16 inch Dob project under way

[Daniel-K]

Hi my names is Danny and my friend is Barry and together we have decided to make a 16 inch F5 DOB

i thought it would be good to show our progress and share it with fellow members to show its really good fun making your own telescope!

we have gone with the method of using a glass tool as we have been told its a bit more forgiving if we make a mistake after all this is our first one.

so we have two pieces of glass both 16 inch's and 1.25 inch's thick plate glass. pritty much on the limit for the size of the mirror.

we also have a turn table that rotates instead of stopping and turning the glass manualy we just turn the mirror every 4-6 strokes. i believe this is to randomise it so you get as near a spherical center (correct me if im wrong )

to start the HOGGING OUT we picked 60-80 aliminuim oxide grit from machine mart. after reading we decide on the chordal stroke to hog out.

our aim is for an F5 or as close as we can get and this requeird us to get a depth of 5mm sagitta.

Day 1

today went very well after about 3 hours of grinding we able to messure down to 1.8mm in the center of the glass as the curve was starting to form.

as we started we realised this was messy so its a good idea to were old clothes and have a towel handy your hands in between the grit.

after about 20mins of griding we began to realise this isnt going to be a one day job. getting the right amount of grit and water is the key to much water and the grit gets pushed to the edge too little water and the mirror just sticks

when you get it right you can hear the grit really working on the glass.

Day 2

more hogging out!!

took around 2 hours to get to 3mm

Day 3

for some reason we had a brain wave to put the mirror on the bottom and tottaly canceled out day 2 so we were back at 1.8mm

Day 4-Back on track

after a trip to machine mart for more grit i really got to stuck in for a few hours with the mirror on top (M.O.T)

and managed to get back to 3mm after 3 hours of hogging out. beacause of this error we have lost 2mm in the thickness of the glass we just hope this doesnt affect the mirror too much but it is our first attempt and its a learning one

Day 5-

after day 3 we decided it was time we both got stuck in today for a few hours of hogging out. so on with the old clothes and away we went of corse interupted by fags brakes from Barry and the fella drinks t like its the last one he's ever going to have

we never encounted and problems today and managed to get to 4.3 mm. 0.7mm left great! the curve in the glass is really starting too show.

Day 6-

today i was determend to get to that 5mm mark. so of i went on my own (barrys was at the hospital with his grandad) it took me about 2 hours on my own to get to 5mm. the next stage was to use 120 grit but me being me when i got the second tub of grit i got 80-120 so thats why its taken us so long to get down to 5mm because we were using finer grit insted of the more abrasive stuff 60-80 but it worked out ok for us, it ment we could get on to the 240 grade grit.

quick summery

7 days into the mirror making and heres were we are at

5mm sagitta-done

120 grit-done

moving onto 240 grit.

Day 7 -

were are starting to work through the grades of grit now and this time we are using 240 so off we go!

1hour 20 mins worth of using the 240 grit and the differnce in apparence and texture is unreal to mirror has a smooth glass like finish unlike the 80-120 which felt rough and has pits in the glass from the grit. the 240 grit has really smoothed out the glss making the pits smaller. we check to see there are no big pits visable using a 32mm 2" eyepiece over the glass doing so tells us that the mirror is the same all over. we can also reflect a small bit of light of the surface, how exciting !!!

at this stage we are happy with what we have done and are waiting for some finer grades of grit to arrive! will keep you updated! with more pictures!

please feel free to give us some advice! ( apart from the spelling:rolleyes:)

[Glasspusher]

Danny, always nice to see people having a go at grinding there own mirror! A full sized grinding tool is not the best way to go at 16 inches, as you have found there can be a tendency to stick, so keep the thing wet. Also you are more likely to pick up fine scratches with the finest abrasives. Might I suggest that you finish on something like a 600 aluminium oxide? Make sure that you rotate the mirror relative to the table it is sitting on on a regular basis, this will hopefully avoid astigmatism. Looking forward to seeing how this project progresses.

John

[Moonshane]

this is an amazing project - go for it!

[Daniel-K]

do i use cerium oxide after the 600? the table rotates and i rotate the mirror the opposit way its turning every 4-6 strokes to avoid that hopefully it will work

[Hypernova]

That is the way I have seen it described in an old (40s) ATM book that resides in my workplace, two glass blanks rubbing over each other, the upper one will go concave and the lower one convex due to the overhang. I suppose not many people use that method these days as it is more economical to make a ceramic tool to do the hogging out, but using two glass blanks seems simpler in my eye.

How do you go about figuring out what the depth of the curve? Probably by employing complex mathematics that are a bit above my head at the moment. And also what do you use to measure the depth of the curve at each stage, just a ruler or some other method?

[Daniel-K]

theres a website that does the math for you will find it later and post it. a straight edge rested on the mirror on a drill bit of differnt sizes is a good way

[Glasspusher]

Yes Danny, after 600 aluminium oxide you can make a pitch lap and start to polish with cerium oxide. Incidentally, the price of cerium oxide has gone through the ceiling, a cheaper alternative is jewelers rouge, it is slower acting and very messy..stains everything. The website for finding the depth of curve, or sagitta, is here....Sagitta calculator

John

[MjrTom]

A very interesting project Danny I will be watching your progress very closely

With the price of 16" figured mirrors these days its a very economical way of getting a big scope for a decent price

[Glasspusher]

Mjr Tom, you are right about the cost savings, particularly with mirrors of this aperture, but making your optics is a very fullfilling experience in it's own right. I think telescope making can sit nicely next to Astronomy, people don't have to be telescope makers or astronomers..they can be both!!!

John

[MjrTom]

Yes definitely

I wish I had the time to do it myself

[Daniel-K]

im very lucky to have a double garage in my new house so lots of room to work with. i think i will get the cerium oxide when i get down to it. hoping dave at L.A.S has remberd the grit

[MjrTom]

Sounds good, there is a wealth of knowledge as LAS regarding mirror grinding since they have the 20" one they made not that long ago and working on the 30"

You off to LAS tonight?

[Hypernova]

Thanks for the link Glasspusher and your response Danny, any help is immensely useful to me

I posted another thread where I asked about mirror kits but I think I may attempt my own 6" f6 mirror from scratch. The blanks should be easy to source from work and Machine Mart has all of the abrasive powder that I'll need. This should keep me occupied over the winter for sure.

[jbk44]

This is a great project good for you for going for it. I'm really hoping you keep us updated on your progress. Plenty of pics please I'm also building a dob and need all the info i can get.

That blank is stunning!

[Daniel-K]

girt ordered and arrived but they got the order wrong so have to wait for them to send the 400 grade out. hopefully get back to grinding this week after nearly 2 weeks

[Daniel-K]

last night we had a go at f400 grit on the mirror and had a very nice supprise after around 20 min of griding we stopped and clean the mirror down and checked to see if it reflected any light wow what a super suprise we got the image is a high power touch pointing towards the mirror which then reflected the light onto the back wall which we were able to focus moving backwards and forwards!!! the result is so clear you can see each l.e.d bulb on the image more pictures soon just very excited

[Anders]

This is very interesting.

I have no knowledge of mirrormaking but to me it looks like the LED lights are pretty concentric so it seems to me like you've got a good figure on the mirror so far, if this is a way to measure that of course.. Is it?

This could be a great way to pass time during cloudy days/nights in the future so i'm going to follow this thread closely

Keep up the good work!

//Anders

[angusb1]

That must be a very satisfying moment. How much farther do you reckon you've got to go?

[Gra]

Hi Danny, great project you got going there. Will have to pop down one day and have a look how it done if thats OK with you and Barry.

Gra.

[Daniel-K]

as advised by glasspusher a fellow member we are going to go to cerium oxide next.

gra it fine if you want to come over and have a look and a chat

[Daniel-K]

we are hoping to do a ronchi test and a focault test on wed hopefully we have a good figure so far

[MjrTom]

Looking very nice Danny

Cant wait to see how it performs

[Daniel-K]

after a week of waiting for the new grit the supplier UKGE sent the wrong one an easy mistake which they rectified straight away and let us keep the other grit very pleased with them.so now we have all the right grits here is a list of what we have used and bought from

60-80 aliminuim oxide - machine mart £12 1.5 KG tub

80-120 aliminuim oxide - machine mart £12 1.5kg tub

240 silicon carbide - we was given this bought you can get it from UKGE

400 silicon carbide - UKGE £7.95

600 silicon carbide - UKGE £ 11.95

cerium oxide 90% purity - UKGE £ 15

now having the grit we spent about ten minuites on 240 grit just to make sure the surface of the mirror was even and the pits we all filled in. next was a big clean down making sure there was no other lower grade gits about as it could be very nasty if a bit was to get on the mirror at this stage.

so now its time to start the next grade which is our 400 grit we spinked a bit on the tool and added some water,using the w stroke method. we soon found out what "sticking" was! we manged to get them apart buy using warm water. we found that the wets worked best with a little grit and a little more water than normal used for a wet. after 20 minuites we washed the mirror down to check the surface and it looked even all over we double checked this with a 2" eyepiece held over the surface to magnifi the surface of the mirror.as this is our first attempt at a mirror we were not so sure how long each of these stage's of grits should be so we both agreed it was even all over and to the edge of the mirror then it must be done (correct me if im wrong here). now its clean down time for the next stage of the girt,once again making sure all of the other grit is washed away.

for the 600 grit we done exactly the same as the 400 and then cleaned again. i have just ordered the cerium oxide so it will be a few days before it arrives. if any one know's if we can use the glass tool for the cerium oxide stage or do we have to make a pitch tool?

[Daniel-K]

the 4th picture shows how much glass the tool has lost through grinding and the 5th shows the curve that the tool gets

[Glasspusher]

600 grit silicon carbide is not the best grit to finish with, you will NOT get the same finish as you would with 600 aluminium oxide, and it will take longer to polish out. If possible I would strongly recommend doing a final grind with 600 aluminium oxide...you will get a better finish to polish from. Basically, silicon carbide is harder than aluminium oxide and cuts quicker......but leaves a coarser finish which will take longer to polish.

John