Lodestar X2 'focal length'

[kerrylewis]

When using the Lodestar X2 with Lodestar Live does anyone know what its equivalent focal length is? In other words what focal length eyepiece would give a similar magnification in any particular scope?

[JamesF]

To be honest it's not really a meaningful question.  There is no magnification when you're using a camera.  Magnification is, if I recall correctly, the ratio of the exit pupil size to the objective size.  As there's no exit pupil with a camera there's no magnification to measure.

The best you can probably do is to give the resolution in, say, arcseconds per pixel.  Or perhaps, if you know the distance of the target, some linear measure per pixel (eg. on a lunar image you might say that one pixel is, say, 100m across).

James

[kerrylewis]

James

I'm probably not phrasing the question correctly. In Lodestar Live the image is obviously magnified and this will vary with the focal length of the scope being used. With magnification being the focal length of the scope divided by the focal length of the eyepiece, I was thinking that there must be an equivalent focal length eyepiece which would give an equivalent view optically through the scope. Does that make any sense?

Kerry

[JamesF]

It is a bit of an odd thing to get your head around, certainly.

Let's say for the sake of argument that you had LL running and displaying the image on two different monitors at the same time.  Only one monitor has a pixel size of 0.2mm and the other had a pixel size of 0.4mm.  The linear dimensions of the image are therefore twice as large on the second monitor as on the first.  If we said there was some sort of magnification, would you say the magnification in the two images was the same, or different?  If it's the same, how come the images are different sizes?  If it's different, how can that be when the optical train is the same in both cases and there's no difference in resolution of the images?  How about if it's on two identical monitors, but one is twice as far away as the other?  Or if you used two cameras, one with twice the resolution of the other, and displayed the image from the camera with the larger pixel size on the screen with the larger pixel size so they both appeared the same size?

It's certainly not the same as when you see an image of something from a microscope; that much is clear.  If you see a picture of an amoeba or a plant cell in a book then you can measure the linear dimensions of both the original and the image and the magnification is the ratio of the two, but how can that make sense with a telescope?  Everything we see through a telescope is considerably smaller than its actual physical dimensions.

I spent some time thinking quite hard about the whole issue of "magnification" some time back and decided that perhaps people don't really think much about what it really means as they only use it in a comparative way -- "*this* magnification compared with *that* magnification", for instance.  We have an intuitive idea of what magnification means in the microscope example, but that breaks down when we talk about telescopes.  I came to the conclusion that the best suggestion I could come up with to describe what, say, "100x magnification" means is that the image appears 100x bigger when presented to the eye by the eyepiece as it would were the eye in the place of the objective.  But that makes the eye and the hardware that presents the image to the eye fundamental to what magnification is.  Without the eye and some mechanism of presenting the image to it that would mean there is no magnification to be measured, and equally that those two things must be considered when one wants to discuss the idea of magnification as it relates to astronomical images.

If we allow that as a working definition then you might say something like "When I display this image of Jupiter on my monitor it appears 300 pixels across.  Sitting with my head one metre from the monitor it subtends an angle of ten degrees.  If I look at Jupiter in the sky it appears fifty arcseconds across.  Therefore at this distance, on this monitor, I'm seeing Jupiter at an effective magnification of 720x compared with the naked eye".  Obviously it needn't be Jupiter -- I just chose that because I happen to know its size.  M31 or some nebula or other would be exactly the same.

And the corollary of that would be that if you say "what's the magnification of the image with this camera and telescope" there can be no meaningful answer, because you've left out the information about how it's being viewed.

I'm very happy to be wrong about this if someone has a better explanation.  I spent a fair while with google trying to find a description of what "magnification" really means as applied to telescopes and completely failed.  Intuitively magnification seems like a physical process of some sort.  Saying that it's "the ratio of focal lengths of the objective and eyepiece" or "the ratio of the exit pupil to the objective size" aren't very helpful therefore, as they're not physical explanations.

James

[HiloDon]

Kerry, I think what you are asking for is "field of view". To answer your question specifically, there is an App called AstroAid available through Apple for $2.99 US. It's a handy little program that will show you the field of view of any object through any telescope and eyepiece or imager. It also has input for a modifier like a Barlow or focal reducer. If you give me the specifics of your setup I can give you the equivalent FOV for an eyepiece and the lodestar. The really nice part of the app is that it has a huge database of astronomical objects and you can see what your image will look like and choose your setup accordingly.

I'll be glad to check FOV for anyone if they post their setup.

Don

[kerrylewis]

I think you're right Don. It's available here for £1.99. Can you confirm whether it has the Lodestar X2 in its database?

Thanks

Kerry

[HiloDon]

I think you're right Don. It's available here for £1.99. Can you confirm whether it has the Lodestar X2 in its database?

Thanks

Kerry

Yes, the SX Lodestar Auto Guider is listed in the database. They list the older 429 chip, but the new 829 is the same from a FOV standpoint.

Don

[Martin Meredith]

I think you're right Don. It's available here for £1.99. Can you confirm whether it has the Lodestar X2 in its database?

Thanks

Kerry

Hi Kerry

You could also try this online FOV calculator which does have the Lodestar in its database:

http://www.12dstring.me.uk/fov.htm

Martin

[allcart]

The fov would be almost the same as you would get in your 6MM Radian Kerry. Not exactly, but very near. A 5mm Radian would be just a slightly better match. Monitor size and resolution doesn't matter. You see what you see. The only major difference is that the monitor image will be rectangular and have more in the corners. The view through the ep will be circular, but the object of your affection will appear to be the same size.

[kerrylewis]

Thanks Don, Martin and Allan - this gives me what I was after

Kerry

[nytecam]

The fov would be almost the same as you would get in your 6MM Radian Kerry. Not exactly, but very near. A 5mm Radian would be just a slightly better match. Monitor size and resolution doesn't matter. You see what you see. The only major difference is that the monitor image will be rectangular and have more in the corners. The view through the ep will be circular, but the object of your affection will appear to be the same size.

As James says the question is somewhat meaningless ... unless the viewing distance to the screen is quoted eg 15" laptop @ 15" or 40" flat TV @ 10ft etc plus the size of the image on said screen. This one pops up time and again but then I don't eyeball ;-0

[kerrylewis]

I've just noticed that there is actually a FOV indicator for the Lodestar on FLO's website. If you go to the camera's details, there it is. There is a whole range of targets and scopes  -  just what I needed