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Why are ota so long ?

spaceboy

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spaceboy

spaceboy

Posted
Posted

Time after time you hear that BV users, imagers or 2" diagonal owners struggle with achieving enough inward travel to focus on objects yet manufacturers continue to make ota's that tad bit too long. Often we are only talking mm but the hassle it can bring to the user is huge. More often than not there is plenty of travel in a focuser draw tube to compensate for a shorter ota and even if it was an issue it is easy enough to employ extensions. Surely it wouldn't make that much difference if they were to start shortening ota by 10mm or so. They claim they do so on the PDS range of newts to allow easier camera focusing.

So am I missing something? I get the whole focal length of a scope and that if your not careful a racked in drawtube could clip aperture but Helios /early SW (both from syntax) used to do slightly oversize drawtubes on their Newtonians (not sure about their fraks) so this would surly elevate any problems. I just feel it would be so much more user friendly to make an ota a tad shorter allowing the use of extensions if there were any problems than to make an ota the length they do where tube chops of barlowing seem the only option.

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johninderby

johninderby

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Have to agree. Annoying that often there isn't quite enough in-focus travel if you make changes such as a new focuser or 2" diagonal. With my Orion 120 in-focus travel,became an issue after fitting a new focuser and using a Hershel wedge. Why not make the OTA 10mm shorter then just have an extra 10mm drawtube travel?

In my case shortened the OTA slightly so problem solved.

           John

 

IMG_1501.JPG

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ollypenrice

ollypenrice

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It's funny but I used to curse William Optics for the opposite tendency. Determined to make their scopes ultra compact they'd use tiny OTAs which left a great long waggly drawtube poking out of the back and sagging. I still have one from that generation.

Olly

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spaceboy

spaceboy

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47 minutes ago, johninderby said:

Have to agree. Annoying that often there isn't quite enough in-focus travel if you make changes such as a new focuser or 2" diagonal. With my Orion 120 in-focus travel,became an issue after fitting a new focuser and using a Hershel wedge. Why not make the OTA 10mm shorter then just have an extra 10mm drawtube travel?

In my case shortened the OTA slightly so problem solved.

           John

 

IMG_1501.JPG

I'm in a very similar boat to you John. Only problem is our friends the other side of the pond have established the Chinese 120mm f/5 fraks clip aperture when using some 2" diagonals anyway. This means for certain you also going to have to shorten the drawtube if you shorten the ota or your going to be no better off having the ST120 over the ST102. That is if the ST102 doesn't suffer the same issues, although I haven't heard of it.

47 minutes ago, ollypenrice said:

It's funny but I used to curse William Optics for the opposite tendency. Determined to make their scopes ultra compact they'd use tiny OTAs which left a great long waggly drawtube poking out of the back and sagging. I still have one from that generation.

Olly

With heavy ep or cameras you always have the option of fashioning some sort of bracing for the drawtube or upgrading the focuser to deal with sag where as you have no other option than to get the hacksaw out on longer OTA. It just seems that more people complain about lacking inward travel over outward. I'm sure there would be more benefit to synta to shorten ota as they would probably sell more BV's for one. I personally really liked the views through my WO BV's but all the hassle reaching focus and having to use barlows resulting in high mags put me right off.

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Macavity

Macavity

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You might also check if the (default anyway) focuser tube is cutting into the light cone? Skywatcher ST OTAs had a reputation for that. "Ray tracing" (or plain old graph paper!) might be useful. Hoping to receive an ST102 OTA for "fun and experiment" soon... I already have a TS Monorail focusser ready! That is a lot shorter, but only has 50mm travel! I need extension tubes... Swings and Roundabouts? :p 

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spaceboy

spaceboy

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1 hour ago, nightfisher said:

This is another plus point for the not so humble Maksutov scope, huge focus range in both directions able to use binoviewer and all in a short tube

I agree that maks and sct are probably best suited to BV but as pointed out so would probably everything else for the sake of 10mm off the end of the OTA (and maybe a slightly larger or shorter focus tube in faster fraks). Just seems daft when there is so little in it to resolve the issue. It would surly benefit more than it would hinder.

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ronin

ronin

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The scopes were designed around the simple idea of an objective of focal length F and an eyepiece sat at the correct position for the object plane of the eyepiece to coincide with the image plane of the objective.

When adding barlows and binoviewers you change the optics and this was not intended. Ever complained that the binoviewer was not designed for a "standard" scope, everyone seems to complain about the scope, buy an oil filter for a car and it has the wrong thread you complain about the filter not the car. Why noit the same for the scope.

The scope is not wrong the barlow and the binoviewers are wrong. Seems that the scope gets the blame. Someone has built binoviewers that cannot be placed at the correct position in order for them to operate.

In term of lumps of tubing a single bit of aluminium tube is less cost then 1 short bit and then a seperate extension tube. So you end up with a fixed length possibly long OTA.

ES and Bresser seem to have gone to the idea of supplying a shorter tube and a couple of extension tubes mainly I guess for the imageing side. Add DSLR as it comes or screw in an extension tube and add an eyepiece.

In fairness if they supplied a shorter tube and then a series of extension teubes and longer focusers then mechanically things sag and people complain of astigmatism caused by the focusr tube not being stiff enough. Not long ago plossl's were the common eyepiece but eyepieces are a lot heavier now.

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johninderby

johninderby

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Problem is people's viewing habits and the accessories used have changed and many scope manufactures haven't kept up with these changes. They are still building yesterday's scopes.  So yes I do blame these manufactures for not updating their designs to accommodate this. 

          John

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Macavity

Macavity

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If it is only a *few* mm of in-focus you need, you can often
play around with (non-solar!) diagonals. Find out the thread
size on the eyepiece holder side and fit a shorter one. If you
adapt to a T2 thread, you can fine tune via extension rings. ;)

http://www.teleskop-express.de/shop/product_info.php/info/p209_Visual-Back-T2-to-1-25----with-additional-T2-male-thread---short.html

Another alternative is the Baader 35mm Maxbright Diagonal.
This can cope with most *normal* 2" eyepiece field stops and
the optical path is significantly less than the standard 2"... :)

http://www.teleskop-express.de/shop/product_info.php/info/p607_Baader-T2-Maxbright-diagonal-mirror---dual-T2-threads.html

In such a situation, lateral thinking? "Never say die" etc. If
long sighted, remove your glasses?? Sadly I'm short sighted!
I have to focus inwards more when I take my glasses off! :D

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spaceboy

spaceboy

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1 hour ago, johninderby said:

Problem is people's viewing habits and the accessories used have changed and many scope manufactures haven't kept up with these changes. They are still building yesterday's scopes.  So yes I do blame these manufactures for not updating their designs to accommodate this. 

          John

Ditto

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SilverAstro

SilverAstro

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If it is just a few mm then yes one does wonder. But if it is for example a Newtonian there is traditionally a balance to be struck between visual and camera* use. If the focal point needs to be further out the side of the tube (*for whatever reason) then the diagonal/secondary needs to be closer to the primary. This means that it needs to be bigger to intercept the wider cone, or you give some of that wider cone an miss. If you do want to intercept that larger cone it means a bigger diagonal which causes more obstruction of the incoming to the primary.

So end result is ye'cana'win :) 'tis a big and perennial head scratch! A compromise will always be that.

 

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Macavity

Macavity

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(DSLR) f/4 Astrograph = Newt with semi-detached refractor. :p
Though I recall TS (DE) will customise them for your needs...

Was it the Borg (collective) who incorporated OTA extensions?
Jolly good idea though. White light Solar with Herschel Wedge
comes to within one mm of the inner limit on my Altair ED66!
The "right side fortunately! The OTA being threaded of course. :happy8:

In fact, see above for an example... ;) 

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