brianb

you would have to find these values each night to account for focus and tempreture?

Exposure for flat frames is "fixed" unless you change something major like the light source but flats need to be redone if you change anything ... even if you could guarantee you'd mount your camera in exactly the same orientation and that focus would be exactly the same, the dust donuts would be different. A change in temperature is probably OK to ignore unless you need to tweak the focusung.

I’m told I need to aim for an ADU value of ~20,000 for my flats, is this the peak intensity (of the central portion say) or the average across the whole frame?

There shouldn't be an immense amount of variation (apart from a few dead pixels which may be very bright or very dark more or less independent of exposure). Take your flat frame, apply a median filter with a radius of about 5 pixels and pick a value.

There's no need to be accurate about exposure for flats. If you underexpose them badly they'll be noisier than they need to be, and the noise will transfer to the finished image, which is why you want to keep the exposure up. But you certainly don't want to be pushing the brightness into the zone where the response is non-linear, since that makes flat subtraction not work properly. About 1/3 to 1/2 full well is usual for CCDs with anti-blooming gates; without the anti-blooming gate, you can go much higher, response should be linear almost up to 100% full. And if you camera has a switchable blooming gate, the flats should be made with the same switch setting as the "light" (image) frames.

I had an enquiry from a guy with a C14 who could just not believe that his enormous telescope was going to be very slow.

But only for extended objects. For stars, so long as they remain points, speed is proportional to the square of the aperture: a 14" f/10 is 16 times as fast as a 3.5" f/5.

Take the ratio of the focal ratios & square it ... (7.5/5.3)^2 = 2, as near as damn it ....

Autumn migration - just about everything is in the air at the moment ... geese & swans often fly at night.

Not less than 1.5 x the diameter

That's the great benefit of rolling your own. Really you need 2 or 3 in different lengths. When the air is still but very damp, 4xD is about the smallest that's any use ... but if there's even a slight breeze such a long tube will cause issues.

This works a treat, but looks ridiculous.

Not when it's dark

Camping mat foam works, nothing else is important.

I'm seeing the proms and granuation easily but other surface detail (except sunspots obviously) are still escaping me at the moment. I'm not sure if the kit needs adjusting or my fledgling solar observing skills arn't up to scratch yet.

How long are you looking? And how transparent is your sky?

With a thin high cloud cover the surface details do tend to lose contrast. It usually takes me 5 - 10 mins to "get my eye in" properly when observing in Ha; a cloth over the head helps, as does avoiding use of high powers and using a good simple eyepiece which gives high contrast (Baader Genuine Ortho highly reccomended, the 9mm is short enough for high power with a f/10 scope, 12.5mm usually works better for me).

Imaging works better with a single stack scope especially when there is thin high cloud because I can tweak the contrast to show low contrast details.

And there's no doubt that individual PSTs differ in the performance of their etalons, though in my experience the prisms in the "black box" are even more variable.

Some suggest their PST etalons are around 0.7A which would be an outstanding performance from a 300 quid etalon,i think in reality they may not be.

The PST etalons are made to a specification - if they turn out a bit better than the design spec, they're not going to junk them, and there is no other market.

The way the thing is constructed gives a theoretical bandwidth of 0.65A. In practice the f/10 cone broadens than to ~0.75A. If made perfectly (not all will be!) The "pass unit" criterion is 1.0A.

0.7A bandpass is not an issue for viewing prominences, but it does make the tuning a bit more critical.

seeing prominences so easily on the disc itself

When they're called "filaments". There have been a lot of notable ones recently.

Excellent note! I won't confuse the issue by saying that I do things a little differently (and compute microrotations when working in altaz mode), just one thing to point out: cover the terminator first as this is the bit with the fastest changing illumination.

My latest "work in hand" I've also started by making a smaller scale image so that any small gaps can be filled in by upscaling the missing bits from that. Doesn't take long & looks a lot better than black holes in the finished product!

A deal breaker for me would be whether the Lunt could be modded in the same way as the PST can be, leading to much better performance at reasonable cost than either of the original configurations.

There are downsides to the modification - the increased aperture PST is not suitable for wide area views due to the "sweet spot".

The LS35 has not been around long enough for anyone to seriously consider hacking it. But I'd respectfully suggest that it might be better to stick to what is known, if that's really what you want to do.

x250 is more than enough with a 6" scope, x200 is more realistic. You do learn to pick detail from the small image. Blowing the image up too large just coarsens it, making it more difficult to see subtle detail.

Otherwise, if the moon is up, use that with the AF, then switch the lens to manual focus.

A distant streetlight will do, if it's more distant than 5000 times the focal length of the lens it's at "infinity" for the purposes of imaging. An ordinary streetlight 250 metres away is plenty bright enough for the AF to lock in to if using a 50mm lens.

With longer / faster lenses, bright stars are enough. I've found with a 300mm f/2.8 that a 4th mag star is enough to work the AF, provided that the centre (most sensitive) point is selected for single point AF and that the camera is accurately aimed.

Keep checking the focus. If using a zoom lens, the focus needs to be reset if you twist the zoom ring. Focus can drift with temperature too, this can be quite a serious issue when using longer & faster lenses, or when using a scope as a lens ... unless you have Zerodur optics and a carbon fibre tube!

Most camera lenses - even the very best ones - give significantly better images if they're stopped down a bit (e.g. use a f/4 lens at f/5.6), and this also makes them more tolerant of small inaccuracies in focusing.

WOW!