in as my newer RA drive failed. The 10" does fine with the 6.6" worm gear assembly put in and I had to modify that to get the gears to mesh properly. A portable dome works to keep the wind off of these. Especially the squatter 10". I am able to image with that in stiffer winds. The 18" sticks out more and I have to remove the glare shield on top at times. I have the old Lumicon field flattener with the Newtonian off axis guider to augment the internal guider with better selection of guide stars especially using the Ha filter. 

 

Dome_EGK_Summer.jpg

 

Joe


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#19 avarakin

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Posted 18 October 2021 - 10:33 PM

I used to be a newt guy, but switched to RCs as soon as I had a chance to try 6'' RC. RCs are not much more expensive but give much better star shapes, no diffraction fur,  shorter tubes, can shoot at native F8 or F5 with FR, have stronger mechanics, very flexible back focus. 

Alex


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#20 Dan_I

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Posted 19 October 2021 - 02:12 AM

 RCs are not much more expensive but give much better star shapes, no diffraction fur

Thanks for sharing your experience. I am curious about what you say regarding star shapes. What do you mean my diffraction fur?

 

My Newtonian has smooth optics and an aperture mask, and I don't see anything weird in the star shapes. Of course I have spikes (so do RC's) but personally I like that.


Edited by Dan_I, 19 October 2021 - 02:12 AM.


#21 happylimpet

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Posted 19 October 2021 - 02:52 AM

I have a 12" f5 skywatcher 300p which i got second hand for £250. It had plenty of backfocus for OAG/filterwheel/camera - though admittedly after i bought a larger GSO secondary and moved the primary up the tube (with adapters that cost about £10).

 

Even with the stock focuser, the collimation was rock solid all over the sky. Its well built. The focuser didnt wobble or tilt. It worked beautifully. Point stars across the field, and clouds resolved on Neptune etc. Its a keeper.

 

It was easy to use, and gave me zero problems, night after perfect night.

 

Cheap newts, at least a good choice of cheap newt, are great choices.


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#22 f300v10

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Posted 19 October 2021 - 07:19 AM

I have been imaging with a F4 Newt for six months now and am really enjoying it.  I originally intended to go with a ONTC from TS, but at the time they were having issues with their carbon tube suppler and could not give me delivery estimate, so I ended up going with a SW Quattro 300P.  Optically the scope is great, but as with most of the lower end imaging Newts it could be better mechanically.

 

As the OP pointed out, the latest generation of CMOS sensors pair perfectly with the 1000-1200mm focal length of a 10-12" F4 Newt.  With a 2600MM the bin 1x1 image scale is 0.65 arcsec/pixel which is ideal for my local conditions, and a very flexible 1.57 degree FOV.

 

Imaging Newts seem to be far more popular in Europe than here in the states where there really are no 'mid range' options.  You have GSO and SW steel tube scopes with excellent mirrors but poor mechanics for $1500 or under on the low end, and Dream $35K+ on the extreme high end with nothing in between.  If someone in the US offered a carbon tubed 12-14" Newt and solid mechanicals for say the price of an Edge 11 I would buy it.


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#23 DeanS

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Posted 19 October 2021 - 07:55 AM

The 3" wynne corrector on my AGO has 79.5mm back focus so will work with a wide range of camera set ups.



#24 Dan_I

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Posted 19 October 2021 - 08:04 AM

I have a 12.5" F/4 AG Optical newt.  the first of about 5 he made.  When I ordered it I said if you can make it as rock solid as my Epsilon 160 I will buy it.  He did, well actually better in the way of mechanics and optics.

 

Too make a high quality newt that has zero flex and holds colliimation well is expensive.  Which is probably why he no longer offers them for sale.

 

I might be interested in selling mine, but it will not be cheap wink.gif

This can be called my dream scopes (if I had a place to host it and the money).

 

This looks certainly like rather expensive to make, but probably not more expensive, or less expensive than truss CDKs or RCs of the same diameter. Maybe you remember the price difference between your scope and a  12" CDK from AGO at the time of purchase? It would be an interesting data point.

 

My hypothesis is that, in this price range, the latter were easier to sell than the former (for the reasons outlined above, common perception of newtonians as inferior grade), hence the business decission to stop making them.


Edited by Dan_I, 19 October 2021 - 08:06 AM.


#25 DeanS

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Posted 19 October 2021 - 08:13 AM

At the time I think they where very similar in price, but the F/4 was the appealing factor of these.

 

Downside is the overall size, which is huge, and the collimation which is tricky.  Although mine has held very well over the years even after several star party trips.


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