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Go Back   IceInSpace > Equipment > ATM and DIY Projects

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  #1  
Old 18-05-2018, 03:39 PM
Billyboy78 (Bill)
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Making a 20" Gregorian.

Hi, new guy to the forum. Why would you want to make a 20" Greg you ask?? Over the last 50yrs I've made an 8", 10", 16", and a 6" Berry refractor, started a 12" classic cassegrain nasmyth plus a purchased 8"Celestron XLT. The 16" was near the size that I wanted but at my age climbing a ladder with a heavy camera was impossible, besides it was too flimsy, all the time adjusting the counterweights and I hated the reverse image in that if the image drifts to the right commonsense tells you to move left, WRONG!, then you've lost it and you start again.

All this started me thinking a Gregorian with Nasmyth/ Coude' focus would be the go, squirting the image down the dec shaft and then the polar shaft of a GEM to a comfy chair where I can hang all sorts of equipment on the eyepiece and not upset everything. At this point in time to test the concept I've constructed a 18 point mirror cell, the 6 sided cage of 25mm square tubing to hold the tertiary diagonal in front of the primary, the dec shaft made of 3" steam pipe inside of 3" roller bearings in pillow blocks. The intersection of the dec/polar shafts will house the 2nd diagonal. The polar shaft is similar to the dec shaft with provision for the slow motion drive and goto encoders. When I described this idea on the CN forum I was politely told to make a CAD drawing and take a course in optical engineering.
They did however raise the question of the secondary and at the time I wasn't 100% sure what an ellipse was, the idea of 2 focal points made my head spin. So, now I've ordered an 8" blank to test whether I can grind it to the desired shape. To this end I made a turntable driven by 2 drill presses, an accessory belt and pulleys off my old Jeep, plus a KE tester. Did I mention I'm on a tight budget?
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  #2  
Old 18-05-2018, 04:01 PM
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FlashDrive (Col)
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Go for it Bill .... should be a great project....


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  #3  
Old 18-05-2018, 05:40 PM
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billdan (Bill)
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Welcome to IIS Bill, my knowledge of Gregorian telescopes is limited to what is described by Wikipedia. I guess making the secondary will be the most challenging being a concave ellipsoid mirror.

Once you are finished it should be an excellent telescope.
Wish you all the best

Cheers
Another Bill
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  #4  
Old 18-05-2018, 06:43 PM
croweater (Richard)
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Sounds good Bill Post a few pics if you get time. Cheers, Richard.
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  #5  
Old 18-05-2018, 08:01 PM
Billyboy78 (Bill)
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Quote:
Originally Posted by billdan View Post
concave ellipsoid mirror.
l
Thanks for the welcome guys.

An ellipsoid is midway between a sphere and a paraboloid and is easier to make than a convex mirror as in a cassegrain. The ellipsoid because of its shape has 2 focal points and when testing you put the light source on the near focal point and the KE on the other, which in my case is quite long, so its a direct measurement.
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  #6  
Old 18-05-2018, 08:18 PM
Wavytone (Nick)
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Billyboy,

I'd ask you think carefully about what you intend to use this for... I've used a 20" and 30" cassegrain before and apart from the planets there's photometry but not much else these were good for - the field of view was way too small, and the focal ratio too slow. I'm not sure if any amateurs do photometry anymore, either. It might be better left as a Newtonian. If you just want a planet-killer a 12" schiefspiegler offers more than sufficient aperture, adequate light grasp and is easier to make.

There is a reason why you will probably be disappointed by a gregorian - the monochromatic off-axis aberrations of the secondary mirror add to those of the primary - rather than to some extent correcting them as in the case of a classical cassegrain. Even if the spherical aberration is perfectly corrected on axis, the off-axis astigmatism and coma are much worse than in a classical cassegrain.

I would encourage you to make a classical cass instead, the secondary isn't impossible to make or figure with patience if you have the scope essentially finished to the point you can star test.

The Nasmyth idea is good, and on that size I expect you would be riding as a passenger on the mount and using electric slewing. The focal length of a 20" will be of the order of 6-7 metres which poses a significant challenge however ... I found non-tracking altaz mounts infuriating to use with a mak of focal length of just 2700mm, so at 6000-7000mm the problem just gets worse and you will have to make this track and probably GOTO just to be useable.

Last edited by Wavytone; 18-05-2018 at 08:30 PM.
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  #7  
Old 19-05-2018, 09:02 AM
Billyboy78 (Bill)
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Quote:
Originally Posted by Wavytone View Post
Billyboy,


The Nasmyth idea is good, and on that size I expect you would be riding as a passenger on the mount and using electric slewing. The focal length of a 20" will be of the order of 6-7 metres which poses a significant challenge however ... I found non-tracking altaz mounts infuriating to use with a mak of focal length of just 2700mm, so at 6000-7000mm the problem just gets worse and you will have to make this track and probably GOTO just to be useable.
Wow, let me clarify some points.

Primary :500mm F5 FL :2500mm.
Secondary : 200mm F4.5 FL : p 920mm x p' 3990mm
Standard german equatorial mount with 75mm shafts, 5mm wall thickness.
viewing angle : 27deg

Yes I've done ray tracing.....on my lounge room floor using pieces of cotton.
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  #8  
Old 20-05-2018, 12:55 AM
Wavytone (Nick)
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Back of fag packet calculation gives effective focal length 10,800 mm - making the scope almost f/22. Even worse than I guessed.

In a 2” eyepiece with an ID of 45mm the available field of view is a whopping 14 minutes of arc and a 40mm eyepiece will give 240X - if the seeing allows that much.

Good luck with that - you’re going to need it.
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  #9  
Old 20-05-2018, 06:45 AM
Billyboy78 (Bill)
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Quite reasonable figures I thought.
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  #10  
Old 20-05-2018, 06:14 PM
Stefan Buda
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There are very good reasons for not building large Gregorian telescopes - that's why nobody does, I guess.
One of the biggest reasons is that they are so much bigger than an equivalent aperture common Cassegrain, with all the resulting extra difficulties.
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  #11  
Old 09-06-2018, 03:24 AM
Billyboy78 (Bill)
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Update: After a lot of good advice I'm thinking the old design was not very practical, so I've redesigned it as an 20" F3 and the secondary 8" f2.75. I've done full sized ray tracing and its shown to drop the overall length by 1.3 meters. I know the f3 will be a challenge but its nothing a bit of patience wont fix.
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Old 13-06-2018, 11:15 PM
Wavytone (Nick)
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Bill how much mirror grinding experience do you actually have ? What you propose is not a small undertaking even for those with considerable experience and a grinding machine to do the hard work.

Two suggestions:

1. Make the primary with a long focal ratio - say f/7, and use the 8” as a flat secondary to reflect the beam back down the OTA. A secondary that big will result in an OTA around 2.5 metres which would be manageable as a pole truss.

From there use a second diagonal flat to bring the beam out to the side. Although it’s not a nasmyth, this would put the focus at a pretty reasonable height for most observers - and without needing ladder.

2. Make it a 20” f/5 conventional newtonian. A whole lot easier, and a lot more practical to observe with as well. Buy the elliptical flat secondary to match. Turn the 8” into an 8” newtonian, say f/4, and it would serve nicely as a super finderscope on one side.

Even when mounted as a dob you’ll need a ladder to use this... and ladders are very very dangerous in the dark.
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Old 14-06-2018, 01:32 PM
Billyboy78 (Bill)
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I got my inspiration from a gent called 'badbobastronomer' whose 20" greg wouldn't look out of place in a flintstones film set. He has a number of Utube videos describing the construction. The lesson here is that it works without much outlay. He's a man after my own heart and I figure if he can do it,so can I.

As for my experience, its all in post #1 1st paragraph, along with the grievances.

Cheers
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  #14  
Old 17-06-2018, 10:04 PM
Wavytone (Nick)
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Hmm... sorry pardon my skepticism, but someone on CN recently proposed making a large gregorian with rather fast mirrors similar to your numbers using sandpaper and a lot of elbow grease, and hoping to literally "eyeball" the figure to the desired shape with no more than a steel rule as measuring equipment...

Not you, by any chance ?
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  #15  
Old 18-06-2018, 07:39 AM
Billyboy78 (Bill)
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LOL! No, rest assured its not me. 50yrs ago I thought of using an army searchlight mirror but I've moved on since then.
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  #16  
Old 18-06-2018, 08:21 AM
sharpiel (Les)
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Quote:
Originally Posted by Wavytone View Post
Hmm... sorry pardon my skepticism, but someone on CN recently proposed making a large gregorian with rather fast mirrors similar to your numbers using sandpaper and a lot of elbow grease, and hoping to literally "eyeball" the figure to the desired shape with no more than a steel rule as measuring equipment...

Not you, by any chance ?
Wow. I hope that project got loads of encouragement. Just for fun reading...
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  #17  
Old 19-06-2018, 06:01 AM
Kunama
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Quote:
Originally Posted by Billyboy78 View Post
Update: After a lot of good advice I'm thinking the old design was not very practical, so I've redesigned it as an 20" F3 and the secondary 8" f2.75. I've done full sized ray tracing and its shown to drop the overall length by 1.3 meters. I know the f3 will be a challenge but its nothing a bit of patience wont fix.
That's the spirit, I would like to see this scope come to fruition Billy

Though I dare say it would not be my first choice for a project
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  #18  
Old 28-06-2018, 01:33 PM
Billyboy78 (Bill)
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Update: Mirror blank ordered plus a truckload of 80 grit. A-grinding we will go ta-da.

Question.. what is the go with grit from a lapidary shop these days? I bought a kilo of 80 from the local shop, mainly because I go past it regularly, with the idea of using any rubbish to hog it out. They tell me they import all their grit from the USA so I'm thinking if there is a high turnover there will be a less chance of it being contaminated. They stock 46 down to 1200. It's a lot easier to buy local. I will use high grade Cerium Oxide.

Last edited by Billyboy78; 28-06-2018 at 01:45 PM.
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  #19  
Old 02-07-2018, 09:36 PM
Wavytone (Nick)
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Billy if in doubt you can grade the grit yourself using water. The procedure is described briefly in one of A.G. Ingalls ATM books, you should own these; if not BUY A SET on ebay.

Personally I wouldn't trust the grit from a source that wasn't a reputable one with a firm connection to ATM - lapidary supplies definitely do NOT qualify.

The test will be when you think you've ground out the pits of a previous grade, inspect with a loupe. If you see scratches, the grit ain't what its supposed to be.

Cerium oxide is for polishing - not grinding - and is an alternative for black-rouge. In my mirror-grinding days I tried both. Cerium oxide polished faster but produced a rougher surface. On small surfaces (secondaries and lenses) it was so aggressive it isn't a good idea as the figure will be terrible and uncontrollable. IMHO not suitable for mirrors under 30cm.

Rouge is slower to polish, but produced a much smoother surface and better figure and ultimately a far better result - with patience and a lot of elbow grease. If you have the patience I would use rouge - not cerium oxide.

I continue to have doubts about your experience... anyone who has ground a few mirrors before would know these basic things.

Last edited by Wavytone; 02-07-2018 at 09:49 PM.
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  #20  
Old 05-07-2018, 02:29 PM
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Carl Zambuto uses cerium oxide. I guess he'll learn once he's made a few more mirrors?
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