13 replies to this topic

[macleod]

Just done 2hours polishing with what I,ve been told is cerium oxide - only its red ! Looks like "ball milled " ferric oxide, because although it is very dense red colour , it doesn,t seem to stain like "rouge ". Its put a great partial shine on the pyrex - I cannot focus on the glass surface with a 30x loupe fitted with LED light so the polish is very fine, but the Laser still shows a very even surface spot all over. Probably need at least another 2hrs to polish out. So please someone, any ideas what this " red Cerium " could Be ? Thank you, macleod.

[Pierre Lemay]

I've worked with two types of cerium oxide. One is white and the other is more brownish or salmon color. I don't know what difference there is between the two but they both work.

 

As for polishing time on an 8 inch mirror, I've done a few 8 inch and I expect you need to polish at least 8 hours to get to a full polish (are you using paper pads to accelerate initial polishing or just pitch?). You want to polish more than less. Nothing more frustrating than having successfully parabolized, especially a difficult f/2.7 parabola, only to get the mirror back from the aluminizer ($$$) and seing the pits near the edge  . Ask me how I know that.

[Pinbout]

 

I've worked with two types of cerium oxide. One is white and the other is more brownish or salmon color. I don't know what difference there is between the two but they both work.

the salmon has more impurities

 

either way you could always follow either with rouge which is slower acting but nice finish.

[Mike Lockwood]

Sounds like Barnesite?  It is red-brown in color.

[macleod]

Thank you all again! Yes " Barnesite " seems to fit the bill -what I have   is a dense red-brown colour , and seems to settle a bit faster in water bottle than the whitish Cerium.The chap who gave it to me says it is quite old ( ?) , and came from a 25kg bulk bag evidently, but they don,t know it,s true nature, just "red cerium oxide " which doesn,t fit the chemistry !

 So if its " Barnesite" will it give me a good initial polish ?

Regarding rouge _ where do you get small amounts of hi quality ? Further, how do you avoid scratches with it etc ? Danny- thank you for the reply, on this fast mirror i,d like to try rouge for figuring, so I creep up on a parabola and catch problems before they get out of hand. My10in f3.4 mirror works well below about 150x , but a bit soft at higher mags., although it does show a bit of detail on Jupiter/ saturn. Still I made it for low magnification wide field, as this one will be.  Mike Lockood - thank you for the comment- I,m a fan of your mirror making pages too !

Just an aside, a few weeks back I had the most perfect Planetary views from my light polluted  balcony - my Zambuto mirror at 500x showing fabulous detail on Jupiter and its red spot.That was with a 10in mirror.Happens only once a year unfortunately.Thank you all, Macleod

[macleod]

Just did first look at mirror in Ronchi . Straight lines, but turned edge. As I have a LOT more polishing to do, will go TOT and  COC short strokes and reduce TDE and slightly lengthen  focus. At least it looks reasonably part of a sphere ! Macleod

[sopticals]

Hi Macleod,

 

PM me as I have some each of both red and black rouge surplus to my requirements.

 

Stephen.(45deg.S.)

[Lognic04]

Can't wait to see how it goes!!!

[macleod]

Hi Lognic04 ! Been looking forward to your CN reports on your new mirror - but nothing! Be a sport and post again ! Yep I,m beating my brains out doing an 8in , 50mm thick pyrex which is reading f2.6 ( 530mm fl ) and Ronchi tells me is a great sphere already. I think that is because I started off making a tile tool from little 12.5mm square ceramic blocks , which gave me plenty of grinding leeway.I started by cutting a tinplate template with a curve scribed at COC or 1060mm, and used this to scrape my dental stone base into a matching curve while it was still a little soft. So right from go I was grinding to a sphere rather than the usual random " hogging" with a metal pulley  as I did on my 10in f3.4 mirror.  Because of the length of time your using a tile tool , the blocks wear down fast at first, so that is why you need the little deep bathroom floor type ceramic blocks.This obviously paid off in the nice sphere I have at this stage .I think I read that Ed Jones makes a template and works his plaster base straight off like this .Hope all is going well with your new large fast mirror. Kind regards, Macleod

[Lognic04]

Sounds good! Also starting figuring on my mirror - I think it's too cold here in Vic! 8 degrees and pouring, last time I tried this cold the pitch got so hard and scratched the mirror!!
Hope it's a bit warmer there

[macleod]

Hi Logan , in those low temps, I have two disposeable  aluminium " thin BBQ roasting dishes" ($3 at hardware /supermarket)  that I fill with water at  about 32deg C and immerse lap and mirror for 10min when its that cold, then press together with "flyscreen gauze " in between until they cool off, to get contact, give the lap a  brass brushing to rough up, warm wash and away we go. Friction will keep them from chilling .So far this winter up here on the NI East Coast we haven,t got that cold (yet!). I warm press my lap nearly every time to get perfect contact in winter.Keep smiling, Macleod.

[XB-36]

Where did you get the stuff to pour a mirror for you telescope.

I am looking to build a 12 inch mak. 

Thanks

XB-36

 

Edit: I am brand new to this and please assume that I know nothing about this. Thanks

Edited by XB-36, 21 July 2019 - 12:20 AM.

[Lognic04]

Hi XB,
What do you mean by poured? I assume you mean ground/ polished? A good place to start is Stellafanes mirror making guide.

[Lognic04]

And... It's really not as hard as some make it out to be! It's worth giving it a go. If you are determined I bet you could make a 12 inch for your first mirror, not too thin or fast though!