Optimized Newtonian - Is it worth it?

Pretty much a big NO. What you would want is a stable primary cell and good lateral support getting rid of mirror clips. For smaller instruments (say 5"-6" in aperture at fast focal ratios) I can't really consider CF tube as a priority given that most of us would have an electronic focuser anyway.  In fact, were it not for stiffness reasons I'd avoid it for anything 12" and below and for above that you'd go for truss configuration.

Tony Gondola:
1.) Lowering the focal plane relative to the side of the tube (eliminate the focuser and focus by moving the primary) This allows a smaller secondary and could provide a very solid mounting point for a heavy imaging train, eliminating a lot of focuser caused problems.

2.) Stepper motor control of the collimation bolts. This could be used for both focusing and collimation and might be the best solution to the issues involved with moving the primary to focus.

3.) Integrate a boundary layer fan to deal with thermal degradation of the image.

4.)Carefully designing the configuration of the spider vanes to control both pattern and length of the diffraction spikes. Taste will vary here but there are a lot of options. That or go with an optical window. 

5.) Minimizing focus shift with temp by carefully choosing the tube material or by using a low thermal coefficient internal structure.

6.) Accurately machined and located baffles to control stray light.

7.) Flock the inside of the tube opposite the focuser

8.) Restrain the primary with an NARROW edge mask rather than clips.

9.) Lastly, it would be great to avoid the use of a coma corrector.

Easy...

1 and 2: Feasible but means a longer heavier tube. The alternative - a really good R&P focuser with EAF - isnt cheap either.
3. Some do this.
4. and 9: Better solution is to use any of the designs that have a full-aperture corrector that can also support the secondary mirror, eg Wright, Houghton, Maksutov, Baker or Schmidt. But this costs $$$.
5. Not necessary - electronic focusing is essential in any case to achieve precise focus and it can deal with temperature change, via a sensor or periodically refocusing - which is desirable in any case to deal with flexure eg after a meridian flip.
6. Desirable, yes - looking at my Russian mak-newtonian, which has baffles. But a solid tube with baffles means $ and weight.
7. Light-trap is even better than flocking, and yes my Russian mak-newtonian has one. $
8. Desirable, yes (looking at my Russian mak-newtonian, which has one). More $.

The issue here is that many beginners focus on cost only, first and foremost.
Without realizing some scopes are built better than others, and several aspects on this list are $$$.

if you buy cheap you won't get what you didn't pay for.

But from the vendor perspective, building a quality product with all of the above would put it at a price none would pay for, as a commercial product, when you have rock-bottom competition from the Chinese building no-frills OTAs. That is not a sustainable business model - all the companies that made a better Newtonian have gone bust - every single one.

That leaves it to DIY efforts to start with something and add what you feel is needed; possibly even rebuilding the whole OTA.

Tony Gondola:
There are a lot of ways I can think of that the current crop of Newtonian Astrographs can be improved. Lets restrict the conversation to apertures between 6 and 10 inches.
 

1.) Lowering the focal plane relative to the side of the tube (eliminate the focuser and focus by moving the primary) This allows a smaller secondary and could provide a very solid mounting point for a heavy imaging train, eliminating a lot of focuser caused problems.

2.) Stepper motor control of the collimation bolts. This could be used for both focusing and collimation and might be the best solution to the issues involved with moving the primary to focus.

3.) Integrate a boundary layer fan to deal with thermal degradation of the image.

4.)Carefully designing the configuration of the spider vanes to control both pattern and length of the diffraction spikes. Taste will vary here but there are a lot of options. That or go with an optical window. 

5.) Minimizing focus shift with temp by carefully choosing the tube material or by using a low thermal coefficient internal structure.

6.) Accurately machined and located baffles to control stray light.

7.) Flock the inside of the tube opposite the focuser

8.) Restrain the primary with an NARROW edge mask rather than clips.

Lastly, it would be great to avoid the use of a coma corrector. I know this is a must for really fast systems but if a smaller FOV and longer F-ratio (F/6) is acceptable, it would be nice to retain the perfect color correction and simplicity that Newtonian optics provide. Of course, a good quality mirror set is a given.

 

I have no doubt that all of the above is possible and would improve the instrument. My question is, would it be worth it in terms of photographic image improvement? Newtonians offer a lot of performance for the money so it seems worthwhile to look for improvements.

I've been building newts for awhile now and I've trialed and errored my way into some design elements that I feel work very well. The current crop of newts available from the big manufacturers is inadequate. If you take one apart you'll notice they use the exact same mirror cells, secondary spiders and focusers as their budget dobs. What might work fine in a f6 visual scope is sorely lacking on an f4 astrograph. It is finally getting a little better with offerings from Apertura and TS with stuff like cnc one piece spiders and marginally better focusers. But while I love newts, I can understand how the commercial options out there cause people to stay away. 

1. Keeping the primary in perfect linear alignment would be a very expensive and unnecessary engineering challenge. Hard enough on an SCT where the spherical primary can move around without losing collimation, much much more difficult with a parabolic mirror. And where would the coma corrector end up? Putting that same precision machining and engineering into the focuser is the way to go.  A slightly smaller secondary would have little benefit. Easy to test yourself, go cut out a circle with an even larger diameter (heck, double it) than the minor axis of the secondary and see how small of a difference it makes.

2. This isn't a bad idea for newts that are operated remotely and will only be touched once a year or so but a well designed cell will not change collimation over time if it lives on the mount. But it also adds a huge amount of complexity, cost and weight... that effort would be better spent on a robust mirror cell. 

3. Absolutely! Constant air movement has plenty of other benefits too. Will keep the secondary and primary (if things are really wet out there) from dewing over even on nights where everything else is dripping. I design my cells so that the single large fan on the cell blows air over and around the mirror, passing over the mirror face and then exiting the front of the tube. A quality fan such as a Noctua running at 100% won't transmit any vibration. Also a good idea to go with the largest diameter fan that you can fit. That means more volume at a slower speed, so less chance of vibration. For a sanity check, I'll through in an eyepiece with the most magnification I can get and on/off the fan to see if I can detect any vibration. 

4. The monolithic cnc cut spiders seem to be the way to go. Keeping everything perfectly square is key here. I've experimented with a few iterations of carbon fiber spider vanes (to avoid CTE issues with aluminum) and while they work well, there is a lot more work involved in making them and the time and energy involved in building them and adjusting for perfect colinear-ness outweigh the potential benefits. Larger newts, 16"+ might see more of a difference. As far as design goes, I like to keep mine as thin as possible but with enough bulk to resist sag. For CF 1.5mm works well, for a monolithic cnc spider 2-2.5mm is more of a realistic machining challenge. 

5. An optical window adds another large layer of complexity for little payoff. Reflections, trapping air, weight and the extra optical element potentially degrading the image have all kept me away from messing with them. 

6. Baffles are a great addition. Very inexpensive and simple to accomplish, just need to cut out some concentric circles and install! Especially worth it if you are imaging from a light polluted area and risk glare from neighbors lights. 

7. Flocking everything is cheap and easy but especially opposite the focuser. I haven't experimented with light traps as MaksPower suggests but it's always been something I'd like to play with. In the meantime, proper flocking and baffling work great. Note that any kind of textured flocking material will always work better than pain as even the flattest and blackest of paints will always reflect a bit at a high angle of incidence. Better to have some texture to "break up" the light. 

8. Exactly how I do it! Makes no sense to have clips AND a mirror mask when the mask could do all the work. Also note that most mirrors are beveled, you can have the mask/hold down cut precisely enough to only contact the mirror at the bevel so you aren't masking any of that valuable aperture. I know a lot of people say it helps with turned down edge but none of the commercial mirrors I've tested in the last decade have exhibited any serious turned down edge... plenty of surface roughness and undercorrection, but not much TDE. Your results may vary.

I've done quite a few 8" f4's and some 6" f4 scopes, usually used at f3 with a Nexus. Very happy with them. Currently planning on doing a 12.5" f4.8 scope for galaxy work. Here is the latest 8". The tube is a CF/Plywood composite (overly long video of my process: https://youtu.be/037iY_Vc1zg?si=e3lym3sMFPIyUNvO), the cell and spider are a mix of CF and aluminum machined on my poor cnc router. 

Tony Gondola:

Anderl:
Have doubts. You will always need a coma corrector for ap, visually it might be ok without. The coma free field will be very small even with an f6 newtonian.
but tbh i never build one so high chance i am wrong.

Almost all of the images on my bin page are from an uncorrected 6" F/6 Newt...

Beautiful pictures but there is visible coma even with an small imx585 sensor so maybe a coma corrector wouldn’t hurt.

Anderl:

Tony Gondola:

Anderl:
Have doubts. You will always need a coma corrector for ap, visually it might be ok without. The coma free field will be very small even with an f6 newtonian.
but tbh i never build one so high chance i am wrong.

Almost all of the images on my bin page are from an uncorrected 6" F/6 Newt...

Beautiful pictures but there is visible coma even with an small imx585 sensor so maybe a coma corrector wouldn’t hurt.

I will try one at some point. My main question is how much if at all will it effect the perfect color correction a newt delivers. That's all in the context of small sensors. Narrow fields interest me more than wide ones do.

I have and used newts between 6 and 12 inch in aperture and I have yet to see sign that thermals have any effect other than just tranisent ones (and this for the larger 12") or that a fan gives any improvements whatsoever other than, transiently, shorten (and not by a lot) the thermal transient. One shan't disturb what should, left on its own just produce a nice boundary layer (with occasional ripples, mind you) but as we're discussing deep sky astrographs (with exposed primary backs) this is of no further matter, at least for the small sizes here presented.  Too narrow a tube is harming the nice development of the chimney effect and so do baffles inside the tube and I wouldn't spend a second on them. Ditto for flocking and because sooner or later you'll get the hair dust onto everything. Good total balckout paint is what you want and what you need. As far the evidence of imaging for over 25 years tells me stray light either comes for the back, from the tube front end or from the focuser and that IF you image from a challenging, in terms of local direct LP, environment . What you want and need is a good light shroud and a light trap at the back end. Now of the CF argument: it looks nice but it is pointless and a pricey overkill ESPECIALLY on a tiny f/4 6". For goodness sake I can't tell the difference with a 12" I can't imagine one doing it for a 8". Monolithic spiders are another waste of machining time (which should be put to better use for the primary cell, by a long country mile) never mind their thicknesses. And here lies the rub: 99% of the primary cells out there are rubbish and their primary for larger sizes too thick. I have one that despite being 12 years old and a simple 3 point design never lost collimation for over one year on end , even when taken down from the mount and put beck on.

Dan Watt:

Tony Gondola:
There are a lot of ways I can think of that the current crop of Newtonian Astrographs can be improved. Lets restrict the conversation to apertures between 6 and 10 inches.
 

1.) Lowering the focal plane relative to the side of the tube (eliminate the focuser and focus by moving the primary) This allows a smaller secondary and could provide a very solid mounting point for a heavy imaging train, eliminating a lot of focuser caused problems.

2.) Stepper motor control of the collimation bolts. This could be used for both focusing and collimation and might be the best solution to the issues involved with moving the primary to focus.

3.) Integrate a boundary layer fan to deal with thermal degradation of the image.

4.)Carefully designing the configuration of the spider vanes to control both pattern and length of the diffraction spikes. Taste will vary here but there are a lot of options. That or go with an optical window. 

5.) Minimizing focus shift with temp by carefully choosing the tube material or by using a low thermal coefficient internal structure.

6.) Accurately machined and located baffles to control stray light.

7.) Flock the inside of the tube opposite the focuser

8.) Restrain the primary with an NARROW edge mask rather than clips.

Lastly, it would be great to avoid the use of a coma corrector. I know this is a must for really fast systems but if a smaller FOV and longer F-ratio (F/6) is acceptable, it would be nice to retain the perfect color correction and simplicity that Newtonian optics provide. Of course, a good quality mirror set is a given.

 

I have no doubt that all of the above is possible and would improve the instrument. My question is, would it be worth it in terms of photographic image improvement? Newtonians offer a lot of performance for the money so it seems worthwhile to look for improvements.

I've been building newts for awhile now and I've trialed and errored my way into some design elements that I feel work very well. The current crop of newts available from the big manufacturers is inadequate. If you take one apart you'll notice they use the exact same mirror cells, secondary spiders and focusers as their budget dobs. What might work fine in a f6 visual scope is sorely lacking on an f4 astrograph. It is finally getting a little better with offerings from Apertura and TS with stuff like cnc one piece spiders and marginally better focusers. But while I love newts, I can understand how the commercial options out there cause people to stay away. 

1. Keeping the primary in perfect linear alignment would be a very expensive and unnecessary engineering challenge. Hard enough on an SCT where the spherical primary can move around without losing collimation, much much more difficult with a parabolic mirror. And where would the coma corrector end up? Putting that same precision machining and engineering into the focuser is the way to go.  A slightly smaller secondary would have little benefit. Easy to test yourself, go cut out a circle with an even larger diameter (heck, double it) than the minor axis of the secondary and see how small of a difference it makes.

2. This isn't a bad idea for newts that are operated remotely and will only be touched once a year or so but a well designed cell will not change collimation over time if it lives on the mount. But it also adds a huge amount of complexity, cost and weight... that effort would be better spent on a robust mirror cell. 

3. Absolutely! Constant air movement has plenty of other benefits too. Will keep the secondary and primary (if things are really wet out there) from dewing over even on nights where everything else is dripping. I design my cells so that the single large fan on the cell blows air over and around the mirror, passing over the mirror face and then exiting the front of the tube. A quality fan such as a Noctua running at 100% won't transmit any vibration. Also a good idea to go with the largest diameter fan that you can fit. That means more volume at a slower speed, so less chance of vibration. For a sanity check, I'll through in an eyepiece with the most magnification I can get and on/off the fan to see if I can detect any vibration. 

4. The monolithic cnc cut spiders seem to be the way to go. Keeping everything perfectly square is key here. I've experimented with a few iterations of carbon fiber spider vanes (to avoid CTE issues with aluminum) and while they work well, there is a lot more work involved in making them and the time and energy involved in building them and adjusting for perfect colinear-ness outweigh the potential benefits. Larger newts, 16"+ might see more of a difference. As far as design goes, I like to keep mine as thin as possible but with enough bulk to resist sag. For CF 1.5mm works well, for a monolithic cnc spider 2-2.5mm is more of a realistic machining challenge. 

5. An optical window adds another large layer of complexity for little payoff. Reflections, trapping air, weight and the extra optical element potentially degrading the image have all kept me away from messing with them. 

6. Baffles are a great addition. Very inexpensive and simple to accomplish, just need to cut out some concentric circles and install! Especially worth it if you are imaging from a light polluted area and risk glare from neighbors lights. 

7. Flocking everything is cheap and easy but especially opposite the focuser. I haven't experimented with light traps as MaksPower suggests but it's always been something I'd like to play with. In the meantime, proper flocking and baffling work great. Note that any kind of textured flocking material will always work better than pain as even the flattest and blackest of paints will always reflect a bit at a high angle of incidence. Better to have some texture to "break up" the light. 

8. Exactly how I do it! Makes no sense to have clips AND a mirror mask when the mask could do all the work. Also note that most mirrors are beveled, you can have the mask/hold down cut precisely enough to only contact the mirror at the bevel so you aren't masking any of that valuable aperture. I know a lot of people say it helps with turned down edge but none of the commercial mirrors I've tested in the last decade have exhibited any serious turned down edge... plenty of surface roughness and undercorrection, but not much TDE. Your results may vary.

I've done quite a few 8" f4's and some 6" f4 scopes, usually used at f3 with a Nexus. Very happy with them. Currently planning on doing a 12.5" f4.8 scope for galaxy work. Here is the latest 8". The tube is a CF/Plywood composite (overly long video of my process: https://youtu.be/037iY_Vc1zg?si=e3lym3sMFPIyUNvO), the cell and spider are a mix of CF and aluminum machined on my poor cnc router. 

Thanks so much for the wealth of information here Dan. Really enjoyed the video and no, it wasn't too long! It's a beautiful build.

Have you ever tried different core materials like closed cell foam? You'd have to add blocking at the fastener locations but could make for slightly lighter and stiffer build (more separation between the CF layers.

Dan Watt:
Wood is a sort of closed cell foam and the flexible plywood costs $30 for a whole 4x8' sheet. Hard to beat. It's not heavy at all and I chose it specifically to avoid having to add inserts to every through hole. In this case I used 3mm thick wood, for a larger scope such as the 12.5" f4.8 I am planning I would use 9mm. 

If I was building something like an airplane wing or F1 body then I would choose different materials but a telescope tube doesn't really need a lot in the way of exotic methods and techniques. Primary goal was a lower CTE and increased rigidity over the typical steel or aluminum tubes and in that I succeeded. Weight ends up being pretty close to the common steel rolled tubes but is much much stiffer.

Makes sense Dan, it's too easy sometimes to go down the rabbit hole of exotic materials when it really isn't needed.

It's nice to see that some of these optimizations are having a real effect and in terms of the original question worth doing. Is your fan on the rear of the mirror cell?