31 replies to this topic

[helpwanted]

Not to hijack this thread however how do you mask a mirror at the mirror? Pictures helpful.

 

My mirror is also turned at the edge and it was recommended to mask it.

 

Clear skies,

Michael

AstroGoods makes them:  

 

https://www.astrogoods.com/cells.shtml

[helpwanted]

I would also add, when you do a star test on your scope, you want good seeing. If your tube has thermals or the seeing is bad, your test will be off. 

[mjgillen]

AstroGoods makes them:  

 

https://www.astrogoods.com/cells.shtml

Thank you for the helpful link! I’ll order one as soon as my mirror returns from getting recoated so I can measure it carefully.

 

Michael

[SteveG]

Eddgie,

Maybe you missed it but I've done multiple star image tests and all give the indications you described and as depicted in your post. So I'm masking and will adjust the mask based on observations. I have zero anxiety and enjoy tinkering. This is a hobby so I don't mind experimenting. For my use on double stars I'd sacrifice some aperture if it gave me a significantly better image. A good performing 7 inch would serve my double star observing needs. If my experimentation doesn't improve things I'll just return to base line and I'll be fine. This is just one of many scopes that I own so I have many other choices.  Thanks for your posts, keep up the good work.  

Bill

I would recommend you follow his advice very carefully. He's been here forever, and the advice is likely far more expert than anyone in this thread. I haven't read the Suiter book yet - but I need to. He's trying to help you get the most out of your scope. I love these posts because I'm learning too.

[Bonco2]

I've been reading Eddgie's posts for years now.  They are always informative based on his deep understanding of optics. But I have limited expectations for the 8 inch. I'll not put any money into it, so if it needs optical correction I'll not do it. So no reason to do other than simple star tests and see if the mask improves things. If it does fine, if not no harm done. Like I said I have several other telescope options most with superior optics. No complaints from me about Eddgie and I enjoy reading his informed comments.

Thanks, Bill 

[Bonco2]

I forgot about this but today reading notes from 30 years ago. I saw that I masked my 10 inch f/6 Cave Newtonian.

Why?  Because I was never satisfied with it's optics compared to my RV6, 6 inch f/8. I'm referring to planetary sharpness and star images at high magnification. Also an optical expert Richard Fagin advised me  the mirror needed simple correction. The mask did improve things after many tests. But I sent the mirror to Richard. When It came back I was impressed with the improvement. My notes said at that point..."no more mask"

The point being is that masking won't reveal the cause of an optical problem but it might be an inexpensive solution for one that doesn't want to do spend money for a correction. If the mask is not too restrictive the light loss will be minimal  and the optical performance might be improved with no cost. It's very well known that masking can improve less than perfect optics if for no other reason than increasing the f ratio. Longer f ratios are more forgiving of less than perfect optics.

[Bonco2]

Here's my report using a 7 inch mask on my XT 8. Jupiter and Saturn were first targets. No mask view was pleasant but not sharp, I would describe it as soft.   Seeing was marginal about a 5-6. 7mm at 171X was max high power view because  the sky wouldn't allow higher power. I put on the mask...Instant improvement in sharpness and contrast on both planets.  The moons of Jupiter became like disks not pointy stars. 

Next was star test using the 7mm on Altair. Since I've owned this telescope this was the best star test images by far at focus, inside and outside focus. Hairy inside focus was hugely improved and was very close to the outside focus image. So what was the optic error? I don't know or care, I'm satisfied that I improved the performance for no cost. For deep sky I can remove the mask and be back to full aperture. These mass produced telescopes are not uniform in optical quality for various reasons. If you have a good one then you are fortunate. If like me yours is not optimal, a simple masking might improve the optics with minimal effort and zero cost. 

Bill