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How to find the focal range for a ST80

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On my camera lens units they give me a minimal focus distance the lens will focus at. 

Now my skywatcher Star travel 80mm ( ST80 ) i use for both astro and terrestial  viewing photography.

my problem is the telescopes infomation does not give the minimal focal distance of the given optics, unlike my camera lenses that do.

this problem really happens in wildlife photography, focus on wildlife far away is no problem, but anything less than 50ft away there is no chance.

if i add an extender (100mm extender ) i am able to focus on closer objects BUT i loose infinity focus totaly , the scope just does not focus anything at distance

IE: not enough inward travel with extender , and not enough outward travel without extender.

is there any way or info on the minimal focal distance for the skywatcher optics, i have looked on the website to no avail 

and am left missing out on some lovely terrestial shots without this minimal distance of focal range. IE how close the lens can be to an object to gain focus of said object.

scope details are 80mm objective @ 400mm focal length

my lens detail are 90-300 focal length and a 4.5ft to infinity focal range

i hope all this does not sound like i am barking up a tree at nothing, but i can see the problem in my head and know what it is but cant explain it in wording very well

my lens gives a minimal but the ST80 does not how do you work out this minimal focal distance

Edited by Mr TamiyaCowboy
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Unlike a camera lens your scope does not have any fixed Optics>Sensor distance (your camera lens might be 55mm for example). You could in theory poke a webcam up the inside of the focuser tube :)  So there can be no estimation of minimum or maximum focus distance, it all depends on the arrangement of camera and the extensions you use - or how close you can get the sensor to the rear face of the focuser tube. Use an eyepiece camera like a QHY5L-II and you can focus as close as you like!


Edited by ChrisLX200
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I dont know about the SW optics but i use my scope down to about 5ft distance that does require a lot of extra extensions though i even have an iris section for doing macro shots.

This is my basic short range setup but like you said it looses any chance of infinity focus.



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Its puzzling me now 

50mm prime lens min focus distance is 1.5ft to infinity 

my st80 is a prime 400mm lens ( 400mm focal length ) so the telescope should be a **ft to infinity 

its like this you focus up orions nebula , and then decide Hmmm i will go view the moon, you swing round, twist focuser a tad and take a shot.

for me its i focus up the magpie at 400m away then see a greater spotted woodpecker at 80ft , you know the better target is that woodie

so you unscrew the kit shove in a extender to find its not good enough add another line up and MEH !!! damn you bird its flown away.

yet if it was orion and the moon instead of a magpie and woodpecker you have no problem at the longer focus range.

what i cant work out is how i find the full min/max focus distance of the objective lens , every lens must have a minimum and max focus distance 

and that should also be the same for a telescope to ?

Edited by Mr TamiyaCowboy
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Thats the downside when using a scope for birding the simple lens arrangement cant compete with the multiple element camera lenses, you can get adjustable extensions (i use an orion one) that does help a bit. 


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It depends on how far back the focuser will allow the eyepiece to go.

As long as the object is more then 400mm away then the objective will form an image somewhere for  the eyepiece to use, it will however get dimmer and bigger as the image distance increases.

The other aspect is a 50mm may go to 1.5 ft, but a a 400 will not got to 12 ft, the relationship is not linear.

The answer is if you look at something at infinity (Moon) then how much more back focus(BF) will the focuser allow.

Then the image distance is (400+BF). = u.

As F=400 you can use:

1/400 = 1/v +1/(400+BF) to determine v, which would be the closest that an object can be to achieve an image for your scope.

So the closest depends on the focuser fitted and it's capabilities.

Edited by ronin
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yes highly annoying, and does not help having to add these adapters and extenders 

dont like my camera hanging off cheap skywatcher focuser tubes and rack n pinion dinosaur tech focusers.

feels like the whole setup will just snap in half, but i really like the ST80 as a cheap Fixed long prime and i mean F5 is a blast.

and if you leave the dust cap on and remove the centre cover your killing that chroma dead in the tracks but you do loose some

brightness and resolution. ( center hole is 46mm apeture if am right ).

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I think you'll just have to live with using a variety of extenders...



i have had a look at extenders, but they all seem to be T-mount when astro work is involved.

they screw onto the t-mount adapter and act as a bridge to the focusers t-mount. 

thats not bad but swapping out would be somewhat a pain having to unscrew and screw new in place.

But trawling amazon i found some lens extenders, they are a EF-S extender set. 12mm and so on.

these are used for macro work with camera lenses, but do you think they could work as an extender for my t-mount. 

link to said items : http://www.amazon.co.uk/gp/product/B00E9M33PY/ref=s9_simh_gw_p23_d0_i5?pf_rd_m=A3P5ROKL5A1OLE&pf_rd_s=center-2&pf_rd_r=1XD9KFDZ7KBFHHSST2WJ&pf_rd_t=101&pf_rd_p=455344027&pf_rd_i=468294

I pop a ef-s lens extender onto camera body, i fit my camera and extender ( sporting ef-s bayonet fitting) to the T-mount adapter and then to the scope.

Now when i need to add/remove an extender, i just unclip the camera body from the extender and add new one to camera body. i can now either remove the extender left on the T-mount adapter

or replace camera with a secondary extender, i am also cutting down clutter having the extenders being flat disc and not thin tube

So guys and girls and the awesome brainiac Olly would this be a better solution to the adding of extenders,

instead of t-mount extentions i use the EF-S macro tube extenders with the added bonus they can be used on camera lenses own to ? 

now if this is a viable option any sugestions on a product, should i go cheapy newwer brand or fork out for something like Hoya / canon branding.

thank you for help, 


the Skywatcher star travel 80mm when mounted Prime to a canon 350D has a minimum focus distance of 30m to infinity 

give or take a few meter , i made it around 25-35m so rounded to 30m

Edited by Mr TamiyaCowboy
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I did some extensive testing yesterday. 

canon 350D @ prime using a china made T-mount adapter for EOS fitting.

this was more to find out how bad the CA is and the best way to remove it None mechanical / filter wise.

at first i shot wide open with the dew cover fitted ( lens hood) bright full sunshine 

and the object a seagull grabbing a ride on the thermals from the local rooftops. so very contrasty bright sky and a seagull not a good choice for CA

image was shot at ISO800 and the onboard saturation maxed out. shutter speed was as fast as she would go

@1/4000 second. 
The image came out washed, 4000s was just to overexposed , i kept the ISO to 800 as the cameras optimal ISO is around 900-1000 ( sensor wise)

you could see the CA evident around the gulls leading wingtips and around its head and beak, it was ugly but still OK ish for a newbie ( pros would have died)

Next i placed on the dust cap and removed the center mask cover. this drops the 80mm lens down to a 46mm lens.maybe this lens was just TO BIG and needed stopping down some

so the 46mm hole would mean something like a 4 stop drop maybe

iso we left at the same 800, exact shame shot taken but this time i was able to drop down to 1600s, bang on

no oversaturation at all infact maybe a little dull but nothing 1250s shutter would not solve and still give a respectable handheld shot.

the CA noticed and evident on the fist image was GONE and i mean its outta there NO CA what so ever. 

so stopping this big monster down to the 46mm with dust cap really did work. 

Extensions, hmmmmm headache for me. i am mostly using the lens for long mid and short focus ranges so the T-mount extenders

are somewhat a chore, but i do hope to grab a set of EOS canon macro extender tubes , they must work in the same was a T-mount extension would

just shifting the sensor away from the focal plane. The major downfall is that YOU WILL loose infinity focus , so astronomy is thrown out of the window if you try to 

extension fit the scope, you may just get away with about 50mm of extension but your going to be ever so shy of infinity.

i need to get a bigger tape measure but can say the focus minimum distance is between 15m-30m to infinity at prime.

Add a 100mm extension tube you can bring that focus minimum down to 10ft  but your maximum focus range is shorted to around 15m max ( even less )

Edited by Mr TamiyaCowboy
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Hard to say. When I've done prime focus with astro scopes on wildlife I've been using 2 inch push fit accessories, including push fit extenders. These are quick to change but if you don't have a 2 inch back then the TS extenders might help.

I know the ST focuser is primitive but rack and pinion are actually the best (non-exotic) focusers, better than Crayfords.


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  • 3 weeks later...

With very basic first go testing today I also observed that the CA went when only the centre cap was removed. I took seaguls also but on rocks with water behind.

I am struggling with focus but am hoping next try I shall remember the tablet and hopefulyl get better results.

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hello again, i return with more helpful information 

Well the ST80 had been sitting for a while, the warmer nights and the somewhat summer days

have had me busy. i am now running one of my FPV drones with a gopro like camera mounted.

well back to the ST80 and some more testing, Last night ( 7th june 2014 ) we had a lovely half moon,

Ok ok .. it was nearly half'ish moon but all the same in that ST80's frame it was a tad small. 

I have the Astro revelation T-mount adapter, but it also came as a A-focal adapter to. you could slip an eyepiece

into the body of the adapter and mount the camera T-ring to it. Now it says a 26mm eyepiece, i have a 25mm eyepiece......

No lets go mad, so i grabbed the standard skywatcher 2x barlow, You know the one that black plastic beast they give you free with telescopes.

Removing the two knurled screw in eyepiece holders, now it gets somewhat shady.......

i have a canon 350D ( note a 350D is the camera, it may work with a 400-450D but others you are somewhat on your own)

when you pop the barlow into the A-focal adapter part you will notice the barlow close to the lip of the bayonet on the T-mount

itself, now on the 350D this is fine and the mirror CLEARS the unit, 

NOW if you notice the top of the barlow protruding above/out of the T-mount when fitted to adpater you may find the mirror smashing into it when shutter is triggered,

so be warned here.
well once i had worked out i was safe, i screwed the whole unit to the T-mount screw thread on the rear of the eyepiece holder on the ST80 Focuser.

gave about five full winds out on the focuser and held the whole unit up, found the usual sideglow of the moon and started to swing scope to aquire the target.

WOW, i mean WOW, the scope is now at 800MM and has a focal of around F10, the 350D in TV mode and set with an ISO800 sweetspot, i was shooting handheld at 1/200s.

She i meaning the ST80 was wide open with the hood fitted, and that bright moon was just awesome. Now we did get some chroma comming in,

the bottom edge of the moon had the blue/purp creep setting in, but not very bad at all, in the canon software chroma reduction area a high number 20 removed all but a touch of chroma.

and now the lunar object fills up the frame, you can start to see lovely crisp clear craters and define some other areas to. 
NOW that 2x barlow is somewhat doubling your F number, but for daylight its all well i was shooting at 1/1250s in overcast sky.

The scope still acts like above mentioned, nothing close is able to focus down, BUT your getting a 2x teleconverter instead, so you can now shoot very long

or mid way with some nice frame filling and less cropping needed.

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I left mine at parents but am itching to try it again.

Did you also try it with just the centre cap removed on the moon add that would have whacked the focal length even longer and you may have still had enough light for a reasonable shutter speed?

Do you have a tripod you can mount it all on?

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I left mine at parents but am itching to try it again.

Did you also try it with just the centre cap removed on the moon add that would have whacked the focal length even longer and you may have still had enough light for a reasonable shutter speed?

Do you have a tripod you can mount it all on?

Sadly no i left the dustcap fully off. i thought it best to test this way as most will use the st80 wide open 

and not stopped down.

i do have a tripod but once you add the camera and a-focal adapter you are bringing the COG of the scope to around where the 

finderscope mounts the scope body. its a long way back from the usual mounting point on scope, so you can find your tripod creeping a bit.

I will drag out an old EQ-1 and pop it into Alt Az mode, if not i will rest the scope and take a couple shots with the dustcap on and the stopdown cover removed

see what the maximum shutter speed could be.

it is a lovely little scope , i just need more wildlife and my savings to go from pennys to pounds so i can grab a couple 50mm extension tubes.

and if the moon is out tonight will see if i can capture something decent in the way of images to show chroma and the larger filled frame

with the 2x cheapy barlow fitted

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      I was keen to do this as have read widely varying reports of ST80 performance on bright objects so the moon was a good "how bad can it be?" test. 
      With a Baader Zoom and stock SW star diagonal (90 degree) quickly thrown on to a lightweight photo tripod I was really happy with the view - very crisp, bright, not difficult to achieve focus and a pleasingly wide field with no appreciable loss of quality from 17x - 50x. Terracing & hills in Copernicus small but really sharp for example.  I can see I am going to use this low magnification combination a lot as you can be up and viewing in 2 minutes. 
      The much complained about chromatic aberration was not to my eye too disturbing and limited to a very narrow green fringe on the brightest edge of the moon's disc & not in evidence along the terminator.  Be interesting to see if this is more evident and the expected red/blue when the moon is at full  brightness not partially filtered by cloud.
      Not that I plan to use it for the moon much given the Apollo 8 experience offered by the Mak, but it's good know the ST80 is perfectly fine for a quick peek  I suspect its one of those things where if I had spent a long time looking down expensive refractors I might be disturbed more, but as I haven't it has just left me kicking myself I didn't pick one of these up years ago.   
      Looking clear tonight so now I have the dilemma of which scope to sneak out to the park... 
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