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  #61  
Old 12-05-2018, 12:47 PM
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I have just passed the 2.5mm saggita mark, when should i be switching to normal strokes?
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  #62  
Old 12-05-2018, 04:51 PM
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I have just passed the 2.5mm saggita mark, when should i be switching to normal strokes?
You are at about F6.4 then. You could stop deepening the curve now if you wish as that's pretty close. With a chordal strike, you often wind up with a deep centre and and edge that lags behind. I suggest you start moving the stroke in towards the centre and start to shorten the stroke. This will help to even up the curve.

I'm assuming you are going for F6 not F5? If you want F5 keep on with the stroke you are using a bit longer.

You can test how good your contact between the tool and mirror are by using the pencil test:

https://stellafane.org/tm/atm/grind/fine.html scroll to the bottom of the page.

Rod
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  #63  
Old 12-05-2018, 05:58 PM
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You are at about F6.4 then. You could stop deepening the curve now if you wish as that's pretty close. With a chordal strike, you often wind up with a deep centre and and edge that lags behind. I suggest you start moving the stroke in towards the centre and start to shorten the stroke. This will help to even up the curve.

I'm assuming you are going for F6 not F5? If you want F5 keep on with the stroke you are using a bit longer.

You can test how good your contact between the tool and mirror are by using the pencil test:

https://stellafane.org/tm/atm/grind/fine.html scroll to the bottom of the page.

Rod
I am aiming for F5 - although i miscalculated and switched to normal already...
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  #64  
Old 12-05-2018, 08:48 PM
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I am aiming for F5 - although i miscalculated and switched to normal already...
There's no problem switching back to chordal stroke. You could try normal stroke for a while and see if it is still deepening the sagitta at a rate you are happy with.

F5 will be more portable and easier to mount. I think it's a good choice. It will be more challenging to parabolise.

Rod
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  #65  
Old 12-05-2018, 08:52 PM
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There's no problem switching back to chordal stroke. You could try normal stroke for a while and see if it is still deepening the sagitta at a rate you are happy with.

F5 will be more portable and easier to mount. I think it's a good choice. It will be more challenging to parabolise.

Rod
I'm not really sure how to know how fast it is going - all i know is i did about 5-10 wets with normal strokes and nothing changed really, though now i realise i was doing strokes FAR too long. if i were to go back to chordal, when should i switch?
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Old 12-05-2018, 10:38 PM
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I'm not really sure how to know how fast it is going - all i know is i did about 5-10 wets with normal strokes and nothing changed really, though now i realise i was doing strokes FAR too long. if i were to go back to chordal, when should i switch?
I would change back to chordal stroke now then. No sense continuing with something that's not working. Test often so you don't overshoot.

Rod
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  #67  
Old 13-05-2018, 09:49 AM
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After some consideration, I actually think I will do F6 - easier to parabolise, and smaller secondary for planetary, although the tube may be taller I am going to make a truss tube, so it is easy to move around
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  #68  
Old 13-05-2018, 01:32 PM
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Hi Logan

There are certainly advantage to the longer focal length but I don't think you'll get significantly better planetary images. The advantage of a smaller diagonal is very over rated. The quality of the mirror and good collimation are what I think makes the differnce.

Consider carefully which scope you can most easily build and transport. You have some excellent mirror makers at the ASV who can help you with the parabolising if that is what worries you.

BTW I have the cardboard tube for my 10 inch F6 sitting in my garage. Let me know if you would like it. It could be used as the tube or as a form for building a more exotic tube, mirror box or secondary cage.

Rod
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  #69  
Old 14-05-2018, 10:07 AM
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I have decided to sperise now. I think that the mirror will end up being about f5 by the time this is done anyway! I have a 10mm ring that has not contact, any ideas on how long this will take? Thanks!
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  #70  
Old 14-05-2018, 02:36 PM
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I have decided to sperise now. I think that the mirror will end up being about f5 by the time this is done anyway! I have a 10mm ring that has not contact, any ideas on how long this will take? Thanks!
Hi Logan

So you have the outer 10 mm not contacting when you use the pencil test? That's to be expected when using the chordal stroke.

It probably won't take long to come into contact at 80 grit but I can't give an estimate. It depends on your technique which I can't see.

I know you are probably sick of rough grinding but you are getting close. I find the early stages of grinding a bit tedious but it gets a little more interesting and quieter as the grits become finer.

Keep up the good work.

Rod
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  #71  
Old 14-05-2018, 03:01 PM
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Hi Logan

So you have the outer 10 mm not contacting when you use the pencil test? That's to be expected when using the chordal stroke.

It probably won't take long to come into contact at 80 grit but I can't give an estimate. It depends on your technique which I can't see.

I know you are probably sick of rough grinding but you are getting close. I find the early stages of grinding a bit tedious but it gets a little more interesting and quieter as the grits become finer.

Keep up the good work.

Rod
Thanks Rod. yep the outer 10%, I was hoping to pour the pitch lap at the ATM section on Sunday but i don't think that will happen in time
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  #72  
Old 14-05-2018, 05:04 PM
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Thanks Rod. yep the outer 10%, I was hoping to pour the pitch lap at the ATM section on Sunday but i don't think that will happen in time
Hi Logan

You are doing much better than I did at your age. I tried making a 6 inch when I was 15. I gave up during rough grinding and finished it in my 20s. I've made quite a few mirrors since but I really admire young people who stick at it. Hogging out a 10 I think is a great achievement.

I wouldn't worry about the pitch lap yet. It's better to do at the end of fine grinding. Pitch dries out and becomes hard so you don't want it sitting around not being used. Aculap (synthetic pitch) might be ok. I think it maintains its consistency better.

The meeting is Saturday BTW, not Sunday. I look forward to catching up with you then.

Rod.
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  #73  
Old 14-05-2018, 05:13 PM
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Hi Logan

You are doing much better than I did at your age. I tried making a 6 inch when I was 15. I gave up during rough grinding and finished it in my 20s. I've made quite a few mirrors since but I really admire young people who stick at it. Hogging out a 10 I think is a great achievement.

I wouldn't worry about the pitch lap yet. It's better to do at the end of fine grinding. Pitch dries out and becomes hard so you don't want it sitting around not being used. Aculap (synthetic pitch) might be ok. I think it maintains its consistency better.

The meeting is Saturday BTW, not Sunday. I look forward to catching up with you then.

Rod.
Thanks Rod BTW, down to a 7mm ring now! how to tell when it is completely done?
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  #74  
Old 14-05-2018, 05:17 PM
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Should I be worried about this bubble? Will it go away?
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  #75  
Old 14-05-2018, 05:42 PM
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Hi Logan

You are done when all the pencil lines grind out evenly and you have the Sagitta you aimed for. Then you move on to 120.

I'm not sure what bubble you are referring to? Is it in the Pyrex? How big is it and how deep down?

Some bubbles in Pyrex are common. Usually they are small and below the curve you are grinding.

Rod
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  #76  
Old 14-05-2018, 06:00 PM
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I have tried to outline it here - it is an air bubble inbetween the disks. It changes shape though so im not sure if i should worry about this?
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  #77  
Old 14-05-2018, 06:47 PM
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I have tried to outline it here - it is an air bubble inbetween the disks. It changes shape though so im not sure if i should worry about this?
Ok now I understand. Short answer - don't worry about it. Its what you expect at this stage. It's more evidence that the disks do not match each other. The centre is deeper on the mirror than the corresponding hill on the tool. As you make the disks more spherical, you should see a more even distribution of bubbles. Don't get hung up on the bubbles though. Use the pencil test to monitor your progress.

Rod
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  #78  
Old 14-05-2018, 07:20 PM
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Ok now I understand. Short answer - don't worry about it. Its what you expect at this stage. It's more evidence that the disks do not match each other. The centre is deeper on the mirror than the corresponding hill on the tool. As you make the disks more spherical, you should see a more even distribution of bubbles. Don't get hung up on the bubbles though. Use the pencil test to monitor your progress.

Rod
Ok cool
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  #79  
Old 15-05-2018, 07:56 PM
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4.5mm left! sag is 2.8mm. Hopefully i can move on to fine grits tomorrow or the day after!
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  #80  
Old 16-05-2018, 08:38 AM
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Seems to be going well Logan. Keep up the good work!
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