Portable 250mm Newt

[Tommohawk]

8 minutes ago, MarkRadice said:

I have taken my 150mm Mak (and 60mm solar scope, eyepieces, ASI camera etc) to Cyprus and Spain as hand luggage while the mount goes in a suitcase in the hold.  6" is enough aperture to be useful but not too big for carry on - and not too expensive second hand.  I even managed to fit the volcano Olympus Mons into the little scope.

 

That's a remarkable result with a 150 aperture - nice job! Makes me wonder if I should forget the aperture fever and think more about my setup location. I could get to Sierra Nevada mountains fairly easily, so perhaps the 3000m would get me something extra. Thats really all I'm after I guess - a better result than I've had before.

But..... a 250mm at 3000m would be even better!

[Peter Drew]

Look up Gladius telescope, a 10" Newtonian version of one of these would tick your boxes. I've made a 12" Cassegrain example and the system works well. 

[Tommohawk]

Hi Peter - thats an interesting design. Not sure how I could make this as a newt though? Focuser would need some kind of sideways strut and would be prone to flex dont you think?

[Peter Drew]

I built a 18", 17" and 16" Newt version without any problems, a 10" would be easier.    

[Tommohawk]

Wow! So do you mind me asking how you attached the focsuer?

[laowhoo]

Well Tom,

I frankly never understood wood for a build, even tho (or b/c?) I grew up woodworking. No, our first and only build, inspired by Bartel's chambered nautilus musing, is entirely from alumin(i)um and JB Weld. Only tool I used was a Dremmel. It's 12.5" doubled (bino) and I think the OTA was 4.5 pounds. And it's a full cage 5.5" tall. Had to keep it light for doubled focusers/secondaries/star diagonals. And we'd (my wife and I) never even looked thru a telescope before that, proving that anyone can do it (whole point of the build).

[Peter Drew]

2 hours ago, Tommohawk said:

Wow! So do you mind me asking how you attached the focsuer?

The main "beam was 2" square aluminium tube, the focuser fitted against a hole in two opposite faces. 

[Tommohawk]

On ‎28‎/‎12‎/‎2017 at 15:36, laowhoo said:

Well Tom,

I frankly never understood wood for a build, even tho (or b/c?) I grew up woodworking. No, our first and only build, inspired by Bartel's chambered nautilus musing, is entirely from alumin(i)um and JB Weld. Only tool I used was a Dremmel. It's 12.5" doubled (bino) and I think the OTA was 4.5 pounds. And it's a full cage 5.5" tall. Had to keep it light for doubled focusers/secondaries/star diagonals. And we'd (my wife and I) never even looked thru a telescope before that, proving that anyone can do it (whole point of the build).

That sounds amazing! I need to review Bartels thoughts on that. Haven't done any welding for about 40 years and never touched ally, so not sure how I'd cope with that. Do u happen to have a pic of the finished scope?  

[Tommohawk]

On ‎28‎/‎12‎/‎2017 at 16:50, Peter Drew said:

The main "beam was 2" square aluminium tube, the focuser fitted against a hole in two opposite faces. 

ok Peter thanks. Can't quite visualise that though. Don't suppose u have a pic?

[laowhoo]

I don't weld w/ ally either Tom. We used JB Weld, a metal laden epoxy. Tools were a Dremmel and hacksaw for cutting struts out of angled mostly. Hardest part was getting it out of the basement. I'll dig out the pics of the build to show the OTA, but don't be put off that it's a double--the only difference we had to account for was being able to adjust for different interpupillary differences among people. Everything else is perfectly straightforward. And neither would you have to build a "3D" cage--you could get away with a flat OTA just as well provided you stabilize your focuser adequately. Maybe you've seen these--they're everywhere, and I wonder for your purposes whether that wouldn't be best? Like here

https://buytelescopes.com/planewave-instruments-20-inch-ritchey-chretien-optical-tube-assembly

Never mind that it's a Ritchey (or that it's $46k LOL), I just did a quick search to show. This OTA is the same for us. Here, like this

https://www.365astronomy.com/Skywatcher-STARGATE-450P-18-f-4.1-Parabolic-Truss-Tube-Dobsonian-Telescope.html

And if you don't feel confident, just double it to the depth of a few inches, to the depth of your focuser

https://telescopes.net/store/explore-scientific-12-inch-ultra-light-truss-tube-dobsonian.html

Either way, it's super light and eminently pack-able.

[Tommohawk]

3 hours ago, laowhoo said:

I don't weld w/ ally either Tom. We used JB Weld, a metal laden epoxy. Tools were a Dremmel and hacksaw for cutting struts out of angled mostly. Hardest part was getting it out of the basement. I'll dig out the pics of the build to show the OTA, but don't be put off that it's a double--the only difference we had to account for was being able to adjust for different interpupillary differences among people. Everything else is perfectly straightforward. And neither would you have to build a "3D" cage--you could get away with a flat OTA just as well provided you stabilize your focuser adequately. Maybe you've seen these--they're everywhere, and I wonder for your purposes whether that wouldn't be best? Like here

https://buytelescopes.com/planewave-instruments-20-inch-ritchey-chretien-optical-tube-assembly

Never mind that it's a Ritchey (or that it's $46k LOL), I just did a quick search to show. This OTA is the same for us. Here, like this

https://www.365astronomy.com/Skywatcher-STARGATE-450P-18-f-4.1-Parabolic-Truss-Tube-Dobsonian-Telescope.html

And if you don't feel confident, just double it to the depth of a few inches, to the depth of your focuser

https://telescopes.net/store/explore-scientific-12-inch-ultra-light-truss-tube-dobsonian.html

Either way, it's super light and eminently pack-able.

OK I get what you mean - truss type thing. I did consider this but the commercial ones seems so heavy I figured I wouldn't be able to replicate this any lighter. The Altair F4 250 would be good, but is 16kg. I guess they're uber solid design and maybe I could get away with some weight saving. I don't need cooling fans really I suppose for a start. Also if I'm going to fabricate the frame myself it would be easy to mail one frame to my preferred foreign destination, and keep a similar one at home, and then just travel with the mirrors. Worth some thought for sure.

Would be interested to see your pics if you can dig them out

[laowhoo]

Ouch. Now I understand my confusion. I thought maybe the UK terminology was slightly different from ours and figured you were just talking about packing the Upper Tube Assembly--that it was synonymous with Optical Tube Assembly (for the optical equipment focuser/secondary)--and that only the UTA was traveling. Sorry. First thing that comes to mind is Royce's Serrurier truss, and he offers freely (had 5 years ago at least) his own plans for a 10" for ATMers. I'll find it in case that looks to fit your need. But I also know I've seen people split full tubes (saw a lotta scopes 5 years ago) and I'll look see if I can find examples of that also.

Here's his build and the plans are still freely available (scalable)

http://www.rfroyce.com/10f6tel/

Off to look for a split full tube...

Not DIY but here's one, links don't seem active, but a search should provide pics of how it was done

http://www.discoverytelescope.com/discovery/pdhq.html

Here's a review and detailed pics by a user

https://www.cloudynights.com/articles/cat/user-reviews/discovery-175”-split-tube-dobsonian-telescope-r3031

(BTW, our mirrors were made by Discovery's former polisher who struck out on his own and enjoys a great reputation--Terry Ostahowski We'd contacted Mike Lockwood for a mirror, he said don't bino-view, build a bino-scope, cuz he'd just made mirrors for Arie Otte, and we could not afford his premium mirrors so he directed us to Terry, whose prices were half of other well-known makers [or 1/3 a Zambuto, but hey...] And we opted for f6 to more easily ensure a great figure, to cut him some slack for the great deal he offered, and to cut ourselves some slack w/ better collimation tolerances and forgiveness on the more affordable eyepieces. We also ordered our flats from Terry, which he sent but wouldn't take money for. Unreal. Terry is now also coating his own mirrors.)

Okay, here's someone who split their Meade for packing purposes with their How To

https://www.cloudynights.com/articles/cat/articles/how-to/creating-a-split-tube-meade-16”-starfinder-dob-r537

Am I on the right page yet?

 

[Tommohawk]

46 minutes ago, laowhoo said:

Ouch. Now I understand my confusion. I thought maybe the UK terminology was slightly different from ours and figured you were just talking about packing the Upper Tube Assembly--that it was synonymous with Optical Tube Assembly (for the optical equipment focuser/secondary)--and that only the UTA was traveling. Sorry. First thing that comes to mind is Royce's Serrurier truss, and he offers freely (had 5 years ago at least) his own plans for a 10" for ATMers. I'll find it in case that looks to fit your need. But I also know I've seen people split full tubes (saw a lotta scopes 5 years ago) and I'll look see if I can find examples of that also.

Here's his build and the plans are still freely available (scalable)

http://www.rfroyce.com/10f6tel/

Off to look for a split full tube...

Not DIY but here's one, links don't seem active, but a search should provide pics of how it was done

http://www.discoverytelescope.com/discovery/pdhq.html

Here's a review and detailed pics by a user

https://www.cloudynights.com/articles/cat/user-reviews/discovery-175”-split-tube-dobsonian-telescope-r3031

(BTW, our mirrors were made by Discovery's former polisher who struck out on his own and enjoys a great reputation--Terry Ostahowski

 

Thanks for that! So it seems my idea of cutting the tube in half isnt original - I searched on this but didnt think to use the term "split tube".

I think the way forward for me is going to be:

1. acquire conventional F4 250mm OTA.

2. for now ship the tube only witohut mirrors to my destination in Spain.

3. travel out with the mirrors out and assemble there

4. figure out a way to make a featherweight portable tube (either truss or split tube) using these mirrors for a longer term solution.

Thanks again for your input. PS would still be interested to see your DIY scope pics if they are around!

[laowhoo]

Lest I forget, these sites were invaluable (sorry for the parsing, but the site is not the most navigable so I took the liberty. And the ray trace program will have you knowing your scope better than Dobson himself. Also included Bartel's secondary calculator...even if only to better relax about the central obstruction/secondary size "degradation" myth.)

https://stellafane.org/tm/atm/

https://stellafane.org/tm/dob/index.html

https://stellafane.org/tm/index.html

https://stellafane.org/tm/newt-web/newt-web.html

http://www.bbastrodesigns.com/diagonal.htm

I'll get those pics. Please let me know if I can help. We've only made one scope, but I did TONS of research first. Maybe one more thing is Nils Olof Carlin's collimation page. It helps to have the complete picture before beginning, and hurts when things put you in the weeds unnecessarily.

http://web.telia.com/~u41105032/kolli/kolli.html

Cheers

[Tommohawk]

1 hour ago, laowhoo said:

Lest I forget, these sites were invaluable (sorry for the parsing, but the site is not the most navigable so I took the liberty. And the ray trace program will have you knowing your scope better than Dobson himself. Also included Bartel's secondary calculator...even if only to better relax about the central obstruction/secondary size "degradation" myth.)

https://stellafane.org/tm/atm/

https://stellafane.org/tm/dob/index.html

https://stellafane.org/tm/index.html

https://stellafane.org/tm/newt-web/newt-web.html

http://www.bbastrodesigns.com/diagonal.htm

I'll get those pics. Please let me know if I can help. We've only made one scope, but I did TONS of research first. Maybe one more thing is Nils Olof Carlin's collimation page. It helps to have the complete picture before beginning, and hurts when things put you in the weeds unnecessarily.

http://web.telia.com/~u41105032/kolli/kolli.html

Cheers

Hey thanks again for all that!. Stellafane's Newt-on-the-web I'm very familiar with and have found it invaluable in the design and limitation of newts generally - all theoretical as yet!

Mel Bartels pages are very handy esp. for basic stuff like C of G calculations for balance points. - I wasn't familiar with them.

Sacek's telescope-optics, which Bartel refers to, is phenomenal! As it happens I have already read this pretty much from start to finish getting the drift of some of it and following the maths in almost none of it! (I was looking to convert a damaged SCT (Schmidt plate broken) to a Klevtzov type corrector)

Bartels comments about central obstruction are interesting but he's comparing a 20% obstruction (linear) with a 12.5% obstruction. Its quite likely that there is minimal difference at these smaller CO's but of course most newts and SCT have CO's much larger than this. 

The general consensus seems to be that at low spatial frequencies CO does have a significant effect and this is nicely summarized here.

And from a subjective perspective my 8" Newt with a 26% CO consistently matches my C9.25 with a 35% CO.

But... that's for another thread maybe!

 

[laowhoo]

I don't understand your Bartel reference. The link is strictly for his calculator. He did do a comparison test here tho

http://www.bbastrodesigns.com/The 1974 Telescope Secondary Experiment.html

I originally used Legault's site to allay my concerns as I wanted to be able to fully illuminate a 1.8" field stop on the 36mm Hyperion 72, even tho I'd rarely use it. Our skies have me using the 1.4" field stop ES 24mm 68 almost exclusively for wide field, and I could have very reasonably justified 2.14" secondary, and maybe optimized for planets. After Legault, Bartel, et al., I knew the 22% CO of the 2.6" was nothing that I'd see, and we'll never image. Anyhoo, here's Legault for starters. There were others

http://www.astrophoto.fr/obstruction.html

Ran across this also wh/ I remember from back in the day, a design that might help for ideas (scroll down a little to see 3-truss scope)

http://www.fpi-protostar.com/ftp/catalog.pdf

We were gonna get our secondaries from them. Seems eminently pack-able, lightweight in any event. Your CO link seems to be for imaging, wh/ we never considered, but his comparo is 20%/33%, and I was only trying to emphasize that a CO of 20-25% is of little consequence, low frequencies notwithstanding? Our MTF considerations were from Suiter's "Star Testing."

In any event, I know better now that you're adept and will confine myself strictly to build considerations.

Oops--I cited Legault to show little CO difference (20% vs 33%), and you used it to argue the opposite! LOL

Here's our build

ATT-44_pgs64-71.pdf

but as I warned, it was designed to show a scope can be built from just about anything. Nothing fancy. For kids at outreach--and if lines are long (I can dream), 2 can view simultaneously.

[Tommohawk]

11 hours ago, laowhoo said:

I don't understand your Bartel reference. The link is strictly for his calculator. He did do a comparison test here tho

http://www.bbastrodesigns.com/The 1974 Telescope Secondary Experiment.html

 

Bartel refers to Vladimir Sacek's site "telescope-optics":

<<

Bartel says: Diagonals act to degrade optical performance, the larger the diagonal, the worse the degradation. A one-third obstruction, a much larger ratio than the customary visual Newtonian uses, degrades the optical quality by one-sixth wave (less than what is commonly quoted - see http://www.telescope-optics.net/obstruction.htm

>>

He also refers to his experiment which seems to show that a larger secondary may give improved views compared to a smaller one - not what one would expect -  and gives some thoughts on why this might be. Interestingly, Legault's work also implies that at high spatial frequencies larger CO's may indeed give improved contrast. But at medium and low spatial frequencies contrast will reduce with a larger CO. He says:

<<

The MTF curve below (see What is a MTF curve ?) shows that the contrast is modified in a complex manner. Indeed it decreases in comparison with the unobstructed instrument, but only in low and medium frequencies (left part of the curve). On the other hand, in the high frequencies, the contrast is not decreased, it is even slightly increased.....

... In conclusion:

The effects of obstruction are:

1) the general contrast of the images is lowered, the instrument (diameter D, obstruction d) has approximately the same efficiency as an unobstructed instrument of diameter D_eff = D - d

2) the resolution power is not modified on high contrast structures:
Moon, double stars, Cassini division, shadow of a ring or a satellite, edge of a planet,...

3) the resolution power may be lowered on low contrast objects: surfaces of Mars, Jupiter and Saturn. The effective resolution depends on the contrast of the object and the technique used, it is at least the resolution of an unobstructed telescope of diameter D_eff

>>

It seems to me that the key thing is that a scope with too small a secondary will not fully illuminate the sensor (or eyepiece field). So when doing wider field work a larger secondary is necessary, as demonstrated/calculated by Newt-for-the-Web.

However, when considering planetary work, even at high magnifications the object may be only a matter of a couple of millimeters across and a larger secondary is unnecessary and will degrade the image.

Leaving all that aside (!) I'm very grateful for your link to FPI-protostar - some great stuff there and its given me a lot to think about.

Finally thanks especially for the link to your bino-viewer build. What an amazing project, especially for a first build!! Your research was obviously meticulous, the execution precise, and the re-purposing of household and general items for such a demanding instrument is really amazing - I love it!

 

[Peter Drew]

22 hours ago, Tommohawk said:

ok Peter thanks. Can't quite visualise that though. Don't suppose u have a pic?

Hope these images help, they are the top half of a "Gladius" style portable Newtonian.  

[Tommohawk]

Hi Peter - brilliant I get the idea now. When thinking of "Gladius" style, does this mean the method of constructing the scope on a single "back-bone" type strut, or does it refer specifically to the method of supporting the secondary. Or maybe just the whole thing?

Certainly looks like it has possibilities for me. Obvious main issue is that with a newt rather than a Cassegrain the focuser/filter/camera mas may cause flexure in the main strut - but that should be fixable esp for a smaller camera.

Need to do some drawings.... and some thinkings!

[laowhoo]

Tom,

Leave it to a Brit to alliterate a compliment (okay, maybe vocalic alliteration). And leave it to an American to bring a knife to a gunfight (visual vs imaging). But yes (and here I go with the visual emphasis), Vlad's site is often cited for things like CO, smaller apertures for bad seeing, etc., and there's plenty there to find what we want, especially if we can do the maths. I read his site twice back then just to get high on the altitudes he can reach. Is there anything comparable? Rarefied air indeed, like I'm climbing with Edmund Hillary, especially for a humble sherpa like myself. He usually sums up at the end, but for CO he sums up at the beginning

Much has been said about the effect of central obstruction in the amateurs circles, most of it being speculation. The common notion is that it reduces effective linear aperture for low-contrast details by as much as obstruction diameter. Informal attempts have been made to find a theoretical basis for this empirical "rule". Not a few amateurs "tested" it and often concluded that it "works". What tends to be neglected is the pretty obvious fact that in any such comparison there is more than just a single factor - central obstruction - affecting low-contrast performance. Most of these factors - seeing error, overall optical quality, sensitivity to miscollimation and thermal errors, light scatter, baffling - favor smaller unobstructed aperture, usually a high-quality apochromatic refractor, over the larger reflector or catadioptric. Consequently, if the rule "works" empirically, it inadvertently proves it incorrect, as long as it insists on the difference in performance coming from the effect of central obstruction alone.

And for MTF I relied upon Suiter as I said earlier. Both address visual observing, and my favorite word by Vlad for both the CO and small aperture vs. seeing questions is "rebound." But I'm happy to leave each to their own, unless they're talking imaging in which case I completely bow out. And heaven help me if I ever start down that road. Suffice to say I hope, a CO under 33% is generally regarded by most as acceptable for visual according to MTF, Dawes Limit and the 90% of viewing we do in most seeing conditions. That's why when we wanted to try to produce binoscopes for kids we tried to get someone to make mirrors of only .92 Strehl as that's all that was practically useful, and why I cringed a little when revisiting Stellafane to hear them say that a 20% CO is acceptable but one should try to aim for smaller as better. Besides, astronomy/telescopes can be off-putting/intimidating enough w/o even getting a chance to step into the weeds. But as you say, maybe best left for another thread.

What a great site this is. I don't forum but had occasion to come here looking for a friend, and I got sucked into the literacy/magnanimity of the posts here. I miss my England--used to shop for my first editions there--and will have to return with my wife. THANK YOU for the meaningful discourse.

[Peter Drew]

@tommohawk.

Yes Tom, the main tube is a single strut beam, the portion shown in the images slides into a 2" square aluminium tube using a linear bearing arrangement for smoothness and accuracy. This design makes it possible to suit visual, photographic or binoviewer focal positions. The opposition of the focuser and the secondary holder tends to balance things out at the front end as far as twisting is concerned. The balance point of the whole unit is quite low and combined with the plate attachment to the mount it is quite rigid. Maximum component length for a 10" F5 would be about 24".  

 

[Tommohawk]

Slight delay over the New Year etc - hope you all had a good time!

OK thanks all for the input. I have been looking for a cheap 10" F4 for some time with a search running on ebay for a 10" Quattro for over 12 months. Bizarrely, given this current thread, a couple of days back one came up on ABS for a good price which was only 10 minutes away!

I'm now in the process of sorting it out a bit - it had lost a couple of ball bearings from the focuser and the focuser alignment and secondary position look a bit off so I'll have a fiddle with that. Looking at the forecast I'll have plenty of time before first light!

So now I'm on the lookout for a duplicate bare tube or some kind of Gladius or truss type thing to mail out to Spain for planetary use later in the year. I'll post back sometime when I've had a chance to check it out.

Thanks to all!