Solar in a 10 inch Dob?

[Size9Hex]

I was hoping to get some advice on whether my current Skywatcher 250px Dob can be used safely for solar viewing please? Also, is it even a good effective choice for solar, or do you get better results with entirely different kit (e.g. refractor & wedge)? I don't recall reading of many (if any) folks using this type of kit on the sun and wasn't sure if that was telling me something? Obviously I'm hoping the Dob is suitable if it reduces the investment (meaning it might actually happen!).

My current setup for the night sky is a stock 250px (closed tube, not truss) with 10x50 straight through finder. My understanding (which might well be wrong) on what I'd need to prepare it for safe solar is as follows:

A blocking filter for the front end. The one I was looking at is: http://www.firstlightoptics.com/solar-filters/astrozap-baader-solar-filter.html

I was assuming I'd get better results with the 10 inch filter rather than filtering through the 2 inch hole/mask in the plastic cover of the scope? Are these filters easy to puncture or tear, or is something you expect to last for years?

Remove the current straight through finder just in case.

Addition of a new (safe) finder of some sort? I'm not sure what the options are, though I guess you could make a pin hole camera style target/finder easily enough?

Any and all opinions/advice appreciated as always please!

[Cornelius Varley]

I think I would make a smaller filter for the end cap. 

[Stu]

You will likely get better results with a wedge and refractor, but that's not to say you can't use your dob.

A full aperture mask is needed to prevent any heat build up in the tube which can damage the secondary. You can either go for the full mask as solar film, or make a reduced aperture which fits between the secondary vanes. There is often a view that solar observing is best with 100 to 120mm aperture because of the daytime seeing conditions. A 10" will in theory give better resolution but only when seeing is excellent.

This is the kind of thing I mean buy a sub aperture mask. Note that it is NOT recommended to use an open truss design without a shroud for solar, it can be dangerous. I know you have a solid tube though so no problem.

In terms of finders, you can either cap, or remove your finder and just check for when your scopes shadow is a minimum to find the sun. Or there are various DIY solutions out there for pin hole/screen finders, or just cover your finder with solar film.

Finally, make sure you get the visual rated film, not the imaging stuff which is too bright for visual.

[vlaiv]

I used such a solar foil filter (baader) with my 130mm newton with no problem.

If using only 50mm opening (2"), you will limit your self to about x100 mag resolution wise. I think filter is quite durable, you can check it by looking directly thru it at the sun (with no optics), but be careful not to look at the sun without it.

I found that I did not need finder when observing sun. Advice is to remove it or at least cover it to avoid accidents. It is easy to find sun - just observe shadow that is cast on ground by your telescope - when perfectly round - you are pointing to the sun.

[bingevader]

I used my 8" dob, etx90 and a 130mm newt at school for the eclipse last year.

The 8" was stopped down to the 2" end cap with solar film, the etx had a 90mm glass filter and the 130mm newt had a full aperture solar film filter.

The 2 film filters were easy to make and much cheaper than the  glass filter, we had the two of them and 12 pairs of solar specs out of an a4 sheet.

The 130mm newt provided clearer, but smaller views than the 8" dob due to the larger aperture.

The etx90 showed the best views, but this was related to the aperture and focal length rather than the glass filter.

I'd go for the larger aperture if possible, but as Stu has said, you may be limited by the seeing conditions then however.

It's easy to test if the filter is damaged by holding it up to the sun and check for holes before fitting.

People have made finders similar to these.

 

[Moonshane]

I have never even thought of using a newt again (for solar) since I got a Lunt wedge and used it with a refractor.

[Size9Hex]

Thank you everyone. Really appreciate that you've taken the time to reply and pass on your experience.

[Moonshane]

The main advantage for me of a Lunt wedge and refractor set-up is that of safety. Whilst 'foil' filters are perfectly safe as long as used correctly and in good condition, consider the two following scenarios:

• Your refractor coupling with the wedge comes apart and your wedge smashes on the ground. Not good financially but you only suffer a slightly warm leg.
• Your film filter is blown off, is knocked off, falls off or is torn and you didn't notice. The consequences if observing don't bear thinking about.

You could buy (used) a suitable refractor for about £50-60 and a Lunt wedge for about £100. To me it's worth the additional cost and to my eyes the views are better too.

 

[101nut]

From my point of view, a well made, well secured and well looked after Baader film filter is perfectly safe. However, any damage to the film will obviously be an issue. The filter area for potential damage, ie the risk, increases rapidly with diameter. A 4" filter has roughly 12"² of potential damage, an 8" filter has 50"² square inches ...

The balance of potential increase in resolution (on days of particularly good seeing) against actual increase in risk needs serious consideration by individual users ... I've seen pictures of a mounted, 30", full aperture filter gently flexing in the breeze and was not afraid to use the word 'stupid'.

AndyG

[LukeSkywatcher]

Use what you have. If that means a 10" Dob with solar film,so be it. I tried a few times to make a filter with the solar film but my DIY skills are less than zero. I have a small 70mm refractor which wasnt getting much use, so i invested in a wedge for it along with the safety filters. I love the views it gives. 

[Alan64]

I just received a 20cm x 29cm sheet of Baader AstroSolar safety film in the mail today...

With a 250mm Newtonian, and a sheet of the above, either an 80mm or 90mm off-axis solar filter is possible...

Click on the image for a larger view.  One might then have either a simulation of an 80mm f/15 or 90mm f/13 apochromatic refractor for high-quality, white-light solar viewing.  If a 90mm solar aperture is chosen, careful measurements will be required to avoid blockage by the secondary.  The diameter of the tube is slightly larger than the mirror itself, therefore the edge of the chosen aperture would need to be as close to the edge of the secondary as possible when positioning.  Avoid the spider-vanes as well.

Do not use the black plastic aperture cover, as the Sun will heat, warp and deform it.

A Herschel wedge is old(circa 1830) but highly-effective technology for white-light solar viewing, and the best.  The Baader and Lunt wedges available today, or any Herschel wedge, can be used ONLY with REFRACTORS; and ideally with a longer-focus 80mm to 100mm crown-and-flint achromat of better quality; a used Towa, for example.  Their use with triplet refractors has resulted in cracked objectives, and is documented.  Many employ ED doublet and even fluorite doublet refractors for such, but at the user's risk.

Be safe, and enjoy observing the "Aten".

 

[Size9Hex]

Thanks for the latest replies. Good to see there's a range of different views on it - all of which are good food for thought. Gives me something to mull over! :-)

I am now starting to reconsider whether I want to trust my eyesight to a large flappy piece of foil!

 

 

[Moonshane]

An air spaced doublet or triplet is fine (I use both) but never use a petzval type with a rear element as this is too close to the focal point which is where the heat is.

[Alan64]

5 minutes ago, Size9Hex said:

Thanks for the latest replies. Good to see there's a range of different views on it - all of which are good food for thought. Gives me something to mull over! :-)

I am now starting to reconsider whether I want to trust my eyesight to a large flappy piece of foil!

Baader's solar film is quite safe when used properly.  As far as I'm aware, there have been no reports of eye damage in the use of it, and over a span of ten years.  It is very important to follow the directions to the letter when mounting the film.

An 80mm f/11 Towa achromat, or better still an f/15, combined with the 1.25" Lunt wedge would make for a great solar instrument.

[Moonshane]

I agree that it's safe in use. I tend to look at worst case. Ignoring the moderate cost a herschel wedge gives the safer views and the best views. For me it was and no brainer.

[bingevader]

I suppose, like with most things, it depends on how serious you are about solar observing.

If, like myself, it's an occasional thing, that enables me to get a bit more use out of the 'scope when the cloudy nights prevent much else, then the solar film is more than adequate.

If you are looking to take up solar observing more seriously, then investing in some half decent kit would be preferable.

Maybe the out lay on a sheet of film would help you decide.

It's not at all dangerous if undertaken properly.

We had to fill out risk assessment forms for school and went through the added safety precaution of taping everything on even though it was securely fastened!

I think you'll manage perfectly well on your own.

[101nut]

12 hours ago, bingevader said:

... went through the added safety precaution of taping everything on even though it was securely fastened!

+1 for that. Your eyesight is your problem, that of others is far more important - tape, more tape and, in my case, a 6" deep dew shield to keep the filters on and inquisitive fingers off ...

[Size9Hex]

On 2 March 2016 at 19:45, Alan64 said:

Be safe, and enjoy observing the "Aten".

Thanks, that was a very helpful post. I confess I hag to google your last point though :-)

[Size9Hex]

12 hours ago, bingevader said:

I suppose, like with most things, it depends on how serious you are about solar observing.

...

Maybe the out lay on a sheet of film would help you decide.

Thanks, that's a really helpful point. I stuck with cheap and cheerful binos for a fair while to see whether this hobby would just be a passing interest. This sounds like a good way of confirming something similar with solar.

[Size9Hex]

Sorry to resurrect a thread from a couple of weeks ago. Just wanted to repeat my thanks to all who replied with advice and update on the outcome in case it helps any visitors from the future who open this thread.

I went for the Baader solar film which was easy enough to put in place over the small aperture in the dust cover that is supplied with the 250px scope. I figured I would do this, and would have plenty of film spare to make a larger custom aperture later if I wanted to. Just used it for the first time this afternoon. Checked for pin [removed word] holes by holding it up to the sun before putting the scope outside. Easy to find the sun in a wide angle (50x) eye piece using the "make the shadow small" method described above. I quickly realised that I was a weak link in the safety chain. It's second nature for me to automatically reach across to remove the dust cover as I sit down to start observing and I was saved from my autopilot actions by my "think three times before doing anything just in case the scope turns into a laser death ray" rule. Failing that, I had at least taped the dust cover down as a second layer of safety. I wouldn't say this was a near miss; Just that confirms the importance of being on the ball, not rushing, not cutting corners etc.

Anyway, just a quick view of the sun through hazy wisps of cloud mid-afternoon today. I was surprised how dim it was (certainly bright enough for good viewing, but dim compared to a life of experience that the sun is extremely bright!). Couple of nice juicy sunspots to marvel at too. Now I'm all set, and looking forwards to seeing Mercury cross the sun in May

 

[Louis D]

I've been using Baader solar film mounted between two craft foam boards with double sided tape since it was first introduced.  I wrapped heavy poster board around the end of my tube and taped about 5 revolutions together to make a tough tube to attach the poster board to using more tape.  I just slip it over the end, and it stays put.  You've got to wiggle it to back it off.  I always double check the tape to make sure it isn't coming undone before each use.  After more than a decade, no issues at all.  I keep it stored in a bag with a heavy cardboard cover to protect the foil.

The views are much better than through my old full aperture glass solar filter.  The color is neutral gray instead of deep orange/yellow and sharper.  However, I always feel like my eye is being cooked over time, so I bought an IR blocking filter a while back to see if I can block the residual IR getting through, but I haven't had the chance to try it out under the sun.  A neutral density moon filter can also tame the brightness if it becomes excessive.  Other than that, the full aperture views are fantastic in either my ST80 or 6 inch dob.  I made custom cells for each from the same sheet of solar film.

[bingevader]

I'm glad it went well Paul.