SkyWatcher Heritage 130p Solar Dob Mod

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SkyWatcher Heritage 130p Solar Dob Mod

Post by thesmiths » Mon Jun 09, 2014 12:53 am

When we get in the modding mood, it's hard to know when to stop. I recently considered getting a 2 inch Herschel wedge but instead decided to try a Solar Dob Mod since it would be a little cheaper (at least nominally in terms of money, certainly not in terms of time). Actually, I also wanted to do some experiments to see if a Solar Newtonian might be a good platform to do imaging below 400 nm (since most refractors don't work so well there but a Newtonian should be unaffected). So here is the story.

I first became aware of this approach from Dave Groski's "White Light Newtonian" webpage a few years ago. The idea is to strip off the aluminium coating from the primary mirror and possibly also the secondary mirror. Around 5% of the light is reflected from normal glass (as in a Herschel wedge). So if both the primary and secondary are stripped, the light is reduced by approximately 2.5 x 10-3. This is equivalent to using a solar filter of ND 2.6.

I have followed much of Dave Groski's guidelines. He has also made a number of posts to Cloudy Nights to clarify some of the issues. I have to admit never having even used a Newtonian before, let alone modding one, so I came to this as a real beginner. After some consideration, I decided the ideal platform to start with would be the Skywatcher Heritage 130p Flextube.
SkyWatcher Heritage
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Despite being classified as a "beginner's telescope", it has excellent reviews. The 130mm F5 parabolic mirror is supposed to be first rate. I could tell from the photos that it would be easy to take apart. I anticipated the tubular design might be useful in terms of achieving focus for imaging (this turned out to be correct). I also guessed that the mirror cell would be made such that light could be able to escape out the back once the aluminium coating had been removed (this also was correct). So I ordered one from First Light Optics (£135 including shipping).

In the meantime, I ordered from Maplin's a 250 ml bottle of Ferric Chloride to strip the mirror (cost of £10, free delivery). This is normally used to etch copper from PCB and it does this quite quickly. It will much more slowly etch aluminium. The etchant Maplin's sells is concentrated and normally diluted 1:1 with water but I used it straight from the bottle. I strongly recommend watching the YouTube video "Stripping the Aluminum Coating from a Telescope Mirror" if you ever want to try to do this yourself.
mirror ferric chloride
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So here is my 130mm parabolic mirror, removed from its cell, with masking tape wrapped around the edges. Make sure to have suitable gloves and plenty of bicarbonate of soda (to later neutralise the ferric chloride acid, as explained in the YouTube video). After about 1 hour, the mirror looked like this, as the aluminium (and the silicon dioxide overcoat) began to dissolve.
aluminium disolving
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It took about 3 hours until all the aluminium appeared to be gone. However, after neutralising and washing in water, there was still a thin aluminium residue in some spots so I had to put a little more ferric chloride back on the surface for 30 minutes. After neutralised and washing again, it was quickly ready to be put back in the mirror cell.
mirror cell
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Taking apart a Chinese-made beginner's telescope made me realise a few things. First of all, the design and components tend to be very good. However, the assembly can be very poor. There were a few minor issues with how the mirror was held (mainly quality control problems so it likely won't be the same if you try this). I put everything back together and collimated the telescope. I had done this when the telescope first arrived (and had all its coatings). With a laser collimator the process was very simple. With the primary coating removed, the process was almost as easy. It turns out there is a circular grinding pattern on the back of the mirror which helps to find the centre spot (the laser spot is now visible from the back of course). With a 1/20 reduction in light, the human eye perceives this as about "half as bright". So collimating was very straightforward.

It was even bright enough to easily do a star test (with an artificial star). At this point I realised that during assembly the secondary diagonal mirror had been put in upside down. A secondary may look symmetrical at first sight but in fact it's not and if it's upside down then a frosted glass side will block some of the incoming light. Again, it was a quality control issue related to the final assembly. Fortunately, the secondary mirror was simply held on by double sided tape so was easily removed. Since I had taken it off, I decided to also remove the aluminium coating on this mirror too. This only took 1 hour (I guess the coating was much thinner).

I reattached the secondary using a useful product from 3M called "Damage-Free Picture Hanging Strips" (available from Ryman's). This is similar to Velcro but stronger and less messy. This way I could play with the position of the secondary and also easily swap it for another one (more on this later). By the way, best to wear cotton gloves (sold for handling things like photographic slides) when handling the mirrors to avoid getting finger prints on them (this is especially true if the mirrors are coated).

But collimating with both mirrors stripped was quite challenging since there was now very little laser light reflected back. With some difficulty, I managed to do this and did another artificial star test. The results looked very good. I also looked at it with a Gerd Neumann Ronchi eyepiece and the pattern looked perfect. Obviously the optical components themselves are very good, despite not always being assembled correctly.
telescope complete
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Here is what the telescope looked like fully assembled and ready for "first light". I decided to use some of the leftover masking tape to wrap around the black tube (to make it slightly more reflective). You can see here that it comes with a nice Vixen dovetail bar and is light enough to put on a very small mount. I actually think the open tubular design is probably an advantage for solar work (in terms of "tube currents").
mirror back
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The view from the back is nice -- no need to buy a solar finder! It's pretty obvious when the Sun is centred. I didn't find any advantage to covering the back in any way. The amount of light coming back into the optical system (compared to the direct sunlight) appeared to be negligible.

At this point it's worth mentioning that the secondary mirror provided with the telescope is quite oversized. The specs on the SkyWatcher website say it is 34.5mm. In fact, the one I got was 40mm (minor axis diameter). This is a 31% central obstruction, which is quite high, even for a fast Newtonian (the secondary mirror gets smaller as the F number increases). In fact, the maximum size of the secondary mirror holder is 35mm and a quick calculation shows that for solar work a 30mm mirror is large enough. I may order a 35mm diagonal at some point and keep the coating on it. With the "Picture Hanging Strips" it should be fairly easy to swap them depending on the application.
secondary mirror
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Here is the view of the secondary mirror and you can see it is fairly oversized versus the diagonal holder. The image of the Sun clearly fits inside the 32mm retaining surface.
telescope front
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This photo shows the view from the front of the telescope. What's interesting here is to note the focuser. It is a very primitive design of helical focuser made of plastic. It was a little wobbly so I wrapped some Teflon plumbers tape around the inside tube. It actually works surprisingly well. Once you achieve coarse focus, you can tighten the screws to hold the camera (or eyepiece). Extremely fine focus adjustments can be made, comparable to a Crayford focuser. There is a slight rotation during fine focus but it is probably only 10 degrees at most. The whole telescope also keeps its focus very well. Most reviewers of the telescope are also shocked how well this focuser works.

With a DMK camera and a 2X Barlow, there was no problem to achieve focus with the tubes fully extended and locked in their normal "click" positions. Without a Barlow, the DMK didn't quite reach focus so I shortened the tube by about 5mm and everything worked fine. That's a really handy feature depending on how you want to image. By the way, if you chose to shorten the tube a lot, you probably will need the 40mm diameter secondary.
sun f5
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Here is an image of the "first light". The conditions were very bad -- lots of clouds and wind -- so not much data could be taken and I didn't spend much time processing it either. Baader Continuum Filter, Baader ND 3.0 and an IR block filter with a DMK 51. This was only taken at 1/120 sec. I needed to use a lower ND filter but I didn't have a suitable one. Qualitatively, it is a similar result to what I get with my 106mm F6.5 APO and 1.25" Lunt Herschel wedge (with the same filters and camera). The combined cost of the 4 inch APO and Herschel wedge is many times the cost of the 5 inch Newtonian. In fact, the Lunt wedge alone costs slightly more than the SkyWatcher Heritage telescope!
sun f10
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Here is a quick attempt with the 2X Cemax Barlow (so 1.3m FL) and a DMK41. I replaced the ND 3.0 filter with something called a "Moon Filter" that I found in my box of filters. It came for free with something and I'm not quite sure how much it attenuates. But using the "Moon Filter" I was able to increase the shutter speed to 1/1000 with the Barlow. This is a 100% crop of a nice sunspot. The seeing was not very good and the processing is quite bad but I think it shows that the resolution of the telescope has the potential to be quite good.

There seems to be plenty of light for imaging with two uncoated mirrors. I subsequently did some tests with the Baader K-Line filter with no ND filter, an IR block filter and could use shutter speeds of 1/5000 at F5 and 1/1000 at F10.

I think this design is, as Dave Groski points out, a great "outreach" telescope since it's safer than most solar telescopes, is so simple and "transparent", and small and cheap. It also gives some additional insights into physics (reflection of light at a change in index of refraction). I hope you enjoyed reading this and I will report back as I gain more experience with this design. In particular, I am in the process of doing some experiments with CaK imaging (which was the original intent of doing the Solar Dob Mod).

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Re: SkyWatcher Heritage 130p Solar Dob Mod

Post by Valery » Mon Jun 09, 2014 3:23 am

Better to paint black the back sides of both mirrors. You will have better contrast.
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Re: SkyWatcher Heritage 130p Solar Dob Mod

Post by swisswalter » Mon Jun 09, 2014 5:17 am

Hi Douglas

what a fine mod and well written story, thanks for sharing and congratulation on the first light. Looking forward to the CaK second light
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Re: SkyWatcher Heritage 130p Solar Dob Mod

Post by thesmiths » Mon Jun 09, 2014 11:24 am

To Valery: I have read conflicting opinions on painting black and leaving clear. There is logic to each of those views. The problem with painting black is the issue of heating of the glass and therefore increased air turbulence. A more sophisticated approach to what you are suggesting is to build a "black box" behind each mirror to trap the transmitted light (but not in contact with the mirror). But this may still lead to undesirable heating effects. I will do some more tests on whether contrast is affected. My initial (somewhat surprising) impression is that it is not.

Arthur Whipple has built a huge 350mm F4.6 version of an uncoated solar Newtonian. You can see his telescope here:

and his amazing images here:

As you can see, he subscribes to the view of keeping the structure as open and well ventilated as possible. He had previously built a smaller 8 inch F10 version along the same lines a few years earlier:

so I assume he spent some time either thinking about or experimenting with the issue of painting, enclosing the mirrors, etc.

A further note on outreach and visual use: an ND 3.0 filter, Baader Continuum Filter and IR block filter leads to a very comfortable and extremely high resolution visual image. No need for further filter (eg with a polarising filter).

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Re: SkyWatcher Heritage 130p Solar Dob Mod

Post by Derek Klepp » Mon Jun 09, 2014 11:31 am

A work of art.

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Re: SkyWatcher Heritage 130p Solar Dob Mod

Post by marktownley » Wed Jun 11, 2014 10:37 am

Fantastic! Really good mod. be interesting to see how you get on with it.
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Re: SkyWatcher Heritage 130p Solar Dob Mod

Post by astroshot » Sun Jul 13, 2014 7:51 pm

Very interesting. Great timing too. I was only thinking about a solar Newtonian today !
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