Physopto

Hi Damian,

You may think me nuts but here goes, please don't shoot me for this. Your pictures are great showing the mirror. Also the Ronchi test results are very interesting to see.

But !!!

How about a short video of you actually figuring/grinding the mirror. It is not for me, although that would be informative. It is just a thought that as you seem to be getting a bit disheartened or fed up with things not panning out. Maybe John could see a possible problem with the method you are using, I.e. Stroke shape. As I say don't shoot me, it took me some time to understand what a "W" stroke was. It is just a possibility as time has passed, that you have slipped into a slight methodical error. (if that is the correct word). I have done the same in other endeavours in the past without realising it. It may, may just save you pulling out some hair.

I'm receding so I wouldn't want that for me

Derek

I have done about another 2 hours during the day, I think I may have a beer now as I think I deserve one 

I would vote for a good malt, it's the Scottish side of me I guess.

Well hi all,

Thanks for you input.

 Just spent a productive half hour or so on this website.

http://www.atm-workshop.com/ronchi-test.html

Very informative. So now I understand what you are doing a bit better. It is a test based on reflected light/shadows through  a grating from a pinhole or a grating parallel to the one being viewed through. Nice idea. The Foucault test is a bit different. Not sure as yet what the knife edge does. I'll figure it out.

I did see the effects of diffraction ( producing a double image in the Ronchi test). My original optical and atomic physics was more to do with lasers, so this is all new to me. It's nice to learn something new. What a really great hobby this is!   

Derek

Oh Heck,

That's done it now I'm off to look it all up.. Hate to be confused about anything for too long.

Derek

Hi Derek.

What I'm looking for is the lines inside and outside the radius of curvature to be completely vertical.

When the edge is turned down (as mine is) it shows as the lines bending inwards near the edge when the image is taken inside radius of curvature (ROC) and outwards outside the ROC as mine do.

Here's how a perfect sphere should look inside and outside ROC

image002.jpg

And here's a TDE outside ROC

image004.jpg

As you can see it's just the very edge that's the problem at present. Once I'm to a sphere I can move on to the parabolising process although for a large mirror like this everything takes an age to alter. Judging change is very difficult so you have to stick with your chosen tactic for a while to see the difference be it right or wrong and that's why it's a back and forth testing game

Hope that explains

Damian 

Hi Damian,

Many thanks,

 Looks like you are getting there. I wasn't sure, only a bit of an idea about what you were looking for. Between you and Ian I now get it. It is definitely a labour of love. As I said., again thanks, the pics  help a lot, "a pic is worth a thousand etc etc".

I'll keep watching.

Regards,

 Derek

Hi Derek,

In really simple terms you should be able to see perfectly straight lines across the full face of the mirror. The dark lines show where the beams of light have cancelled each other out. Others, including Damian will give a more definitive answer.

Sent from my Fone

Hi Ian,

Nice to hear from you. I sort of guessed a bit about it, I'm familiar with constructive and destructive light effects, but not with this test. So that was very helpful, thanks. How are you doing? hope all is well and will see you again shortly I hope.

Look after  yourself.

Best regards,

 Derek

Hi Damian,

What are we expecting to see exactly when it looks right to you? I'm completely in the dark so to speak with that.

Looks like you will be able to arm wrestle a bit when finished

Derek

Hi James,

Had one on my NEQ6 helped a lot when imaging by giving a lot of extra clearance for the camera, i.e. cleared the tripod legs.

Very solid and  stable if set up correctly and well worth the cost. Just be careful of the ball bearing, make sure it is centred correctly or the tube will move slightly as it does not tighten up properly. Done correctly it's a great addition.

Derek

I looked up turned down edge and the effects. Sounds like an awful lot of effort to get rid of it!

Still you got this far so I know you will do it. Just remember a bit done here and that is just that bit less to do in future. Every bit counts and soon adds up!

Derek

Hi Damian,

 I have been looking at a chap ( Gordon Waite) over in the USA doing a 20" f 4.5 mirror. He got 1/30 wave I believe in the end. Some feat! I also watched his measurement of Ronchi and Foucault tests. Now I am beginning to appreciate how much work goes into your mirror.

Best of luck,

 Derek

Hi Damian,

I see you are still at it. Keep it up, Mike and I are expecting to see it finished  up at Galloway next time.

We are watching with interest.

Derek

I'm still very     :Envy:     If I had the time I'd be pulling out my hair as well.

I'm dying to see the outcome. Mike and I talk about your efforts a fair bit!!

Derek

Costco sell wide aluminium foil, it is very thick/heavy and far better than any of the cheap shop bought stuff. Does not rip easily!!. 450mm wide and comes in long rolls. You could do many more mirrors with it ( years of hell sorry  fun), if you can keep it hidden away from the wife.

Thanks for dropping by Derek and I agree it's very tedious at time's and enough to test the patience of a Saint.(although being an imager patience shouldn't be in short supply)

The pitch is poured onto aluminium foil on top of the mirror, so come's off the mirror quite easily unless like mine it goes under the seam in the foil and sticks to the glass itself.

I think the stuff just doesn't like me although the feeling is mutual

Damian

I get it now. Silly me. Yes I've been through similar heart stopping moments. I should be used to the sick feeling but it never diminishes. just out of interest where are you getting the foil? i.e. what size are you using since you mentioned a seem.

Derek

Hi Damian,

Been reading some of your troubles. Glad it's not me doing it.

Question for you. Can you not spray the Glass with a release oil before the pouring of the pitch? Please don't shoot me down as I know nothing about it.

I have a product called Sabre. It is sold up here in the North East. It is primarily used to clean and polish UPVC window frames. But I use it for al sorts of things. It can be used to clean up oil etc from carpets, Clean Glass and I am sure that it contains some form of Acetone. It will melt car paints. It may be a good way to clean off the residual pitch without damage to the surface. Comes in 5 ltr poly containers I think.

Best regards,

 Derek

Hi Gina,

Which ever way you do it incorporate circuit breakers/switches in the  winch power line. With any winch of that type it has a massive power or pull and could easily rip everything to bits if the controller stuck in the on position. So I suggest two microswitches at each end in serial. They can be change over types. If one fails because of damage or damp the other will cut the power. I once watched an overhead crane rip fittings off a tank for such a reason. Thousands of pounds damage! If you can get pressure sensitive strip even better as it should prevent finger/telescope damages.

Best of luck,

Derek

Hi All,

 Lead acid batteries are really a pain in the rear. What ever the stated capacity please take with a pinch of salt! It depends upon age,  temperature, type of lead acid makeup and various other things. Most lead acid batteries really can only supply about half to three quarters of their stated capacity at the sorts of currents we tend to ask of them.  A 100 Ah battery should on paper give 100 hours at  a 1amp discharge rate. At 10 amps discharge you may expect 10 hours. Not true! The higher the discharge rate the less you will get out of them. Lead acid batteries give a constantly reducing voltage slow curve discharge. Think of a hose held horizontally discharging water, the water leaving the hose curves downwards until it hits the ground. That is what you will see as the voltage reduces at a constant current discharge. The higher the current the quicker the fall off of voltage. As astronomers we want the battery to give nominal 12 volts. A fully charged lead acid battery shows 12.8 volts approximately (dependant upon battery type). At half charge it will show 12.4 volts. But fully discharged it will show around 10.6 volts. In other words not a linear discharge voltage fall.

Personally I try to keep my batteries on constant charge whilst in use. I charge them up, then they are kept on a maintenance charge of 13.6 volts. Trouble is that CCD cameras often state that the voltage needed is to be around 12.6 volts but not above. Whereas mounts such as the NEQ6 require above 12 volts to be stable but not above 14 volts (I believe). Instructions for QSI CCD camera state that you should only use the power block to supply power. I don't like 240 volts in the field, not safe! The camera alarms when below 11 volts, minimum rated voltage  or above 14 volts the maximum rated operating voltage. Either case can cause a failure in the electronics. Remember in the field it would be easy to drop below the 11 volt range in the cold, (batteries hate cold and their output drops significantly).

My answer is to us a DC to Dc convertor. Any input from the battery from 10 to 15 volts will give an output about 12.6 volts at up to 5 amps (60 watts). These are available from several sources on the internet. They were initially made for the caravan/motorhome market I think, as above 12.6 volts can damage so called 12 volt  TVs. There is plenty of info on the web. They are available from about £13/14 up to £40, dependant upon maker. These devices are about 75 to 80% efficient.

Lastly lead acid batteries hate to be discharged. Even worse they may fail if left in a discharged state for even a short period of time. Each time they are partially discharged they lose capacity.

Leisure batteries are the best at recovering from partial discharge, car batteries need to be fully charged at all times, they are only designed to give large currents to start the engine then be recharged immediately when the engine runs the alternator.

If you look at the makers statistics for any battery it should give the required info. From a brand new 100 Ah leisure battery I do not expect to get more than 30 to 35 Ah before the battery falls below 12.4 volts if it is not being charged in any way! After that it will fall below 11.5 volts very quickly and we all know how that feels when we hear the anguished cries *!@%$*  dead battery etc.,

I hope this may help anyone reading.

Derek

Regards,

 Derek