Astrophotography – deep sky

Needle and M13

Ebay has finally yielded some new tougher bolts and replacement battery for my mount so I couldn’t resist the lure of some clear skies on Sunday night. Given that it was a school night and it didn’t get dark enough until after 11, I thought I’d try and do it Roger-style with shorter exposures on my little ZWO camera (sadly I didn’t have any double glazing facing the right way for this).

As well as borrowing Roger’s approach I also borrowed one of his recent targets as I’m quite taken by the Needle Galaxy at the minute, along with M13.

Needle is a stack of 35x 20 second exposures. I had gain at 500 for this and it was very noisy and I had to process it to within an inch of it’s life to get something out of it.

M13 is a stack of 80x 7 second exposures- the same gain, but the shorter exposure/more subs seemed to help, and in comparison it fell out of the camera and processed itself (OK- I exaggerate).

Started imaging at 11:15 and done and dusted and in bed an hour later! Hopefully will get another go at this on a darker night and with more time to play…

M13 5.5.18

With the good weather last weekend a couple of us from the group decided to spontaneously meet up for some astronomy.

I always find that phd2 guiding performance is a direct indicator of conditions and on this occasion guiding was below 0.5″ all evening showing that we had exceptionally good seeing.

I started off with 35 images of M3 which will follow, and then when it was higher in the sky  i switched over to M13. Using Rob’s 14″ dobsonian we had some amazing visuals of M51 with the spiral arms just visible. Nick turned up later and whilst Rob was setting up his imaging scope we had a go at star hopping to M3 which we couldn’t find. It just shows that myself and Nick are far to reliant on technology! Rob took over and quickly found our target which was amazing! Later in the night we also had some fantastic views of the ring nebula, different shades of grey clearly showing where the various colours would be with an image.

On this occasion i didn’t take any darks or flats thinking that M13 would be an easy target given its brightness. That was clearly a bad decision but its also a learning experience.

This was taken with the Baby Q and canon 6d with an Idas d1 filter.

NGC 6207 is also clear in the image as well as several other NGC, PGC and IC objects.

 

Foraging for Faint Fuzzies – – – 06-07/05/2018

Before imaging Jupiter in my previous post, I went hunting faint fuzzies in the Virgo / Coma / Serpens area.

Nothing really spectacular, but satisfying, nevertheless. So,we have:

M84, M86 & M87 in Virgo, I reprocessed the one with M87 to show the “jet” from the supermassive black hole at its centre.

NGC4526 in Virgo.

NGC4477 & NGC4479 in Virgo.

M58 in Virgo.

C38 / NGC4565 in Coma Berenices.

NGC4596 in Virgo.

NGC5838 in Virgo.

NGC5921 in Serpens Caput.

Observing and Photography 19-22/4

So after a month of absolutely nothing doing at all, along- like buses- came several clear nights. Bit tricky to fit around work and family commitments, but at last had the chance to get out and really try out the new Dob. Despite a lot of turtle wax it’s still a little bit sticky, especially on the azimuth, but otherwise it now feels to be working really well.

On the 19th it was a bit hazy- I’ve started using M51 as a gauge of sky quality, and on this occasion the 2 cores were visible, but not the spiral arms. Nonetheless I pressed on to go Galaxy hunting around Virgo. I’m using Sky Safari to help me with this and find that for the most part I can manage to star hop by using overlay tool and I was able to explore the region around Vindemiatrix picking up 6 new galaxies for me. For the sake of comparison I got the 8 inch Dob out as well to see if I could achieve the same, but I simply couldn’t find them. My back garden certainly has a fair bit of light pollution, and it seems the extra aperture enables me to find things which I otherwise wouldn’t see.

On Thursday and Sunday I also had some time later on with slightly more mixed results. I spent some time on Leo- the M66, M65, NGC3628 triplet was easily found and a nice sight, but I struggled to get to M96/95/105 and really want to have another go at that. More satisfyingly, the Beehive looked wonderful, M92 was great and M13 was stunning- the heart of it was like a shimmering circle of sequins. Gorgeous. The best was yet to come- late on, Jupiter appeared over the rooftops. Unfortunately for me I’m looking through the light pollution and rising heat of Burton in that direction, but even with it dancing in the haze it was a wonderful sight, the bands strikingly clear and colourful. At times I could see the Great Red Spot.

Whilst doing this I had the 5 inch newt set up and pointing at Markarian’s Chain. I hadn’t managed to hop to this with the Dob, and trying to frame it was a challenge (I lost quite a bit of time trying), but I’m quite pleased with the result. It’s 15 4 minute subs, 5 darks and some flats & bias. I can find 15 galaxies in this shot which blows my mind…

The clear skies have been a long time coming but it’s been worth the wait!

 

 

DSO hunting in Bootes – Canes Venatici – Coma Berenices 18-19/04/2018

A fine night at last! Haven’t been outside for quite a while with the trusty PD camera, so here are a few DSOs I imaged last Wednesday night. Apart from the obvious ones, NGC5466 is a small obscure globular cluster in Bootes not far from the better known M53. We have also got M100 in Coma Berenices, NGC4298 & NGC4302 in Coma Berenices, NGC 4618 & NGC4625 in Canes Venatici, NGC 4214 in Canes Venatici, NGC 5371 in Canes Venatici & C45 / NGC 5248 in Bootes,

Solar Image processing. Myths and Legends.

I have just read an (unattributed) article in April’s Sky at Night magazine on solar imaging, and I have to say, from my own experience I disagree with a lot of it!

It says

“— requires a monochrome high frame rate camera set-up” and “use of a colour camera is inefficient”

Who cares? There is plenty of light from the Sun, efficiency isn’t a problem!

It also says that Ha features change quite quickly. True. As do the atmospheric “cells” that cause image wobble. It suggests you take a 1000-1500 frame avi. The time this takes immediately cancels any advantage there might be from a high frame rate. When you stack all these, you get a blurred image. The only reason you would take so many frames is to reduce noise. Again there is plenty of light from the Sun, so this isn’t an issue.

It suggests that you might need a flat frame (possible) and that you take a defocussed 500-1000 frame avi to achieve this. Why? It is much easier and more accurate if you need a flat frame to simply blur an image you have already acquired.

My images use a £50 colour camera with a not particularly high frame rate. I find a good compromise is 200 frames. This takes around 7 seconds.

Click on “Solar” on the blog and judge for yourselves!

While still in Victor Meldrew  mode, in the same magazine there is a review of a new Skywatcher 20” goto dob for £5499. I am sure that this is a splendid scope, but following my earlier post it is worth remembering that it is only 1 stop faster than Rob’s new 14”! I am pretty sure Rob didn’t spend that amount on it! In fact, in the review there are pictures of M42 and the Trapezium. There is also a picture of M51 of recent discussion. They look nice, but I would invite you to compare the pics with these window-sill images with a scope costing £100 ish.

Moral – Just because something is in print does not necessarily mean it is correct. This is a hobby, it is whatever floats your boat. You can spend a fortune if that is what you want to do, but you don’t HAVE to!

Some Musings on Aperture and DSOs

Rob’s recent post on his new scope led me to reflect (!) on my own experiences. My main interest is deep-sky objects, or “faint fuzzies”. I have a rule of thumb that says that to see any appreciable difference in object  brightness, you have to go up 2 stops in focal ratio (f-numbers). This translates to a doubling in aperture (doubling the aperture increases the light grasp by 4X). So when I upgraded from my old 4”, I went to 8”. Doing the same again would suggest a 16” would be needed. For me, a 16” would be unmanageable, so I went to a 12”. This is only 1 stop advantage to the 8”, and visual results were a bit disappointing. This more-or-less coincided with getting the PD camera, and even using this “live” as an electronic eyepiece gave views so superior to the 12” that the 12” virtually never got used.

So I wondered why this was, so here is a bit of basic physics.

Firstly, the eyeball. This iris opens to about 7mm when fully adapted to the dark. Or maybe 5mm in an old fogey like me. Given the size of the eyeball this is around f/3. See http://www.faculty.virginia.edu/rwoclass/astr1230/human-eye.html

To get the brightest image from a scope, the magnification from the scope must produce an exit   pupil (the beam width leaving the eyepiece) pupil less than 5-7 mm. Otherwise some of the light leaving the scope doesn’t enter the eye and is wasted. This defines the minimum magnification you can use for a given aperture. For example with the 8” and a 5mm exit pupil, this minimum magnification is 200/5 = X40. (The SCT has a focal length of 2000mm, so this is an eyepiece of 50mm FL). A lower magnification than this is not harmful, it is just that you are then not making use of all the available aperture. 7X50 binoculars are known as “night glasses” for exactly this reason – their exit pupil is 7mm, making the best use of all night-time light. Therefore, going up in aperture does not necessarily make the view brighter, but rather allows you to us a higher magnification for the same brightness. This might be a huge advantage for small objects like planetary nebulae, but less so for extended objects. Bigger aperture also improves resolution, allowing you to split closer binaries, but this is usually not the critical issue for faint fuzzies. The other issues affecting brightness are the eye sensitivity (more of this later) and its “integration time”, or the time period over which the eye sums the image it sees. This is its “shutter speed” and there is some literature that suggests in the dark, this is about 0.2 seconds. Again. see http://www.faculty.virginia.edu/rwoclass/astr1230/human-eye.html

Now, from here, I have NEVER conclusively seen any galactic spiral arms visually, although sometimes I have persuaded myself I can. So when I first coupled up the PD camera to the 8”, and turned it on M51, this is what I saw, live, with no processing at all:

Bingo! Spiral arms!

So if we are now talking about imaging, rather than the eyeball, what comes into play for image brightness?

3 things.

  • Focal ratio (not aperture)
  • Integration time. The PD single image is 1/50 second. Its “senseup” parameter allows it to internally stack up to 1024 single images, giving an integration time of about 20 seconds, or about 100 times the eyeball.
  • If the CCD had the same sensitivity as the eye, the brightness of the CCD image would be the same as the eye if a senseup of 4 were used. An experiment is called for!. I attached a lens to the camera, set it to around f/3 (similar to the eye), then in a darkened room compared the image from it to that I could see with my eye for various senseups. Although this is a very crude experiment, I reckoned that about senseup=2 was about right-not miles away from the predicted value. So a senseup of 1024 suggests a sensitivity about 8 stops faster than the eyeball.

There is also another factor involved, and that is contrast. This is the brightness of the faint fuzzy compared to the sky “background”, and I have found that with our local skies, that is the controlling factor. You can improve this visually using filters (UHC or OIII for example). These tend to dim the whole view, but the right one can improve contrast. Light pollution filters used to be good in the days of low-pressure sodium street lighting but are not much use with LED lights. On the other hand, the camera has a “gamma” setting that allows you to “stretch” the contrast, live, or if you post-process, the sky is the limit, as they say! For example, stacked 11 of the basic frames of M51 (11264 frames in total), processed it with GIMP, and here is the result. It is flipped vertically to get the orientation right (See http://www.thornett.net/Rosliston/Astrophotography/DSO.pdf for the details). Stacking 11 frames with a senseup of 1024 gives another 3 ½ stops faster than the eyeball or 11 or so altogether.

Interesting as all this might be, let’s remember that this is a hobby – and you do whatever you enjoy!

As a final thought, Lord Rosse used a 72″ aperture reflector to first identify the spiral nature of M51!