going prime focal

[silv (Annette)]

I am thinking about going prime focal… (how does that sound? )
8" newtonian, 1000mm focal length, f/5 and my Sony Nex-5N.

This afternoon, I pointed my Sony Nex-5N with the 18mm lens into the focuser. At a height of about 10-15cm above the telescope tube, the image of the tree was visible for the camera and focusing with the focusing ring on the cam was possible. Mind, I had to hold the cam at a certain angle so that the secondary mirror was not filling the most of the view through the barrel.

Any closer to the focuser and the image got more and more blurry until - when the lens almost touched the focuser - apart from the mirrored focuser, there was only green color left with no chance to get that more into focus.

Then I took the lens off - as would be the case at prime focus with t-adapter and ring thingies.
My camera then only shot an even colored picture - depending on the brightness of the room: blueish green in my room and whitish when pointing outside into the garden. A pic from close up to the focuser also was bluish green, same as the photo of the room in general.


Is all that normal?

How do you focus with your camera once connected with t-ring and t-adaptor - and the lens is off?
The Sony Nex-5N has a live display and it is pretty much showing what you'll get in the photo; it's also taking ISO, Aperture and shutter speed into consideration.
Should anyone be in doubt: the Sony is used in prime focus as can be seen here:
http://sonyalphanex.blogspot.co.nz/2...on-c8-sgt.html

[silv (Annette)]

the green view with the lens on and close up to the focuser:
that must be a case of focal point alteration:
either by moving the primary mirror (eeek!) or by getting a device that somehow shifts that focal point mechanically/optically/virtually to the observed 10-15cm above the tube?
and to what cost? I mean - cost in light an weight?

but I wouldn't HAVE the lens on once I connected the naked camera. So how would one go and find out the correct focal point for a lens-less camera?

Any input - just some keywords, even, would be very helpful! Thanks

[arifmshaikh]

You can focus same way you focus by putting eyepiece. You may be able to bring the camera focus during daytime, but you will need more in focus.

I did my 200mm f/4 Newtonian by moving primary mirror, but then you will need extension tube for normal observation.

[silv (Annette)]

but then you will need extension tube for normal observation.

ah! I had come across that term "extension tube" but only now I understand what it is for. thank you

You can focus same way you focus by putting eyepiece.

hm. that means I would use the focuser wheels. okay. I understand.

i don't really want to move the primary mirror... was it difficult?

[silv (Annette)]

found this article and am reading it right now...
http://www.iceinspace.com.au/forum/s...ad.php?t=55226

[arifmshaikh]

Originally Posted by silv

ah! I had come across that term "extension tube" but only now I understand what it is for. thank you

hm. that means I would use the focuser wheels. okay. I understand.

i don't really want to move the primary mirror... was it difficult?

It is not difficult to move primary. First you test your setup on star and see if it comes to focus or not. I have skywatcher 200mm f/5 which comes with extension tube, so I believe I can get the focus with prime focus(still need to try).

I used long screws and spring bought from bunnings to achieve focus the my old f/4. The other alternative is cut the tube.

[silv (Annette)]

bugger - so many good questions there and almost no answers...

read this article - and can not - will not - must not - move the primary. Cutting the tube? Never!
But it mentions the other gear to achieve something similar:
low profile focuser and Coma corrector.
Intriguing: the focus point is closer to the secondary - contrary to my afternoon experiment.

Arif, you solved it with springs and long screws? Did you follow any internet article when doing it?


magical optics ...

[arifmshaikh]

[QUOTE=silv;876213]bugger - so many good questions there and almost no answers...

read this article - and can not - will not - must not - move the primary. Cutting the tube? Never!
But it mentions the other gear to achieve something similar:
low profile focuser and Coma corrector.
Intriguing: the focus point is closer to the secondary - contrary to my afternoon experiment.

Arif, you solved it with springs and long screws? Did you follow any internet article when doing it?


magical optics ...[/QUOTE


I read the following review for my modification http://www.cloudynights.com/item.php?item_id=1677

If need to calculate how much back focus you need and then church the low profile focused height. I did mine because I wasn't sure the low profile focuser will solve this profile.

[silv (Annette)]

cheers arif!
your input was very helpful and I feel I can get it done, somehow, now.

I did mine because I wasn't sure the low profile focuser will solve this profile.

this was an eye opener, too.

[silv (Annette)]

one last question:
if it turns out that I can not get focus with the lens off (just a feeling),
how do I then subtract the 18mm lens from the measured length of the focal path to get the focal point of the assumed lens-off situation?

[arifmshaikh]

Originally Posted by silv

one last question:
if it turns out that I can not get focus with the lens off (just a feeling),
how do I then subtract the 18mm lens from the measured length of the focal path to get the focal point of the assumed lens-off situation?

The simple method is find the difference of focus. Use any eyepiece and focus on object 100/200 meter and mark on the focus tube. Now use the same eyepiece and focus on star and mark on the focus tube. Now you have difference of let say 20mm. Now mount your camera on telescope bring same object 100/200 meter away to focus. You have to take focuser all the way down to achieve focus. If get the focus and you still have few mm to travel in, minus this few mm from the focusing difference that you get from the eyepiece, whatever difference you get that much you have to move your primary.

Even if you can't get focus, then move the primary with existing collimating screw, this is how I did to find the difference. I had the same GSO ota that was mentioned in the review.

[ZeroID (Brent)]

Silv, I had to add 25mm to my mirror supports to bring it to focus on the DSLR at prime. About the additional distance of the CCD sensor to the EP as opposed to where your eye would be.
I use an extender for a few EPs but as I am heading into Astrophotog that will be a minor issue.
I just added three equal distance 25 mm spacers I made up on the lathe, longer adjustment bolts and shifted the mirror plane up without cutting or major alterations to the OTA. The mirror got a wash at the same time.

[Shiraz (Ray)]

should be a really good scope/camera combination.

at prime focus, you will use the scope as the lens for the camera, so you will need to run the camera in manual, and set all parameters by hand. Focusing will be manual by way of the scope focuser.

Its not clear how much of this you have already done, but to check if you can get focus, focus the scope through an eyepiece at something a long way distant (you need something a few km away to keep this test simple) from inside a darkened house (through an open door/window, not through glass). Measure the distance between the top of the fixed part of the focuser and the top surface of the eyepiece adapter. Then take out the 1.25 eyepiece adapter and wind the focuser in as far as possible - measure the distance from the top of the fixed part of the focuser to the top surface of the moving part. Then hold the camera (without lens) up hard against the end of the focuser tube and adjust the focuser outwards. If you can get a good image, you may be in with a chance of getting it to work without any scope modifications. Measure how far out you have wound the focuser to get focus and if that distance is greater than the height of the camera adapter, you will be OK.

If you cannot get focus, the CCD in the camera is too far away from the front of the camera and you may need to move the main scope mirror up the tube a bit to compensate - longer bolts, spacers and springs as Brent suggests if you don't want to modify the tube, but be very careful that the longer bolts cannot possibly touch the mirror. A lower profile focuser might also give you enough extra focus, without moving the mirror, if you can get get to focus, but don't have enough focuser travel to fit in the adapter. With an f5 system, you will probably need an MPCC to reduce coma at the edges of the image - really depends how fussy you are. The MPCC will also help by adding 10mm extra back focus, but it adds extra problems of its own (getting the right spacing for your camera). To make the best decision, you really need some measure of how much extra focus your system needs. If you cant get it in focus in the above test, post the focuser distances you measured and someone will help you work it out. Would also be helpful to know the distance from the front face of the lens mounting ring (lens off) to the CCD chip (should be visible looking into the camera from the lens mounting) - just guess by eye, don't put anything inside the camera body.

If you want some idea of what you will get at prime focus, put in a 15-20mm eyepiece, fix the 18mm lens on the camera (set it at infinity in manual focus mode) and hold the camera+lens in front of the eyepiece (in the darkened room looking at something distant through the door/window as before). You should be able to focus an image using the scope focuser - the eyepiece and camera lens act as transfer optics to get the image formed by the scope onto the CCD. This method puts lots of glass components in the imaging train, so it is not the best way to go, but it will at least show you what you can expect to see when you get it going in prime focus.

[silv (Annette)]

oh, sooo angry! grrrrrrr!

What am I missing???

in order to find a focal point for the lens-less camera in the telescope I would have to be able to find any focal point anywhere, right?

So I take the lens off, outside (where it's not raining atm ) I aim at my finger at 10cm distance (or closer or further) from the chip:
all I see in the life view is whitish light with maybe a shadow of darker white caused by the finger.
I shoot the pic: same.

There is no option to focus without lens.
There is no option to set AF to "infinity" - a term I have read often, used by people with Point-And-Shoot, I think. While mine is no point-and-shoot - lenses are interchangable but it doesn't have mirrors.

What are the camera settings I need to use without the lens to get focus?
Without that, I can not measure the focal point distance from the secondary - hence can not decide for a focuser.
Can't decide for a possible primary mirror move, either.

Grrrrr!

(I don't have any t-mount acessories, yet, figuring they will be determined by the focuser I might eventually buy.)

[silv (Annette)]

Arif, Zero - thank you very much for your help. It will be useful in the next stage of my learning...

[Shiraz (Ray)]

Originally Posted by silv

oh, sooo angry! grrrrrrr!

in order to find a focal point for the lens-less camera in the telescope I would have to be able to find any focal point anywhere, right?

No Silv, you don't get an image without a lens - the lens takes the jumbled light from a scene and makes an image of that scene on the sensor chip of the camera. Without a lens all you get is the jumbled up light. At prime focus, the scope is the "lens" and it forms the image on the camera sensor - you don't need any other lens. The camera plays no part in focusing anymore, since it does not control the scope - you focus by hand with the scope focuser.

to set the camera lens to infinity when doing the demo suggested in earlier post, use "manual" focus mode, not AF, and physically move the focus ring on the lens to the infinity mark.

When using the camera with the scope, you should only need to set ISO and exposure time in "manual" exposure mode (eg ISO200 and shutter around 1/500 will probably give you something recognisable in daylight). Aperture is determined by the f5 scope, so you can ignore that - focus should be set to "manual" and also ignored.

[silv (Annette)]

shiraz, that's odd.
your post from this morning only shows up now for me. ....
hadn't seen it when I posted my previous one.
will read this now. cheers!!

[Shiraz (Ray)]

I might have been editing it when you looked maybe. anyway, let's know how you get on - I tend to be a bit longwinded

[silv (Annette)]

should be a really good scope/camera combination.

that sentence has had a motivating effect. cheers shiraz.

the distance from the top of the lens mount (lens off) to camera chip surface is ~1.8cm, maybe less .

*

the e-mount lens I have doesn't have an infinity focus marker.
the camera focus settings don't have an "infinity" setting.

But I understand that this is of no consequence for prime focus. Thank you for clearing that up!

if you can get focus, focus the scope through an eyepiece at something a long way distant (you need something a few km away to keep this test simple)

I will drive somewhere and test from inside the car.

Measure the distance between the top of the fixed part of the focuser and the top surface of the eyepiece adapter. Then take out the 1.25 eyepiece adapter and wind the focuser in as far as possible - measure the distance from the top of the fixed part of the focuser to the top surface of the moving part. Then hold the camera (without lens) up hard against the end of the focuser tube and adjust the focuser outwards. If you can get a good image, you may be in with a chance of getting it to work without any scope modifications. Measure how far out you have wound the focuser to get focus and if that distance is greater than the height of the camera adapter, you will be OK.

I understand. I can do that.

[silv (Annette)]

I tend to be a bit longwinded

I suffer from that infliction, too.

But in this case your winding was necessary and very very helpful.