150 mm Solar Newton

[bart1805]

Yes, I would like to have a 150mm TOA. But then again: that is a dreamscope! So to move up in aperture for imaging the sun in CaK a Newton seems to be quite ideal. It has been done before, with bigger and smaller mirrors. When you de-aluminise only the primary and not the secondary, the effect can be compared to using a solar wedge. So approximately 4% reaches the filters and camera.

I found a second hand 150mm f/8 Skywatcher, interesting because of its focal length and small secondary. It has a 1,25 inch focuser (that is not so positive) and does not weigh much.
Today I started with stripping the mirror. I followed the same route Douglas described here:
http://solarchatforum.com/viewtopic.php ... ic#p117999

Here is the original mirror:

20190315_093632_resized by bart moors, on Flickr

Masking tape around the edges and poured some ferric chloride on it.

20190315_112922_resized by bart moors, on Flickr

After about 10 minutes the dissolving started.

20190315_120530_resized by bart moors, on Flickr

It took about 2,5 hours to dissolve all the metal. And here is, after cleaning, the now completely naked mirror:

20190315_160315_resized by bart moors, on Flickr

Clouds and rain this weekend so enough time to apply a new center marker on the mirror, put everything back together and collimate.

Questions? Feel free! Thanks for reading, Bart.

[MalVeauX]

Nice Bart, I've been interested in a solar newt for a while, with respect to white light & calcium, and short wavelength imaging in general at higher resolution without the need for a costly ERF or sophisticated design. Newts can be open and well ventilated and are inexpensive. Other than the weight when you go larger (8", 10" +) they're really a great alternative, albeit, dedicated for the task. I'd love to do a 10" for 430nm, but I can't support the weight on my mount currently.

I look forward to your results!

Very best,

[bart1805]

The same here. I am going to start relatively small and see for myself what are the good and bad points. And yes they are cheap. This complete scope will be less than a holder for a medium sized ERF.
If all goes as planned I will be making my own 9 inch f/7 mirror in the last quarter of this year. Looking forward to learning how to do it. And I have time to experiment with this one and try to get an idea how the bigger one should look like.

[MalVeauX]

I've looked at the mirror sets you can get for the primary & secondary, nothing else, from Agena Astro and they're inexpensive. I looked at those because while most 10" F4~F5 reflectors are 34~35lbs minimum, most of that weight is the tube (at least at those sizes). The GSO 10" mirror set is like 8~9lbs total. And it's the same mirror in the generic 10" F4/F5 OTAs being sold, so over 20+ lbs for just the OTA tube and accessories. I'm sure the mirror holding cell weighs a bit too. But still. I was thinking of what it would take to build an ultra-light-weight truss. I've seen lots of single truss designs for much bigger scopes using wood and/or aluminum. Could probably shave the weight that way to build an ultra-light-weight 10" F5 (so it's F10 with a 2x). Getting that weight down under 30lbs maybe. That would be a sweet inexpensive entry point to 430nm/Gband imaging that would also be handy for CaK and even 532~540nm.

Very best,

[Carbon60]

Interesting project, Bart. Good luck with this. At least it gives you something to do whilst it's raining.

Stu.

[skyhawk]

Unsafe for visual ????

[krakatoa1883]

A solar newtonian is a very useful addition to a solar setup. I made my own and I am very happy, It is useful for both WL, K-line and Ca-H without the need of expensive ERFs. Diameters larger than 5 or 6 inch require an open tube and a careful design to avoid internal turbulence.

It is safe also for visual with additional filtering.

[bart1805]

A solar newtonian is a very useful addition to a solar setup. I made my own and I am very happy, It is useful for both WL, K-line and Ca-H without the need of expensive ERFs. Diameters larger than 5 or 6 inch require an open tube and a careful design to avoid internal turbulence.

It is safe also for visual with additional filtering.

Thanks for the link Raf. I saw you left the center marker on the scope when you de-aluminized the primary, that is a good idea! It should be possible to left the center also aluminised, that would make the collimation proces even easier.
Could you please explain this one:
Since the sunspots are high contrast details, the obstruction caused by the secondary mirror is not important as the lens works on the right side of the MTF graph and therefore the "traditional" advantage of the refractor on the reflector is almost missing from the all.
I thought it would be better to work with a secondary as small as possible, if not for the contrast, then to minimise the aperture loss.

[bart1805]

@Skyhawk: yes totally unsafe when you use it this way. You could also de-aluminize the secondary and use filters or work the way Raf described. For me it will be a imaging instrument, not a visual one.
@Stu: the rain is (unfortunately) true.... Everything is ready now. Flocked the opposite of the focuser and the focuser tube, collimated the instrument. So waiting for clear skies!
@Marty: don't use a spherical mirror, but a parabolic one. And the faster the scope, the more critical is the quality of the mirror. It is just a whole lot more difficult to make a 10 inch f/5 than a 10 inch f/7. That is the reason "my future mirror" will be 9 inch f/7 or 10 inch f/6,5. A Serrurier Truss concept seems the way to go.

[Rusted]

How do you deal with the equally strong reflection from the back of the mirror blank?

A wedged blank is possible, but difficult. Rough ground? A conical blank? Obsidian? Black paint wouldn't work.

[marktownley]

A solar newton is a very good idea.

[bart1805]

How do you deal with the equally strong reflection from the back of the mirror blank?

A wedged blank is possible, but difficult. Rough ground? A conical blank? Obsidian? Black paint wouldn't work.

Hi Rusted, here is more information by Christian Viladrich about the back of the mirror.
http://www.astrosurf.com/viladrich/astr ... /N300.html

[krakatoa1883]

I thought it would be better to work with a secondary as small as possible, if not for the contrast, then to minimise the aperture loss.

in general the smaller the secondary obstruction the better the performance for high resolution work. However sunspot umbrae and penumbrae represent black-grey details on a bright white background, much more contasted that typical planetary details. In the MTF graph that represents the performance of the scope in terms of contrast the system works toward the lower-right corner, where it is closer to the contrast curve of an unobstructed telescope. In fact the WL images provided by my solar newt are very similar to those I have from my 100mm ED through the Baader wedge. In calcium light a good filter provides enough contast and the results are similar, I would say even better due to the perfect correction of the newtonian in the UV.

However be prepared to cope with more turbulence, from this point of view the refractor is always better unless the tube of the newtonian is totally open to air. In particular I found calcium imaging with my non-optimized solar newtonian (I kept the original tube) really challenging.

How do you deal with the equally strong reflection from the back of the mirror blank?

A wedged blank is possible, but difficult. Rough ground? A conical blank? Obsidian? Black paint wouldn't work.

a rough ground is enough, no problem of unwanted reflections

[Rusted]

Thanks. So how does the thick, glass blank behave when constantly bathed in sunlight?
Does it "warm up" above ambient and then need forced cooling?
THis may be the problem rather than passing air currents through the OTA.

You raise some interesting issues with open structures.
Refractors can also be built as open frames. As is my open, folded 180/12 OTA.
I haven't tried this instrument with a Herschel wedge let alone for H-a.
Would it suffer from increased thermal current issues if I simply removed the full aperture foil filter?
I'm just trying to level the playing field here between mirror and lens, solar telescopes.

[bart1805]

Thank you Raf.
Will first try it like it is now. Then I will make a simple wooden box 21x21 cm. 3x3 cm bars will function as the trusses. It will be an ultra simple wooden Serrurier Truss telescope. In combination with the SSM I think it will work. In fact, thanks to the SSM, I kind of lost my fear of bigger aperture when imaging the sun.

[krakatoa1883]

Does it "warm up" above ambient and then need forced cooling?
THis may be the problem rather than passing air currents through the OTA.

it depends on the mirror material as explained in the link to Viladrich's newtonian. I never found my mirror hotter than ambient temperature, at least by touching it, but I don't have thermometric mesures.

Would it suffer from increased thermal current issues if I simply removed the full aperture foil filter?

closed tube refractors perform very well with a Herschel wedge, if your scope has an open frame it should behave even better.

[Rusted]

Thanks for the useful responses.

I could imagine the mirror blank's temperature being easily monitored with a cheap and simple Indoor/Outdoor digital thermometer.
The outdoor sensor on the lead could be taped to the side of the mirror blank but shaded to avoid direct sunlight skewing the readings.
Avoiding local metal parts and placing the Indoor case in the shade is vital for accurate comparison.

Anyone who has tested an uncoated mirror on the Moon will confirm there is still a great deal of light reflected.
One should be very careful about filtering such telescopes since apertures can easily exceed most amateur's refractors.

[krakatoa1883]

Anyone who has tested an uncoated mirror on the Moon will confirm there is still a great deal of light reflected. One should be very careful about filtering such telescopes since apertures can easily exceed most amateur's refractors.

indeed this is the main advantage of solar reflectors, they makes available large apertures for WL high resolution work at a fraction of the weight and the cost of a refractor of comparable aperture. Mine is perfect for both imaging and visual if additional filtering is applied, larger newtonians should probably be checked for suitability. Btw the filters needed for visual use with my 130mm newtonian (ND3.0 + ND06 + UV/IR cut) are the same that are mounted in the Baader CC wedge for visual observing with my 150mm refractor.

[Rusted]

Excellent. Thanks.

[christian viladrich]

Hello Bart,
This is an interesting project !
This being said, and for Ca K - H imaging, and considering you have an F/8 mirror, another option would have been to keep the primary mirror coated.
The setup would have been a litlle bit more complex because you have have needed the addition of a subaperture filter just in front of the secondary mirror. A blue Astronomik or Baader filter could do the job for Ca K imaging or G band. The optical quality of these filters is about L/4 P-V.
The issue with the uncoated primary miror and Ca K, is that the level of light is a bit low.
In any case, this is something you can experiment at a later stage of the project. And you are going to have a lot of pleasure with the current configuration.
Cheers
Christian

[bart1805]

Thanks Christian. I talked about this project on a Dutch site and there someone suggested the same. But that was also after I had bathed the mirror in the ferric chloride..... It is not an exotic mirror, so someday I will find one to experiment the way you describe here. Mounting it before the secondary would be possible, but maybe making another spider lower in tube to fully use the two inch of the filter is the way to go. The amount of heat will increase a lot when you go bigger than 150mm, would it work (or would you dare it) in a 200 or 250mm telescope?
It will be interesting to see how half a Baader K-line plus one or two of the PST#1 filters will work in this de-aluminised version.
CS! Bart.

[christian viladrich]

Hi Bart,
Testing the concept with a 200 mm F/8 mirror (indeed a 300 / F5.5 stopped to 200 mm) is on my to-do list. I think it should be OK.
Before that, I am working on another project related to very narrow band Ca K imaging.

[bart1805]

Hi Bart,
Testing the concept with a 200 mm F/8 mirror (indeed a 300 / F5.5 stopped to 200 mm) is on my to-do list. I think it should be OK.
Before that, I am working on another project related to very narrow band Ca K imaging.

Interesting! And looking forward to see how you managed to apply the Blue filter before the secondary.
Perhaps it is possible to de-aluminise the secondary and apply a coating that reflects CaK and nothing else. Maybe MCM would be able?

[marktownley]

Exciting to hear what you're doing in CaK Christian, are you able to talk more about it?

[christian viladrich]

Bart, the idea of removing the coating of the secondary, or of having an MCM-like coating on the secondary, is not a good idea. This is because the secondary mirror support would get all the heat load. There would be no mechanical problem. This is just you will have the equivalent of a 40 Watts heater (for a 200 mm aperture) just in front the focus. The seeing would be awfull.
On the other hand, having a 200 mm primary mirror coated by MCM (Ha + CaK) would be OK. The only issue is to gather a group of four 200 mm mirrors to fill the coating chamber.
Unfortunatly for my 300 mm telescope, 250 mm is the max possible size for MCM coating chamber

Mark : not yet ;-)