Is it safe to use a spherical mirror for a solar newtonian ?

[Dave_astro6572]

I have just learned about solar newtonian recently. Looking back at my telescope collection, I still have a 4.5" f/8 spherical mirror unused. So my plan is to turn that mirror into a solar newtonian. Searching online, I see people only use parabolic mirror for such scopes; I think because parabolic mirror focuses light into a single point and a spherical mirror don't; which makes it unsafe to use. But do a small 4.5" f/8 mirror safe enough to be use in a solar newtonian ? Has anyone built a solar newt using a spherical mirror before ? Let me know your experience. Thank you !!!

[Dennis]

Hi, if you strip the coatings of the mirror there will be less then 5% remaining light that hits the secondary. That sounds safe to me (with additional filters at the eyepiece).
But why a spherical mirror in a newton, i think it will be optically not so top.

[Dave_astro6572]

Re Dennis:
Thank you for the information !!! I think I will also strip the secondary mirror as well. That gonna lower the amount the light that hit the eyepiece even more.
For the filters, I think of using just a normal UV/IR cut combine with a ND or polarizing filter is safe enough for visual.

[DavidG]

I made a solar newtonian from 4.25 f/10 mirror that I stripped the coated, a secondary I stripped the coating and homemade rotatable Herschel wedge. Since the light reflected from the uncoated secondary is partially polarized, by rotating the Herschel wedge the light is then cross polarized and you can adjust the brightness.
You can find my design at www.considine.net/dgroski

Here is a link to thread of a person that built my design https://www.cloudynights.com/topic/4832 ... newtonian/

- Dave

[Dave_astro6572]

I made a solar newtonian from 4.25 f/10 mirror that I stripped the coated, a secondary I stripped the coating and homemade rotatable Herschel wedge. Since the light reflected from the uncoated secondary is partially polarized, by rotating the Herschel wedge the light is then cross polarized and you can adjust the brightness.
You can find my design at www.considine.net/dgroski

Here is a link to thread of a person that built my design https://www.cloudynights.com/topic/4832 ... newtonian/

- Dave

Hi,
I have read about your design and found it extremely interesting. Sadly, it is very hard to get my hand on a Herschel wedge from where I live. I have some questions about your design though. Do you any kind of filters ? Especially an UV/IR cut filter. The UV / IR behavior in such scopes still bugging me.
After I stipped the secondary mirror there was apparently back reflection. Do you experience any in your scope ?
Thanks.

[allhoest]

Re Dennis:
Thank you for the information !!! I think I will also strip the secondary mirror as well. That gonna lower the amount the light that hit the eyepiece even more.
For the filters, I think of using just a normal UV/IR cut combine with a ND or polarizing filter is safe enough for visual.

Keep in mind that you need to collimate your scope.
You probably need to collimate during the day also.
With a stripped mirror I can laser collimate, but I use for that a very powerful green laser.
When the secondary is also tripped, the collimation becomes a real issue.

CS
Alex