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Ships and Stars

24" on up mirror projects or mirror blanks?

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22 hours ago, GTom said:

Watching a dozen mirror making videos and reading reports, I am tempted to grind a 20-24" piece.

I waited over a year to get my 24" f4.1 from a reputable maker. It is 2" thick which was agreed on because a thick mirror is so much easier to work on and test. There are few professional makers that can actually produce VG large mirrors around f4.

Not trying to sound discouraging but if these guys are challenged by making them a first time amateur maker might just have some challenges too. A truly thin mirror like Lockwood produces might be almost impossible for a beginning amateur to make with any sort of accuracy.

then there is the structure...

Edited by jetstream
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I've been wanting to grind a mirror but right now even a 12" is not in the budget.

Have you researched any clubs near you that grind mirrors?

Could be you might find one with a lot of experience to give you a helping hand. 

We have one near me that even has their own testing equipment.  They have made some large telescopes. They do recommend your first to be 8 or 10 inch. That would not be much of a jump with me having an 8 already.

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I managed 16" at f/5 on a full thickness plate glass blank but when I decided to regrind to F/4 I ran out of patience.
The F/5 went surprisingly quickly. A low expansion blank is far easier to work with than plate or float glass. No endless waiting for testing.

A thin mirror probably needs to be mounted on a multi-point supporting cell and rotated regularly to avoid astigmatism.
That's how I worked mine on a 24" lathe faceplate, face up and driven on its edge by a long v-belt from a gearbox motor.

I almost always worked by DIY reciprocating machines. Hand work is far quicker and arguably provides a smoother surface.
Machines can cause rings and zones from repeating exactly the same length of stroke without care.
With handwork, no two strokes are the same but you have to avoid the heat of your hands affecting the glass.
Machines have the advantage that you can provide more power and relax during wets but are not compulsory.

Mike Lockwood suggests that thicker blanks take such a long time to cool that they should be avoided in all sizes.
Not just for the change of figure, when cooling, but the thermal currents rising from the surface.

Fans can break up thermal currents and force more rapid cooling or equalization.
But need power and may cause vibration.

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3 hours ago, andrew s said:

You don't need to go that extreme why not a Watec 910HX for £500.

Regards Andrew 

This is interesting! I must confess I know nothing about this area. Does it use the dob as a prime lens, or does it attach to say a 55mm plossl or 2X powermate? With the right filter(s) would I be able to see Hb areas like the Cocoon or HH? £500 is certainly reasonable in comparison to my other options, besides doing some single 10sec AP subs.

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2 hours ago, scarp15 said:

It seemed to go on for ever and then just seemed to inconclusively completely fizzle out with no further updates. I assumed he had lost interest in the polishing and subsequently dropped out of the forum all together.

Little bit more to it than that mate....but it was such a shame.

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23 hours ago, Ships and Stars said:

Wow! Love the stainless steel. I've just had a quick look at your build thread bit will definitely have a closer look. I do a little bit of MIG on mild steel, haven't tried stainless yet, but have been meaning to. Great to hear someone built a big dob from scratch like that, I'm a lot more comfortable working with metal than wood myself. Good stuff!

its a brilliant project, creating something from a blank. looking back saying, i made that 😀

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1 hour ago, Ships and Stars said:

This is interesting! I must confess I know nothing about this area. Does it use the dob as a prime lens, or does it attach to say a 55mm plossl or 2X powermate? With the right filter(s) would I be able to see Hb areas like the Cocoon or HH? £500 is certainly reasonable in comparison to my other options, besides doing some single 10sec AP subs.

You could do any option. Prime, Barlow or eyepiece projection with or without filters.  There are cheaper £250 low light video cameras.

Regards Andrew 

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Not a project to be taken lightly. 300+ hours went into mine and I had the help of a very experienced mirror maker 😉

Edited by mapstar
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4 hours ago, jetstream said:

I waited over a year to get my 24" f4.1 from a reputable maker. It is 2" thick which was agreed on because a thick mirror is so much easier to work on and test. There are few professional makers that can actually produce VG large mirrors around f4.

Not trying to sound discouraging but if these guys are challenged by making them a first time amateur maker might just have some challenges too. A truly thin mirror like Lockwood produces might be almost impossible for a beginning amateur to make with any sort of accuracy.

then there is the structure...

Agreed. Structure is the easy bit in the scheme of things. 

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2 hours ago, faulksy said:

 looking back saying, i made that 😀

I sometimes say that when in my observatory 🤣  I would quite liked to have built a telescope from scratch and even looked into it at one stage, including M-o-M but really would have needed to start it at an earlier age!

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58 minutes ago, Gina said:

I sometimes say that when in my observatory 🤣  I would quite liked to have built a telescope from scratch and even looked into it at one stage, including M-o-M but really would have needed to start it at an earlier age!

Look on the bright side Gina, it would have soaked up a lot of your time and taken you away from some of your other wonderful projects. It's good to have some unfinished works, just like the great composers :) 

Jim

 

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8 hours ago, Peter Drew said:

I have a 30" Dob and EAA equipment (not intensifier tube type).  Used in conjunction with a 12" SCT at F3.3 it easily outperforms the 30" used visually on DSO's.

We have a 42" pre- generated ceramic blank, that cost nearly 5K several years ago!  

Hi Peter, what type of EAA would you suggest? Video? Andrew above mentions the Watec video camera, reasonably affordable. I will look into this route to allay my aperture fever!

 

7 hours ago, scarp15 said:

There was someone on here a few years ago, I think went by the name; crash test dummy. As I recall he went from a 12" shop bought dob to a 16" (I think) mirror grind. He meticulously kept an ongoing blog recording the stages from a blank. It seemed to go on for ever and then just seemed to inconclusively completely fizzle out with no further updates. I assumed he had lost interest in the polishing and subsequently dropped out of the forum all together. I can only think that a 24" plus would become a very dedicated quest.

It is certainly great to be inspired by aperture, yet also perhaps being able to use what you have, repeatedly over a succession of years, will develop your observing skills and familiarity with the equipment you are using, as well as keeping costs which can start to get a bit insane in this hobby more reasonable. Indeed a 20" mirror taken to the Cairngorms if transparency is good will be exceptional I would expect. Above all else there would seem few opportunities to fully appreciate venturing to the best dark sky location you can get to when transparency is favourable. In those rare circumstances- of very good transparency, taking what you have, with the knowledge and ability to gain the most from this, whilst investing using the best range of filters and eyepieces is highly engaging.  

That said it is good to know that there are a few who are inspired and go for these monster dobs.    

Yes, I've been know to start a few projects, get about halfway into it and...

I reckoned a few years grinding and polishing a large mirror with no promises it will have decent quality is naturally a real gamble. Finished mirror sets in that range on up are decent used car money and thus out of my league. The EAA route certainly has appeal when looking at it like that, but I've always been a dreamer. Sometimes it works, sometimes it doesn't! I thought mirror making would be a good summer hobby when it's too bright here between c. mid-April to mid-August. 

 

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I think as said, if it were possible for you to link up with / collaborate with an experienced mirror maker, perhaps to learn the skills or at least mentor, then it may be a more reassuringly potential, if the cost were acceptable, venture. In terms of seasonal, well, I would pitch at all four seasons, the weather and all that being the way it is. Sometimes members at astro societies, experienced observers, may get together to build a large club dob, although the mirror would be often sourced elsewhere and collectively there will be the finance to invest in going ahead with this type of project. You definitely sound like someone who needs a project though. Perhaps if the truth be told, life has many obligations and complications, family of course, work, weather health etc; yet if and when we find ourselves next time at a very dark place on a cold winters night just getting on with observing, the rest will be washed away, forgotten. 

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23 minutes ago, Ships and Stars said:

I reckoned a few years grinding and polishing a large mirror with no promises it will have decent quality is naturally a real gamble. Finished mirror sets in that range on up are decent used car money and thus out of my league. The EAA route certainly has appeal when looking at it like that, but I've always been a dreamer. Sometimes it works, sometimes it doesn't! I thought mirror making would be a good summer hobby when it's too bright here between c. mid-April to mid-August. 

 

Ships n Stars I think it can be satisfying for sure, even cathartic, but looking back at my unfinished 12 inch mirror I think I would start with something smaller if I were to do it over again. Who knows,maybe I'll go back to it and maybe finish it in my retirement :) 

Jim 

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@ S & S.   All my EAA work was with video captured on VHS yet the results were just as good as computer based imaging albeit in monochrome which I actually prefer.  I went through an ever upgrading sequence finally settling on the Watec 120n+.   This was a highly sensitive camera with theoretically infinite integration, realistically limited by mount and setup accuracy. One minute exposure with a 5" refractor showed the "Pillars of Creation".  I believe that this camera has now been superseded by the 910HX, however this camera has on screen display for functions, a system I dislike.   😀 

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2 hours ago, scarp15 said:

when we find ourselves next time at a very dark place on a cold winters night just getting on with observing, the rest will be washed away, forgotten. 

Yes, its all about the experience, at least for me.

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3 hours ago, Ships and Stars said:

I will look into this route to allay my aperture fever!

Eventhough I'm a traditional astronomer I do believe that being able to observe nicely, from home and under light polluted skies is a great option to have and NV gives this option. They also work well under dark skies. I'm all for anything that allows us the observe the night sky despite that mentioned fact that I'm traditional. Personally if I were you I'd forget a bigger dob, your 20" is perfect.

S&S I would explore NV and other options but would not give up the experience of being "one" with your dob under very dark skies.

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NV gives you an instant “eyepiece view”, but prefers fast f-ratio for nebulous objects (the 55mm plossl afocal method is popular to achieve this). AlanJGreen uses one on his huge dob. Video/stacked short CCD needs tracking and some other hardware and then waiting for the result. All options give a different result, depends what you want to observe.

PEterW

Edited by PeterW
Correct typo
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21 hours ago, Ships and Stars said:

Hi everyone! The EEVA route is appealing I must say. Recently saw images of someone using a relatively small refractor from a suburban/urban LP zone - the results were simply astonishing. Was that you @Cosmic Geoff?

I originally bought a 55mm TV plossl to adapt to an intensifier tube, but never got that far and sold the plossl. Can you even get Gen 3 or 3+ in the UK? I think that's the top end of the spectrum as far as I know. I'm guessing £3k if I'm lucky?

 

 

I've no idea if that was me, but EEVA from my semi-urban backyard easily matched what I could see from a dark site with the 8" SCT.  In the past few months I have been following the brighter comets with EEVA - I have not even tried to sight them visually (except a few nights ago I tried to find C/2017 T2 PanSTARRS with an 8" GoTo SCT - total failure.)

When you get into the detail, there are various ways of accomplishing EEVA - I just used what I had to hand, including a good quality planetary video camera. 

When I was a youth, I made an 8" Newtonian from scratch, including grinding the mirror and making a fork equatorial mount.  It did get finished and it did work, but at this remote I can't say how good it was. Would I want to do the same today? ROFL😁  Times have moved on.  About three or four years ago I bought a complete 8" Newtonian OTA for peanuts.  It was okay, but I eventually re-sold it as I heartily preferred the more modern and hi-tech in the form of an 8" GoTo SCT.

Each to their own, and many people (including me) need a Project from time to time to satisfy their creative and crafting urges.

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21 hours ago, Ships and Stars said:

Hi everyone! The EEVA route is appealing I must say. Recently saw images of someone using a relatively small refractor from a suburban/urban LP zone - the results were simply astonishing. Was that you @Cosmic Geoff?

I originally bought a 55mm TV plossl to adapt to an intensifier tube, but never got that far and sold the plossl. Can you even get Gen 3 or 3+ in the UK? I think that's the top end of the spectrum as far as I know. I'm guessing £3k if I'm lucky?

 

 

I've no idea if that was me, but EEVA from my semi-urban backyard easily matched what I could see from a dark site with the 8" SCT.  In the past few months I have been following the brighter comets with EEVA - I have not even tried to sight them visually (except a few nights ago I tried to find C/2017 T2 PanSTARRS with an 8" GoTo SCT - total failure.)

When you get into the detail, there are various ways of accomplishing EEVA - I just used what I had to hand, including a good quality planetary video camera. 

When I was a youth, I made an 8" Newtonian from scratch, including grinding the mirror and making a fork equatorial mount.  It did get finished and it did work, but at this remote I can't say how good it was. Would I want to do the same today? ROFL😁  Times have moved on.  About three or four years ago I bought a complete 8" Newtonian OTA for peanuts.  It was okay, but I eventually re-sold it as I heartily preferred the more modern and hi-tech in the form of an 8" GoTo SCT.

Each to their own, and many people (including me) need a Project from time to time to satisfy their creative and crafting urges.

Edited by Cosmic Geoff
Deleting
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21 hours ago, Ships and Stars said:

Hi everyone! The EEVA route is appealing I must say. Recently saw images of someone using a relatively small refractor from a suburban/urban LP zone - the results were simply astonishing. Was that you @Cosmic Geoff?

I originally bought a 55mm TV plossl to adapt to an intensifier tube, but never got that far and sold the plossl. Can you even get Gen 3 or 3+ in the UK? I think that's the top end of the spectrum as far as I know. I'm guessing £3k if I'm lucky?

 

 

I've no idea if that was me, but EEVA from my semi-urban backyard easily matched what I could see from a dark site with the 8" SCT.  In the past few months I have been following the brighter comets with EEVA - I have not even tried to sight them visually (except a few nights ago I tried to find C/2017 T2 PanSTARRS with an 8" GoTo SCT - total failure.)

When you get into the detail, there are various ways of accomplishing EEVA - I just used what I had to hand, including a good quality planetary video camera. 

When I was a youth, I made an 8" Newtonian from scratch, including grinding the mirror and making a fork equatorial mount.  It did get finished and it did work, but at this remote I can't say how good it was. Would I want to do the same today? ROFL😁  Times have moved on.  About three or four years ago I bought a complete 8" Newtonian OTA for peanuts.  It was okay, but I eventually re-sold it as I heartily preferred the more modern and hi-tech in the form of an 8" GoTo SCT.

Each to their own, and many people (including me) need a Project from time to time to satisfy their creative and crafting urges.

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21 hours ago, Ships and Stars said:

Hi everyone! The EEVA route is appealing I must say. Recently saw images of someone using a relatively small refractor from a suburban/urban LP zone - the results were simply astonishing. Was that you @Cosmic Geoff?

I originally bought a 55mm TV plossl to adapt to an intensifier tube, but never got that far and sold the plossl. Can you even get Gen 3 or 3+ in the UK? I think that's the top end of the spectrum as far as I know. I'm guessing £3k if I'm lucky?

 

 

Computer issue - can Admin delete the duplicate posts? TA.

Edited by Cosmic Geoff
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On 14/01/2020 at 09:47, Cosmic Geoff said:

I've no idea if that was me, but EEVA from my semi-urban backyard easily matched what I could see from a dark site with the 8" SCT.  In the past few months I have been following the brighter comets with EEVA - I have not even tried to sight them visually (except a few nights ago I tried to find C/2017 T2 PanSTARRS with an 8" GoTo SCT - total failure.)

When you get into the detail, there are various ways of accomplishing EEVA - I just used what I had to hand, including a good quality planetary video camera. 

When I was a youth, I made an 8" Newtonian from scratch, including grinding the mirror and making a fork equatorial mount.  It did get finished and it did work, but at this remote I can't say how good it was. Would I want to do the same today? ROFL😁  Times have moved on.  About three or four years ago I bought a complete 8" Newtonian OTA for peanuts.  It was okay, but I eventually re-sold it as I heartily preferred the more modern and hi-tech in the form of an 8" GoTo SCT.

Each to their own, and many people (including me) need a Project from time to time to satisfy their creative and crafting urges.

Yes indeed, I think a lot of the motivation is simply the desire to build something impressive (to me anyway) and do something astro-related in the summer. I'm increasingly intrigued by the EEVA route, a decent video camera in the 20" dob should produce some good results, and allow me to see things that I'd struggle to find or observe on visual alone even with a monster mirror. Plus the 20" already has goto and decent tracking. Think I'm going to give that a try. Thanks everyone for the advice!

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On 14/01/2020 at 01:46, jetstream said:

Eventhough I'm a traditional astronomer I do believe that being able to observe nicely, from home and under light polluted skies is a great option to have and NV gives this option. They also work well under dark skies. I'm all for anything that allows us the observe the night sky despite that mentioned fact that I'm traditional. Personally if I were you I'd forget a bigger dob, your 20" is perfect.

S&S I would explore NV and other options but would not give up the experience of being "one" with your dob under very dark skies.

 

On 14/01/2020 at 07:00, PeterW said:

NV gives you an instant “eyepiece view”, but prefers fast f-ratio for nebulous objects (the 55mm plossl afocal method is popular to achieve this). AlanJGreen uses one on his huge dob. Video/stacked short CCD needs tracking and some other hardware and then waiting for the result. All options give a different result, depends what you want to observe.

PEterW

Excellent advice really! I've decided to stick with the 20" and try the EEVA route. If I can view the fainter objects from home when the businesses turn off their lights in the evening, I'd be delighted and would get more use out of the 20", plus I'd always be able to take it to my dark site when time and conditions allow. 

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