Refractor baffles and heat

[davhei]

Hi everyone,

I have an 80 mm f6 apo triplet that I have done some white light solar with using a 1.25” Lunt solar wedge with ND3 filter and a baader continuum filter at the eyepiece. Got some very crisp views in august and early september with granulation effects (macrogranulation i suppose) and groups of sunspots and faculae during perhaps a dozen sessions. Great!

The scope is aluminium and the focuser is metal, something I took care to confirm prior to using it as a WL solar scope. The other day I started thinking about the baffles though, and the manufacturer confirmed they were plastic.

So. Stop or don’t stop.

What makes me uncertain is the fact that the heat sink on the wedge hasn’t gotten warm during my sessions so far, indicating that the beam at that spot anyway isn’t searing. And since the beam narrows to a point the closer you get to the focal plane it should be less concentrated further up the OTA towards the objective lenses.

What are your thoughts on this. Should I stop using the scope for solar with a wedge? Obviously don’t want to burn the baffles, but on the other hand, being overly cautious would prevent me from using the nice gear I have sitting around.

[Franklin]

Doubt it would make a difference, all the heat is concentrated at the focus and 95% of that is out the back of the Lunt. The baffles will get warm inside the tube, but only like sunlight through a window. I have a similar set up and have had no problems. See what others think.

[Stu]

@davhei I agree with @Franklin

The light only gets more concentrated further back in the scope, and if everything is right it should not even be hitting the baffles with the sun fairly well on axis.

I would just continue to enjoy the scope as you are 👍

[davhei]

Seems reasonable.

Just to be clear, it is primarily when the sun is off axis that concerns me. Say you let it drift and it starts to come near the edge.

Edited October 28, 2021 by davhei

[Stu]

1 hour ago, davhei said:

Seems reasonable.

Just to be clear, it is primarily when the sun is off axis that concerns me. Say you let it drift and it starts to come near the edge.

I still think that is fine as the light is still reaching the eyepiece, not clipping the sides. Where you may want to be careful is actually beyond that, where you lose the image from the eyepiece because then there is a potential risk of it clipping the baffles. I don’t think it’s likely to cause problems as I doubt the light would be focused, but I always cap my scope when not using it (as you should for safety anyway), but this at least stops any issues as the sun drifts beyond the fov.

[davhei]

11 minutes ago, Stu said:

I still think that is fine as the light is still reaching the eyepiece, not clipping the sides. Where you may want to be careful is actually beyond that, where you lose the image from the eyepiece because then there is a potential risk of it clipping the baffles. I don’t think it’s likely to cause problems as I doubt the light would be focused, but I always cap my scope when not using it (as you should for safety anyway), but this at least stops any issues as the sun drifts beyond the fov.

Best not to idle too much when finding the sun at the start of the session. I find that even after my best efforts to align the scope using the shadow I may still require some searching at the eyepiece before I can acquire it.

[Stu]

2 minutes ago, davhei said:

Best not to idle too much when finding the sun at the start of the session. I find that even after my best efforts to align the scope using the shadow I may still require some searching at the eyepiece before I can acquire it.

I doubt a bit of searching will cause any problems, just avoid leaving it for longer periods uncapped when not observing, should be fine.

[davhei]

4 minutes ago, Stu said:

I doubt a bit of searching will cause any problems, just avoid leaving it for longer periods uncapped when not observing, should be fine.

Sometimes you need a bit of reassuring or a sanity check at least. Would hate to give up on sharp solar views now that it is becoming more active.

[Stu]

7 minutes ago, davhei said:

Sometimes you need a bit of reassuring or a sanity check at least. Would hate to give up on sharp solar views now that it is becoming more active.

Of course yes, caution is always the best policy! But once you’ve done your checking, do enjoy the views. I love the sharpness and contrast you get with a wedge.

Quite right to be cautious though. I was using a 150mm scope for Ha observing with an internally mounted ERF some years back. I was experimenting with some aperture masks to see if it sharpened the image up, but when the sun got a bit off axis, the slight tilt in the ERF was enough to focus the light on the mask and burn a hole in it. No harm done but it did smell a bit! Always worth checking what you are doing 👍👍

[Floater]

17 hours ago, davhei said:

 I find that even after my best efforts to align the scope using the shadow I may still require some searching at the eyepiece before I can acquire it.

I agree with what seems to be a consensus that you will be fine. But to allay any lingering doubt regarding ‘searching’ I recommend using/fitting a finder. There are several on the market (including reputable enough suppliers on eBay) that cost very little.

It is surprising how difficult it can be to get that Big Fella in the eyepiece … 😎

Edited October 29, 2021 by Floater

[Highburymark]

On 29/10/2021 at 10:44, Floater said:

 

It is surprising how difficult it can be to get that Big Fella in the eyepiece … 😎

I remember my first solar session well: a small Maksutov, Baader solar film, and no finder. The perfect storm for a newbie - an F14 scope with jerky manual movement and a tiny fov. It took about an hour to find the Sun…..

 

[Peter Drew]

If no finder is available, searching for the Sun is made easier by removing the eyepiece.  You can then readily see the approaching glow whilst observing from a safe distance.  Once the image is found, pop the eyepiece back in.       🙂

[Rusted]

Despite being a solar imaging obsessive I still haven't built a solar finder.
The field of view through my 150mm f/10 with GPCs [working at up to f/20] is absolutely tiny.
Not much larger than a group of sun spots. I still use the shadow of the second telescope as my finder.
Make the shadow round and the sun usually appears on the computer monitor via the camera.

The other day I was checking the 150mm for internal dewing and deliberately pointed it slightly away from the sun.
Looking into the objective I could see the brightly focused sun falling on every baffle.
My home made telescopes are always fitted with thin, blackened, aluminium baffles.

To suggest that the sun's light will not fall on the baffles is slightly silly. IMHO.
A few seconds of non-driven drift or inattention will easily put the bright spot on all of your baffles.
So I would err on the side of caution. A long dewshield will help to block off-target sunlight.

Try to find some thin, "Funky Foam" in black from a crafts shop. It's lovely stuff to work with using a pair of scissors.
Roll a tube at least 3 times the aperture in length. To fit snugly over your existing dew shield.
Electrical tape will hold it in a tube form forever. The foam weighs almost nothing. Lighter than paper and much longer lasting. 
It won't change the telescope's balance much but may catch the wind. In the "old days" dewshields were always made this long.

I too have set fire to a temporary cardboard aperture stop in the dew shield of my 150mm from internal reflections off a D-ERF filter.
I'm talking about real flames inside the 10" Ø aluminium dewshield!  The smoke was filling the dome and making me choke!
There I was blaming our pyromaniac, waste wood burning neighbours.
 

[johninderby]

Now that I use a Baader solar filter on a standard 9x50 finder find it works so well. Wide fov so easy to find the sun but so accurate I can zero in on individual sun spots using just the finder.

Edited October 31, 2021 by johninderby

[Highburymark]

Nice set up John. Actually I find the TeleVue solar finder isn’t 100% accurate with some scopes. Even on my TV85, it’s well off centre. A proper finder with solar film is a good idea.

Must say the thought of heat damage to baffles never occurred to me before. I‘be tended to observe with slo-mo manual mounts for perhaps 20 minutes at a time, so the scope isn’t too exposed to the heat over long periods. But now I have the option of a tracking mount, I make sure not to leave it unattended for too long.

 

[johninderby]

If leaving the scope unattended I turn it at right angle to the sun or put the lens cap on. 

[Rusted]

2 hours ago, johninderby said:

If leaving the scope unattended I turn it at right angle to the sun or put the lens cap on. 

I would never leave a telescope unattended out of doors in daylight.
You never know who might become curious and take look at where the bright light is coming from.

There was a danger with antique, Herschel, solar wedges.
They passed the blinding, focused light out of the bottom of the open, prism housing.
Straight into the eyes of any passing child who glanced upwards.

Unless a solar foil, full aperture filter is well secured it could blow off in a gust of wind.
I make mine from close fitting, household cleaning containers.
They slide well down over the telescope's dewshield with a close fit.
Let's be careful out there!

 

[johninderby]

I meant unattended for a few minutes of course and that’s in my back garden with no one else around.

Edited October 31, 2021 by johninderby