Planning a 20" (500mm) Scope (Nasmyth??)

[J_M_Franklin]

For a long time i have planned building my own scope, I did a small 150mm Newtonian when I was 14..even ground the mirror, but I have wanted to make a BIGGIE for years, but life always got in the way.

As I am making a concerted effort to get back into observational astronomy, and my kids are certainly interested, as is the better half, I have made the decision to plan and build a 20" or 500mm monster...that was the easy part of the decision..

I have used many scopes over the years and always found the observing position to be a pain...literally sometimes as I am 6ft 4", as a result I am considering making my scope using the Nasmyth design

I think the design has numerous advantages that are often overlooked by amateurs, with the observing position always being at the same height as it passes through a hollow declination axis it is certainly comfortable, however this also means that loads on the scope can be heavier as supports can be rigid and solid in their construction thus supporting some pretty decent kit without the danger of impact with the scope, housing or mount.

Now I understand that this is a not a simple design project as the two supporting mirrors (secondary and tertiary) have to be properly positioned and collimated to ensure optimal performance.. and I have to work out the distances of the mirrors to ensure it all works as I want it too.

This is at the planning stage, and I am projecting a total build time to "first light" of about 3 years, maybe longer depending on costs and time factors as I work away a lot.

Does anyone here have any experience of building a Nasmyth or anyone with greater experience of building a telescope have any comments on this project..I am open to all advise. I don't think the construction is an issue, being an engineer, albeit electrical, it is within my abilities and I have 99% of all the tools I need...I need to buy a new router as my old one is dead..

Does anyone know the formula for calculating the optimal distances of the mirrors on a Nasmyth as so far my research has drew a blank.

Thanks.

Jim

[JamesF]

It sounds like a fascinating project. I find myself wondering if it might not be possible to use a primary mirror with a hole in the centre (like a Mak or SCT) and pass a post through that hole to mount the tertiary mirror on. The collimation screws could then be accessible from the back of the OTA.

James

[John]

Having had some brief views through a quality 20" scope I can vouch for the stunning views that are possible with this sort of aperture.

The one I used (made by David Lukehurst) was a conventional design but the focal ratio had been selected to ensure that the eyepiece height was comfortable and did not require a ladder but just a low step to view the zenith. The eyepiece seemed to remain at a comfortable height for me standing (I'm 6.0 foot) even when the scope was declined around 45 degrees.

I suppose I'm wondering if, with careful focal ratio selection, the need for the complexity of the Nasmyth design could be avoided. I guess it's possible if you are OK with a fork-type alt-azimuth mounting such as a dobsonian.

[J_M_Franklin]

It sounds like a fascinating project. I find myself wondering if it might not be possible to use a primary mirror with a hole in the centre (like a Mak or SCT) and pass a post through that hole to mount the tertiary mirror on. The collimation screws could then be accessible from the back of the OTA.

James

Thats a damn fine train of thought there James, the obstruction would be less that a standard, but it would have to be firm to avoid flexing as the scope moved...Titanium?? (Joking on the titanium)

[J_M_Franklin]

Having had some brief views through a quality 20" scope I can vouch for the stunning views that are possible with this sort of aperture.

The one I used (made by David Lukehurst) was a conventional design but the focal ratio had been selected to ensure that the eyepiece height was comfortable and did not require a ladder but just a low step to view the zenith. The eyepiece seemed to remain at a comfortable height for me standing (I'm 6.0 foot) even when the scope was declined around 45 degrees.

I suppose I'm wondering if, with careful focal ratio selection, the need for the complexity of the Nasmyth design could be avoided. I guess it's possible if you are OK with a fork-type alt-azimuth mounting such as a dobsonian.

Definitely will be Equatorial mounted because to not allow such a beastie to be used for astrophotography would be criminal, I was thinking of a German Mount..

[John]

Definitely will be Equatorial mounted because to not allow such a beastie to be used for astrophotography would be criminal, I was thinking of a German Mount..

Ah, I see. Thats going to be some mount as well as the scope !

Good luck with the project

[bottletopburly]

good luck

[John]

I've just re-read your post. Do you mean 20" (500MM) or are you also considering a 16" (400mm) as an option ?

[HD-AP]

Hi

I have built a 16 inch f6 /f15 naysyth newtonian /cassegrain on an equatorial fork mount . The optical design and optics were done for me by the late david sinden of the sinden optical company they are zerodur to 1/30th wave surface he gave you better accuracy than quoted because he could .

The tube assembly is a serriur truss on a polar disc mounted equatorial fork .the fork and mount were a collaboration between myself and barry watts of beacon hill telescopes ,drives are by awr technology .The newtonian focus /top half of the tube rotates to allow a convienient eyepiece position .I haven't put the naysmyth cassegrain focus through the declination axis although i could have but didn't as it leads to problems instead i have four focusser positions at the lower end of the tube .although i think one will suffice .

The focusser is a large 90 mm crayford 2 stage focusser designed and made for me by john wall .The project has taken 9 years so far the design of the telescope hardware has changed over time and is always a work in progress .Telescope is currently dismantled pending building a new observatory to house it .I can post some pictures of it during construction if anyones interested .john walls crayford is a work of art !!

I researched the design extensivley and consulted with many experts who gave me advice freely which is greatly appriciated harrie rutten nasa optical engineer's schott glass david sinden john wall to name a few .

So if i can help with your design or answer any quetions you may have ask away.

[J_M_Franklin]

I've just re-read your post. Do you mean 20" (500MM) or are you also considering a 16" (400mm) as an option ?

Yup, should be 500mm, I noticed my typo after the post but cannot edit?????

[ronin]

My concern is the picture you have chosen, the secondary and tertiary mirrors look flat and if that is the design you intend to follow I suspect that the light loss due to obscuring the path will be significant. The light rays depicted are not right for a flat mirror and look identical to one for a hyperboloid mirror at the secondary position.

If flat then the secondary would need to be larger then normal, I suspect it will be about 35-40% the diameter, then the tertiary mirror will be blocking a significant proportion of the light from the primary to the secondary, say about the same.

Using 40% dia blockage by size we would have 16% at the secondary so down to 84%, then another 16% so 70%. That means you are down to a 330mm mirror effective. I would not be surprised to find that the secondary and tertiary block closer to 50% meaning a smaller effective diameter ~300mm.

If you use a hyperboloid mirror at the secondary position then this, I think, has to be matched the primary so until you have made the primary I do not think you can get a secondary, and having a custom built hyperboloid will not be inexpensive.

Do you intend to have a dual truss system as here?

Example

With the number of obstructions on this design and the number of spiders I am not sure how good it will be on astrophotography, you could have 8 spikes to any stars.

[J_M_Franklin]

Hi

I have built a 16 inch f6 /f15 naysyth newtonian /cassegrain on an equatorial fork mount . The optical design and optics were done for me by the late david sinden of the sinden optical company they are zerodur to 1/30th wave surface he gave you better accuracy than quoted because he could .

The tube assembly is a serriur truss on a polar disc mounted equatorial fork .the fork and mount were a collaboration between myself and barry watts of beacon hill telescopes ,drives are by awr technology .The newtonian focus /top half of the tube rotates to allow a convienient eyepiece position .I haven't put the naysmyth cassegrain focus through the declination axis although i could have but didn't as it leads to problems instead i have four focusser positions at the lower end of the tube .although i think one will suffice .

The focusser is a large 90 mm crayford 2 stage focusser designed and made for me by john wall .The project has taken 9 years so far the design of the telescope hardware has changed over time and is always a work in progress .Telescope is currently dismantled pending building a new observatory to house it .I can post some pictures of it during construction if anyones interested .john walls crayford is a work of art !!

I researched the design extensivley and consulted with many experts who gave me advice freely which is greatly appriciated harrie rutten nasa optical engineer's schott glass david sinden john wall to name a few .

So if i can help with your design or answer any quetions you may have ask away.

Sounds like a great project, I am aware of the issues one can face with taking the Nasmyth focus out of one of the drive axis amd that this would mean over engineering the axis mounts and shafts to ensure they remain rigid under drive and do not impact the light path as a result.

I love the fact that so many professionals have been happy to help, I think that is great. At this stage I am planning the physical structure of the scope and whilst materials are an easy choice for the most part, it is the calculations for the light path that is currently stumping me..I know that it's going to be an F/10, but it is the break down of the path to ensure it is correct and can focus that is important...

[HD-AP]

Hi jim

Thanks for the kind comments. The main problem you encounter when putting the naysmyth focus through the declination axis is field rotation in that the field of view rotates as the telescope tracks an object , which needs mechanical or derotation optics to compensate which is an engineering optical challenge and can be expensive .I can help you with your calculations if you want ?

But it will be a great project i'm sure.

private mail me if you want .

I notice that you are in swindon i live on the outskirts of bristol so not a million miles away .

best regards

[J_M_Franklin]

My concern is the picture you have chosen, the secondary and tertiary mirrors look flat and if that is the design you intend to follow I suspect that the light loss due to obscuring the path will be significant. The light rays depicted are not right for a flat mirror and look identical to one for a hyperboloid mirror at the secondary position.

If flat then the secondary would need to be larger then normal, I suspect it will be about 35-40% the diameter, then the tertiary mirror will be blocking a significant proportion of the light from the primary to the secondary, say about the same.

Using 40% dia blockage by size we would have 16% at the secondary so down to 84%, then another 16% so 70%. That means you are down to a 330mm mirror effective. I would not be surprised to find that the secondary and tertiary block closer to 50% meaning a smaller effective diameter ~300mm.

If you use a hyperboloid mirror at the secondary position then this, I think, has to be matched the primary so until you have made the primary I do not think you can get a secondary, and having a custom built hyperboloid will not be inexpensive.

Do you intend to have a dual truss system as here?

Example

With the number of obstructions on this design and the number of spiders I am not sure how good it will be on astrophotography, you could have 8 spikes to any stars.

That picture is a very simple depiction of how a Nasmyth design is laid out, not a technical spec. The secondary mirror is always a Hyperboloid and the tertiary is always a flat. Properly designed I do know that the total obstruction should be no more than a classical Cassegrain. Actually a Classical Cassegrain mirror does provide the ability to mount the tertiary mirror in such a way that you do not have two spiders, John actually asked the question and when rereading some info I have I realised that this is actually how it is done...In fact by still installing the baffle required for a Cassegrain with the end modified to hold a mount for the tertiary, you have the option of removing the tertiary mirror and using the scope as a classical cassegrain..

[Scott]

Hi

I have built a 16 inch f6 /f15 naysyth newtonian /cassegrain on an equatorial fork mount . The optical design and optics were done for me by the late david sinden of the sinden optical company they are zerodur to 1/30th wave surface he gave you better accuracy than quoted because he could .

The tube assembly is a serriur truss on a polar disc mounted equatorial fork .the fork and mount were a collaboration between myself and barry watts of beacon hill telescopes ,drives are by awr technology .The newtonian focus /top half of the tube rotates to allow a convienient eyepiece position .I haven't put the naysmyth cassegrain focus through the declination axis although i could have but didn't as it leads to problems instead i have four focusser positions at the lower end of the tube .although i think one will suffice .

The focusser is a large 90 mm crayford 2 stage focusser designed and made for me by john wall .The project has taken 9 years so far the design of the telescope hardware has changed over time and is always a work in progress .Telescope is currently dismantled pending building a new observatory to house it .I can post some pictures of it during construction if anyones interested .john walls crayford is a work of art !!

I researched the design extensivley and consulted with many experts who gave me advice freely which is greatly appriciated harrie rutten nasa optical engineer's schott glass david sinden john wall to name a few .

So if i can help with your design or answer any quetions you may have ask away.

i'd love to see some of those pics please.

Scott

[HD-AP]

Hi jim

Thanks for the kind comments. The main problem you encounter when putting the naysmyth focus through the declination axis is field rotation in that the field of view rotates as the telescope tracks an object , which needs mechanical or derotation optics to compensate which is an engineering optical challenge and can be expensive .I can help you with your calculations if you want ?

But it will be a great project i'm sure.

private mail me if you want .

I notice that you are in swindon i live on the outskirts of bristol so not a million miles away .

best regards

Hi jim late night moment Doh. The field rotation problem was when i had the telescope configured as an alt azimuth design which is why i went for the equatorial polar disc fork.

[HD-AP]

Hi scott

pictures of scope build

[laser_jock99]

pictures of scope build

That's some feat of engineering! Love those huge worm wheels.

[HD-AP]

That's some feat of engineering! Love those huge worm wheels.

Thanks 14 inch aluminium brass , a bit steampunk

[J_M_Franklin]

That is a serious bit of kit John, love the worm drives, nothing like a bit of over engineering to ensure reliability..

I take it that picture 3 on the bottom row is the one of the drive motors..??

With regards the image rotation problem..trying to get my head around that because the way I was thinking about this it would not be an issue..have I missed something?

[HD-AP]

That is a serious bit of kit John, love the worm drives, nothing like a bit of over engineering to ensure reliability..

I take it that picture 3 on the bottom row is the one of the drive motors..??

With regards the image rotation problem..trying to get my head around that because the way I was thinking about this it would not be an issue..have I missed something?

Hi jim

I had a senior moment on the field rotation ,it was a problem when i had the telescope configured as an alt-azimuth ,but i have since mounted it as an equatorial polar disc fork mount which eliminates the problem .

which picture do you mean the awr drive motor is shown in a few of the uploaded pictures .

best regards

clive

[Ptarmigan]

As I am making a concerted effort to get back into __observational__ astronomy

,,,

I have used many scopes over the years and always found the observing position to be a pain...literally

The Nasmyth (modified Cass) has a large (long) f/ratio.

Another design you might consider is the Springfield/Porter mount in which a "normal" Newtonian is mounted with the dec axis coaxial with an extended Newtonian egress, and a tertiary mirror or prism directs the light cone up the final RA axis to a fixed eyepiece.

This results in a lower f/ratio but a more elaborate (unwieldy ?) mechanical design.

However, for __observational__ use you might consider an alt-az design pivoted around the Newtonian egress for the Alt axis, the observer remains fixed at this eyepiece whilst the whole observatory (including his chair !) rotates about the Az axis.

An advantage of this (super Dobsonian!) is that an internal wall can be mounted in this observatory such that the telescope is in the cold half, the observer is in the warm half, and only the eyepiece tube joins the two !

[Scott]

thanks for the pics Clive, am I right in thinking the polished alloy pic is the focuser? just love seeing these grand engineering projects. sorry for digressing from your build jim, looking forward to seeing some of your pics.

Scott

[HD-AP]

thanks for the pics Clive, am I right in thinking the polished alloy pic is the focuser? just love seeing these grand engineering projects. sorry for digressing from your build jim, looking forward to seeing some of your pics.

Scott

Hi scott

Yes it's the crayford focuser john wall made for me .

cheers

clive

[HD-AP]

Hi jim

Here are a couple of links you may find useful

http://www.google.co.uk/url?sa=t&rct=j&q=mike+lockwoods+naysmyth+cassegrain&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http%3A%2F%2Fwww.loptics.com%2FATM%2Fmirror_making%2Fcass_info%2Fcass_info.html&ei=LwOyUJXRMoSZ0QWI0oGgAg&usg=AFQjCNHvFJrEFNnCDJXb8vLrdePN7rDbrw

http://www.google.co.uk/url?sa=t&rct=j&q=mike+lockwoods+naysmyth+cassegrain&source=web&cd=6&cad=rja&ved=0CE8QFjAF&url=http%3A%2F%2Fwww.cloudynights.com%2Fitem.php%3Fitem_id%3D1127&ei=LwOyUJXRMoSZ0QWI0oGgAg&usg=AFQjCNFFuMRNyhzGPVDOEy3z_GLdT-mxJA

cheers

clive