Captain Scarlet

I’ve discovered my astronomic oiii fits and sits quite neatly inside the detachable eye cup of my 10x50 bins. There may even be some gaps in clouds tonight so Cygnus and the Veil beckons!

Very nice read. You've reminded me to use my Oiii filter. I've had one for ages but never used it, and somehow I never remember it when I go for my quick stroll outside before bed with just binoculars. I must tie a knot in my fingers or something. M

Superb to see such delight, I also revel in naked-eye views. re binoculars. If you can afford to spend a bit more, I would very strongly recommend Nikon Prostaff 3s 8x42. I recently researched this area to try to find a very good pair for as little as possible to give to someone who’s done me great favour recently, and these came up trumps. The Prostaff 3s give lovely wide field of view great sharpness and very light-touch focuser. The person I gave them to is delighted with them. M

Also a small piece of camping mat, perhaps 60cm x 30cm, to stand on whilst observing. It's amazing how quickly your feet lose heat through the bottom of your boots, and how much your feet hurt when very cold! I went to watch the World X-Country Ski Championships in Finland in 2004-ish and my cousin suggested that trick. To test it out I tried for a few hours NOT standing on it, and he was right! Cheers, Magnus

There's something like this just up on ABS... https://www.astrobuysell.com/uk/propview.php?view=165022

The position of the camera in this case depends on the placement of the secondary mirror. Because the secondary is simply a flat mirror, it's just bouncing the primary's light-cone out sideways. As my crude picture here shows, different positions of the secondary result in different positions of the camera and focuser, but the main focal length remains unchanged. Cheers, Magnus

Bang on my number, that's a relief!

... however I want to now photograph an area of sky at 3 different amounts of back-focus just to check the numbers accord with reality.

so I can produce a table like this:

Those are the numbers that come out from the equations and measurements, and they combine to produce the following table, which accords with reality as far as how many turns of the knob gets me to focus:

The whole point of this exercise for me was to try to establish as accurately as possible the true F1 and F2 of my two mirrors given the two equations EFL = -F1.F2/(F1 - F2 - x) B = -F2.(F1 - x)/(F1 - F2 - x) I can rearrange them to be F1 = x/(1 - B/EFL) and F2 = B.x/(EFL - B -x) I can physically measure x and B by taking the scope apart and applying a micrometer, which I have done (my other thread). The last bit was to accurately and independently measure the EFL for a given configuration, which was the whole point of the annotated star photo. I arrived at values for F1 and F2 with error estimates for each of 2.5mm and 1mm respectively (F1 = 463.28mm and F2 = 115.71mm as it happens). I now want to do it all again as I'm only 99% certain of the adapters and hence BF I was using, and also do it for 2 or 3 different amounts of back-focus

Sorry. attached the diagram I drew to try to understand it:

I've basically used two equations: one the standard 2-mirror catadioptric equation, and the other is straightforwardly derived by drawing out the geometry of what's going on. EFL = -F1.F2/(F1 - F2 - x) is the 2-mirror equation (for 2 convex mirrors convex secondary and concave primary in mak config) where x is the mirror separation B = -F2.(F1 - x)/(F1 - F2 - x) where B is the distance from secondary mirror to backfocus The first immediately shows the inverse relationship between x and focal length

I'd be surprised if there weren't apps that could do that, I've just not used any before. But it's such a simple exercise and calculation if you have something like photoshop and access to very precise star-coordinates that I did it myself.

OK seeing as nobody else is going to take the bait, my own calc is that Separation on the sensor is 4081 pixels, between the two stars that lenscap chose, equating to 26.72mm. The angular separation between them now (as opposed to J2000) is 1976.90 arcseconds taking account of their proper motions between 2000 and now Leading to a focal length of 2788mm (+- 24mm) for the scope used to take the photo. I've tried all the combinations of stars and all yield the same result. Interesting as the scope was my Skymax 180, nominal FL 2700mm, and the back-focus used to take the pic was somewhat less than that taken up by the standard visual back and diagonal, which should have meant a FL less than 2700mm for the photo. I'm going to re-check this result as I forgot to photograph my set-up at the time I took it (schoolboy error), by taking the pic again before I update my EFLs upwards by 200mm on my big post on EFL of the Skymax 180. M

I have a couple of Oklop bags which are very good Cheers, Magnus

The circularity is about secondary positioning: if perfectly circular (from the focal point), and if the offset has been set correctly, that circularity is pretty much the only way you can be sure the secondary is in the right place to intercept the primary's light cone optimally, before you move on to the axial alignment. As you say, axial alignment is different: the axes can still be (in extreme case) aligned even close to the edge of the secondary, but you'd then only be intercepting less than half the primary's light-cone. And as you say if you can see the primary's edges concentrically then you're there anyway: the initial circularity helps get you there much more quickly and allows you to "get there" without further having to touch the central screw, only the tilt adjusters.

If the secondary’s edge, from your eye’s place at the focal point, sits on the edge of a cone (centered on the focal axis) emanating from your eye, then and only then should it appear perfectly circular. Assuming it is a “root-two” ellipse and the “bounce” is 45degs. Subsequently, if the edge of the primary is concentric to that, you’ve “coincided” the primary cone with your “eye-emanating” cone. That circularity and concentricity are beautifully convenient optical tricks making Newtonian collimation so much easier.

I wonder if it's a coincidence that "light time" from Jupiter is around 45 minutes

I think you have the angular separation a bit wrong ... the SIMBAD database shows it as 1976.64 arcsecs (J2000), using which would bring your calc to 2802mm

Hi You'll find below an annotated image I took with one of my scopes recently. The image is a full-frame off a Canon EOS 6D (mk1). I need to know as exactly as possible the focal length of the scope, and this image can provide that. Although a .jpeg, I believe it has all its pixels. Can someone here kindly work out, independently of me, what FL the image was taken with please? I have already made the calculation myself but I'd like it confirmed. Cheers, Magnus

Welcome to SGL. I too am currently based in SW Ireland, not far from Baltimore. The skies are truly spectacular when the weather permits. I’ve just temporarily adopted a 130p from my bro in law while his house gets some work done and it’s very good!

Very nice to read. A scaled-up “pro” version of a session I had 3 weeks or so ago, lovely, totally reminded me of that night. M

Cloudy for me here tonight but if you could put an indemnity in the post guaranteeing similar conditions for me tomorrow here in Ireland I'd be grateful please M

I had a not dissimilar experience with them when I ordered a secondary mirror. It took 13 weeks...