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white card trick to measure inward travel


Atreta
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About a month ago i sent a message to steve( Steppenwolf) about what could be done to maybe help get my newtonian to focus with a dslr.

The answer wasvery detailed  but i can't understand one thing he mentioned:

he said to place a white card in the light path from the focuser and move in and out until i see a focused imaged in the card.

Would it be reflected in the bottom of it? or didn't understand at all.

I tried this but nothing is showing.

here's what i tried:

place the card in front of the focuser and moved away from it, is it right?

Anyone tried this?

ps: thank you again Steve

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I think he means point the scope at something fairly bright then remove the camera or eyepiece/diagonal.  Hold a piece of white card up to the back end of the tube and move it around until you see a focused image on the card.   The plane of the card is where you need the camera sensor to be for imaging - so measure the distance and buy the right adapters to achieve this.  On the other hand you may well find that the focal plane (represented by the card) is so close to the scope that you can never get the camera in the right position to achieve focus on the sensor.

Mark

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Why wait for a full moon?  A distant horizon during daylight will work and a crescent moon (or very distant streetlight) at night will work.  If those objects are not quite at infinity then be aware that a slight further inward movement of the focuser will be required to reach infinity. 

Mark

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Although my new scope is advertised as having sufficient focus for DSLR, it only does with 4mm to spare, so no way to use even a thin OAG or reducer, etc. The answer seems to be a low profile focusser.

From my experience when using equipment - cameras, filter wheels etc sometimes there is not enough inward travel to gain focus. The only answer without moving the primary mirror is to fit a low profile focuser.

A low profile focuser is quite expensive, and i can't even find one that fits a 4.5" tube.

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From my experience when using equipment - cameras, filter wheels etc sometimes there is not enough inward travel to gain focus. The only answer without moving the primary mirror is to fit a low profile focuser.

What about an extension tube??

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Good stuff. :cool:

(In similar way, to gain a few mm focus, you can move the mirror up the OTA

with the well known Newtonian "stronger spring" mod. (qv) Use longer thread

"lobe knobs" plus longer / stronger springs etc.)

It can be a bit tricky to see a projected image on white card. It's easier with the

old culinary standby "grease proof paper" (poor man's ground glass screen)!

To see the image in daylight, close the curtains and poke the scope through...

Give your neighbours something to worry about...  :D

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I think he means point the scope at something fairly bright then remove the camera or eyepiece/diagonal.  Hold a piece of white card up to the back end of the tube and move it around until you see a focused image on the card.   The plane of the card is where you need the camera sensor to be for imaging - so measure the distance and buy the right adapters to achieve this.

This is exactly what I was suggesting in my exchange of PMs!

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thanks everybody who helped.

What about an extension tube??

Alex, the extension tube would work if it was need more back focus, the newt needs more inward, but maybe i'll need one to use with the eyepieces when i finish the project of modifying the focuser.

Good stuff. :cool:

(In similar way, to gain a few mm focus, you can move the mirror up the OTA

with the well known Newtonian "stronger spring" mod. (qv) Use longer thread

"lobe knobs" plus longer / stronger springs etc.)

It can be a bit tricky to see a projected image on white card. It's easier with the

old culinary standby "grease proof paper" (poor man's ground glass screen)!

To see the image in daylight, close the curtains and poke the scope through...

Give your neighbours something to worry about...  :D

I'm not confortable with this mod, i'm afraid to mess beyond my hand skills(wich is near zero :D)

This is exactly what I was suggesting in my exchange of PMs!

:) i was able to project a street lamp, but couldn't the landscape during daylight.

I'll wait for the moon to be available in my view to give it another try. i think this weekend will be fine.

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thanks everybody who helped.

Alex, the extension tube would work if it was need more back focus, the newt needs more inward, but maybe i'll need one to use with the eyepieces when i finish the project of modifying the focuser.

Thanks. I always get that muddled! :blush:

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