Captain Scarlet

There's something like this just up on ABS...

https://www.astrobuysell.com/uk/propview.php?view=165022

6 hours ago, Pryce said:

This might be a stupid question but as far as I understand focal length is the distance between the lens/mirror and the focal point(eye piece)

Then,  my question is this; how can the listed focal length of the Explorer 150P and 150P-DS(or any of the P/P-DS siblings for that matter) be the same (750mm) when the primary mirror of the P-DS is closer to the secondary mirror AND the P-DS has a shorter(or lower) focuser? 

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

39 minutes ago, inFINNity Deck said:

According to Astrometry.net:

Size:    44.2 x 29.4 arcmin
Radius:    0.442 deg
Pixel scale:    0.484 arcsec/pixel

In fact the pixel size is (44.2 x 60)/5472 = 0.484649 "/px

The sensor has a pixel size of 6.55micron, so the focal length would then be (6.55 x 206.3) / 0.484649 = 2788.1mm

Of course I could be wrong...

Nicolàs

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.

9 minutes ago, wornish said:

Can I ask why you need the effective focal length? 

What will you use the information for?

so I can produce a table like this:

11 minutes ago, vlaiv said:

Shouldn't focal length of secondary mirror be longer?

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

4 minutes ago, vlaiv said:

It's sort of not obvious to me, so I'll need a little help.

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

56 minutes ago, vlaiv said:

I just have one question - are you sure that creating larger separation between mirrors is going to shorten focal length?

In fact - I'm even not sure if focusing closer is done by making separation between mirrors larger (it's just a hunch that I have).

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

41 minutes ago, JamesF said:

Won't astrometry.net tell you the focal length if you feed it the original image?

James

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

9 hours ago, markse68 said:

I think I understand the geometry it just seems odd to me to stress getting the outline of the secondary perfectly circular when that seems a very imprecise thing to try to attain? What's important surely is that the secondary is positioned so that its opposite edges are symmetrically spaced within the focuser tube circular outline- that is that the minor axis is central and the major axis too, getting it roughly circular, then the precise alignment happens when you tilt it around the minor axis to see the primary evenly spaced within its outline. Isn't that far more sensitive than trying to eyeball the perfect roundness of the secondary? Getting the secondary roughly circular and therefore roughly to 45deg helps to judge the centrality of the secondary though, barring offsets

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.

On 09/08/2020 at 20:51, markse68 said:

Why is it so important to get it perfectly circular? I’ve often wondered this. What if your secondary isn’t a perfect ellipse? If its “in the middle” then when you tilt it to get the whole primary in view you’re going to get a more sensitive and accurate 45degrees aren’t you? Not knocking the technique btw- it looks excellent and useful- just wondering what’s so magical about it being perfectly circular 🤷‍♂️

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

42 minutes ago, lenscap said:

Measuring the seperation of TYC 3122-2059 and -0032 on my screen and assuming a sensor width of 35.8 mm gives a seperation of 26.85mm on the sensor.

From Skysafari those stars are 757" apart in Dec & 158 seconds ( = 2370 arc sec) in RA, which gives an angular seperation of 2488 arc sec.

F = 26.85 x 3600 x 57.3 / 2488 = 2226 mm

Probably completely wrong though. 😀

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...

Will you be giving us a step by step guide (yes please) or is it so easy as to be unnecessary?
M

On 04/08/2020 at 22:36, Robindonne said:

...    Do you guys always use the electronics? And related to that, is it possible to open the clutch and just look around without first having to look at an orange screen?

yes you can open the clutches and freely move it around but it's not very satisfying or easy to fine-control, it's not designed to be used that way.

When I want to use it in purely manual mode, I use the controller, switch it on, quickly flick through all the settings without paying attention to anything, set the tracking mode (in "Setup Menu") to "no tracking", set the Slew Rate to a low number, say 4, and it's very satisfying to manually slew around that way.

If you remove and examine the secondary baffle, you will understand why the meniscus might be responsible. The baffle is certainly not a thin-walled cone. At the point where it's glued to the meniscus it is a rather thick-walled tube, 2mm thick in fact, and the silvering itself shows outside the baffle. If the full diameter of the unobscured meniscus is, say, 40mm, at least 1mm is OUTSIDE the bottom of the baffle, losing 2mm diameter, and the baffle itself consumes a further 4mm, leaving only 34mm doing any work. Add to that, that the baffle itself is likely not centered (look at my photos, mine was MILES off) and it'll be worse.

A good upgrade would be to replace the stock baffle with a thin-walled one, properly centered.

Wow can't think how I missed this! Fantastic result Stu, I look forward to 1st Light.

Re the retaining clips and side-grubs. I cleaned my 12" mirror about a year ago, and when I re-assembled I had precisely the same concerns about how much pressure/clearance between the mirror and its retaining clibs and side-grubs. Initially, my thinking was "well they wouldn't be there if they wren't necessary" so I had all 3 clips and 3 grubs slightly touching the mirror to keep it in place.

That night, I had my bro-in-law with me and naturally I needed to show off my (then) new 12" reflector, so we observed together. Quite literally his exact first reaction on looking through the eyepiece, being a total newcomer to astronomy and observing, was "Why are all the stars triangular?". They were indeed. That learned me, as they say.

So the lesson: ensure there's a tiny amount of fresh air between any retaining screws clips and grubs and the mirror, otherwise I can assure you it's highly noticeable!