Increasing the image scale.

[Ant]

I've been trying to increase the image scale with the scope/camera combo that I have... 

This image shows the increase I've managed, the Jupiter on the right is the native scale with the SkyMax127 (which is F11.81). The Jupiter on the left is the new image scale using the same camera and telescope.

Right image is Price Focus, left image is using EP projection. Now clearly I have issues to overcome (a lower power EP would be a good start), I need to brighten it up and lower the gain. But I am really pleased with the first indications. 

I estimate that this works out to around F52 (6.6m FL). This was using a Hutech Astro 12mm Ortho, exposure for the AVI was around 1/4s about 3 FPS.

Cheers

Ant

[Ant]

Don't look in detail at the scope image - the stuff that I had to use to get everything collected lol

[JamesF]

There's an eyepiece in there somewhere?!

James

[JamesF]

It is actually possible to work out the focal length directly.

Your large image is near as anything 200 pixels across, or 1.12mm on the sensor.  Jupiter is currently 41 arcseconds across, so that's an image scale of 41/1.12 = 36.6 arcseconds/mm.  That gives a focal length of 206265 / 36.6 = 5635mm.  That's 3.76 times the native focal length, or f/44.4.

James

[Luke]

Ah, eyepiece projection! I wonder if NickK holds the SGL F-ratio record, with his 2xPowermate double stacked with a 5xPowermate :

http://stargazerslounge.com/topic/208396-having-some-fun-4-months-and-back-first-light-at-6750mm-fl/

[JamesF]

Interestingly, your small image is almost exactly 50 pixels across which would make the "native" focal length 1408mm which at first looks odd given the stated focal length of the OTA.

It is the case that the focal ratio of a Mak or SCT changes as the focal plane moves.  I wonder if the stated 1500mm focal length of the 127 Mak is actually calculated assuming the focal plane is (say) 100mm behind the collar for the visual back or something like that.

I've just done a solar image using my 127 and 450D, so I'll measure the size of that and see what results I get for focal length there.

James 

[JamesF]

Ok, my image of the Sun from today is 2700 pixels across.  If we assume the Sun is currently 32 arcminutes or 1920 arcseconds in diameter and my 450D pixels are 5.2um that gives an image of 14mm on the sensor or 136.8 arcseconds/mm and a focal length of 1508mm which is pretty close to the stated focal length for the OTA.

However, in this case the camera sensor is probably close to 55mm behind the end of the visual back whereas with the DMK I'd guess it is only 20mm behind the end of the visual back.  I wonder if moving the mirror to shift the focal plane an additional 35mm out accounts for a one metre change in effective focal length?

(I am approximating the angular diameter of the Sun here.  I can't find a definitive source for the current size.)

James