Jessun

Thank you Philip R. Kind of you.

I'm not back in action per se. I have yet to take my first image with the 16200... and I've had it for some years now...

The walk of shame in GOT springs to mind....

/Jessun

Things go funny when we include the mystical spot target or the (almost as confusing) baffling choice of sensors and pixel sizes.

In essence F ratio is simply dealing with the theory of optics. It has only a number, and no frills to it. F5 is F5. Not F5 Special because of this or that.

Draw a simple telescope on a paper, one lens, one tube, one focal plane with an image circle. No measurements needed. Imagine it's a small telescope then the image circle is small. And vice versa for a big one. A simple division is all the maths needed. Point what ever sized telescope you envision to the mother of all flat panels and then put a photon counter, the size of a plank length (or half lol) and for any sized version of your design the photons will come in at the same rate at the counter wherever you count, lets say smack in the middle or 10% off centre or anywhere, proportionally.

There is no real mystery to this. Confusion kicks in when there is a large fix sized pixel at the end.

Aperture determines the max theoretical resolution and surely doesn't dictate the faintest object you can detect. A photon is a photon. It won't aim squarely at only large telescopes if they come from afar...

A lower f-ratio is per definition always faster than a higher one.  There is no way around this. One only has to do away with the idea of finite sampling points in the image circle. Imagine an infinite number instead to better understand F ratio.

/Jessun

It looked like this for a while too, when I moved to a different flat with no need for the overhang bit, so I chopped it off.

Yes I'm here but haven't followed up on my threads.

/Jessun

Wow, what a blast from the past!

I'm happy that it cleared up some issues. The basics of optics is never determined by one parameter alone, they are all linked.

Think about this: The Hubble Space Telescope is as far as I can gather f24.

The focal length of the HST is something like 60 meters vs a 2.5m mirror, give or take. You can build the same f-ratio telescope at home, scaled down to a few lengths of toilet rolls and if you achieve same f-ratio, (A two inch toilet roll would have to be stacked to a length 24 times the aperture  meaning a 48 inch telescope for that tiny aperture). 

If you built this and put it in orbit, or indeed kept it in the back yard, it would fill the wells of a given CCD or CMOS at the same rate as the Hubble. OK, perhaps only for the very central pixels as the light cone fades rapidly towards the edges, but the f-ratio does not take this into the equation...

When that fundamental penny drops, there is no longer a mystery concerning f-ratio ever again.

/Jessun

Is it not simply easier to align something with three points of contact? Much like a three legged stool self aligns without wiggling? 

The roundness in general puzzled me somewhat though. Sure things screw on and off with ease but just as easily do they unscrew for various reasons... 

I'd be all for a non rotating assembly system that clicks in place. 

I'll buy your first focuser Olly 🙂

/Jessun

Tricky one, since guiding traditionally relied on a human turning little wheels to keep a star in the cross hairs. These days with various software calculating centroids to a fraction of a pixel it's perhaps a different story.

Any too precise theoretical calculations might perhaps tempt you to chase the seeing rather than stay put and guide on an average position - which is something that software can do for you over a vast range of scope 'ratios'.

Don't know what you're talking about ;-)

My ol' friend:

The beauty that was turned into an R2-D2 that had been through a traffic accident...

Basically moved flats and didn't need the overhang anymore...

/Jesper

Yes, in a way. It paved way for the triple rig and taught me most of the basics that I still use.

/Jesper

This is what I ran for some years, with the mount and OTA hanging a foot over the edge of a balcony rail.

Learned a lot from this experiment.

/Jesper

In my opinion, for it is nothing more, a flip simply flips and mirrors (turns it upside down if you like) so any camera misalignment is doubled. If you're leaning 5° to one side in the draw tube the resulting stack will have a 10° discrepancy between pre and post flip subs, so the black triangles show up along the edges.

What causes the misalignment in the first place could well be all sources of cone error tricking you to think that the camera is aligned with RA and Dec on that side of the meridian.

PS - I take this all back. It's not the case... Camera retains orientation as good as the mount can do it after the flip. Perhaps it lies with internal bearings or axis alignment allowing a little tilt or something else not squared up internally.

/Jesper

I simply turn the camera as best as I can to align it. Then I pick a bright star and expose whilst slewing. The trail will tell me how to adjust.

I always have North up. No reason for it whatsoever, just my way of doing it - it just matches my mental picture of the targets I suppose. This means that my M31 is often said to be up side down... You can't win!

/Jesper

On ‎29‎/‎06‎/‎2016 at 17:13, wimvb said:

But since it's a movable (kind of) rig, how do you polar align such a beast?;-)

Hi Wim, quite easily. The rig (~220kg) rolls out from inside, then I use three electric actuators to lift the rig off the wheels and level it. The actuators move at about 8mm/second so it's simple to align using a bubble level. Since the AP mounts were originally meant to be set up each night the rest is plain sailing adjusting the alt/az to nail Polaris in the polar scope using an app to pinpoint the position as seen through the reticle. Personally I welcome a bit of random misalignment from night to night as it's just as powerful as dithering. (Something that I can't currently do with three OTAs).

/Jesper

It's the Astro-Physics 1600GTO, Varad.

/Jesper

A peaceful ADM 60mm guide scope does that job here, tucked in out of view...

Works a treat, just like most guide scopes :-)

/Jesper

PS I have a 31*24" aluminium print that I think you'd be keen to see. I'll start a thread once I have things nailed down.

PPS the rig is effectively an f2.68 rig had it been a solo OTA :-)

/Jesper

To an acceptable degree they were initially just bolted down with an option to pan the outer ones via a central pivot bolt. I had no tilt function available and lost som 3-5% of CCD real estate.

I just recently put the outers on FLO 'heavy duty' adjustable saddle, much like the Cassiday ones.

No test under stars yet so I have no info if they can carry the load of the APMs.

/Jesper

Three OK too?

/Jesper

It's a real shame that this happened. Bendy bolts adressed but this introduced. I suppose it still works using a bolt (M6?) in the remaining holes?

I hope you get this sorted, seems a fine mount in all other respects.

/Jesper

That plastic head looks pretty roughed up too... Is it really hard to move this clutch?

/Jesper

We don't have the budget of professional astronomers .

Very few segmented mirrors come up on AstroBuy, hardly any guide star lasers and just very vew active optics systems. Interferometric imaging kits are even more rare .

We're stuck in the amateur section of things, and resolution down to what seeing allows is for many of us easily reached with a DSLR and an APO small enough to carry in one hand. And I know I'd prefer a fast one of those APOs.

It's all linked of course, change one parameter and another one will change too. For a given focal lenght the only way to decrease f-ratio is to up the aperture, and while it's true that aperture determines the faintest object you can resolve, chances are that the pixels of your average CCD will set the hard limit for that anyhow, so it would make sense to talk about how to fill those pixels up as fast as possible. (I don't talk about planetary imaging here, where tiny pixels and a whole different data capture process is used).

I think this is the reason aperture alone isn't really mentioned all too often in discussions - despite the inevitable link to all other parameters. A little Takahashi at f3.9-something is a bigger buzz these days.

/Jesper

Well it is, but it's hidden away in the f-ratio discussion for imagers.

You can determine a sampling scale, which is arc seconds per pixel on your CCD. Call it resolution if you like. This is determined by the pixel size of your CCD and focal length only.

Once that is done it's down to how fast you want to fill the CCD with signal. By making the aperture bigger - keeping the focal length fixed - the f-ratio decreases to a smaller number, and by that the exposure times decrease drastically, meaning that you collect more photons per time unit, due to the bigger aperture. The resolution remains the same.

Time is of the essence, you want loads of data, and quickly. So the aperture is there, as a factor just as vital as any, but it's the f-ratio that gives an imager a better clue of the performance of a telescope, so discussions tend to stay there.

Sadly, large lenses are difficult to make and can budge under their own weight, and large mirrors are also very expensive. Weight plays a big role for what we can possibly carry and set up, and the mount has it's limits too. So again the big telescopes are out of reach for most and we settle for smaller. Then it's a race to make the small telescope as 'fast' as possible - at a premium price too....

Since we expose for hours on end we can still pick up faint stuff that the naked eye looking through the same telescope couldn't.

/Jesper

Great write up Ian, many thanks for that!

Unhelpfully in my example, the OSC index is greater than 0.5, which suggests the mount is correcting more in RA in one direction than the other over time. I'm man enough to admit I don’t know what that means!

Could that not be simply refraction towards the lower east or west?

/Jesper

I think it's actually steel, rather than aluminium. (A fridge magnet test suggests that to me anyway). That is good news though as steel expands about half as much as aluminium.

The dovetails are aluminium though so that will put some stress on the system unless one end is allowed to slide as in more expensive systems.

I put two Losmandy bars on my steel 8"RC, with the idea that the differences in expansion would at least be symmetrical, and not cause some sort of collimation drift.

/Jesper