ollypenrice

The professionals are at Calar Alto in Andalusia.  https://en.wikipedia.org/wiki/Calar_Alto_Observatory  You can easily see Africa from there. Anywhere reasonably close to that should see you in a good place but local conditions vary on a small scale. The Alpujarras range, south of Granada, is lovely but I remember from cycle touring in the area that there is a very sudden transition from desert to green. The dry end is the North/East end, towards Alméria.  In no distance at all it greens up, literally in less than 1 km from memory. This may be a rain shadow effect. The fact that there is a film set for shooting cheapo westerns there tells you what you want to know!  For the seeing I guess altitude matters. It can be windy there, too.

You're in the high Alps. I'm in the French pre-alpine mountains west of the really big stuff and our winters are very good.

Olly

There is a demon tweak available in Photoshop for boosting Ha signal and it may also work in GIMP, I don't know. Simply go to Image-Adjustments-Selective Colour and it will open in the Reds. Just move the top slider to the left to lower the cyans in red. Out pops the Ha.

Olly

It is a great field and that one-armed spiral is one of my favourite galaxies.

Olly

Wow, that's really well resolved and the colour is convincing as well. An exceptional image.

Olly

Altair are wrong. They say, 'All major mount manufacturers recommend setting up an Equatorial or Alt Azimuth mount as level as possible,' but they don't. Takahashi mounts have no facility for leveling on their expensive GEM mounts. The only advantage presented by a level mount comes when drift aligning when a level mount has no interaction between axes but a) will you be drift aligning? and b) the effect is trivial.

It is folly to introduce flex into a mount by making an adjustable top for it. It serves no purpose.

The usual problem is that you need to get a tool under the mount to attach it to a pier but that is bad design on the part of the mount designers. Modern mount designs are now trying to avoid this.

Olly

The size of the chip has nothing to do with resolution. The object just needs to fit on the chip. Daytime photography talk of 'crop factor' is 100% meaningless in AP.

What resolution will each camera give you in arcseconds per pixel? That is the number which matters. If it is around 1.2 or 1.3 "PP it will be good on galaxies.

Olly

This is an interesting (and rather worrying) idea. Why worrying? Because when you make 40 panel mosaics you'd need an almighty computer to work at full size. 

But is binning used purely for the purpose of noise reduction? The idea is to get more signal per pixel and bigger 'effective pixels' do that.

Since it's wet outside I think I'd better get experimenting...

Olly

Nice! Must have a look at this one.

Olly

Ooh yes! But this really does need all the clicks to bring it up to full size where it really shines. In case people won't do that, I might be inclined to post a crop to show off your excellent resolution of detail and the nice Ha jet.

Olly

I do like an awful lot about this one but have a couple of reservations. Firstly I think that that the Ha has given the whole field a faint pinkish blush. I think Ha is best added to galaxies in a way which does nothing more than open up the HII regions. Here, I sense its presence even in the two bright stars, one to the right of, and one below, the galaxy. Secondly, I think the galaxy is very blue. This was the subject of a long discussion on SGL a few years ago and I was convinced by Vlaiv's argument that spiral arms were getting ever more blue!

I only say this because the image is top drawer and I'm being very picky as a consequence.

Olly

It's hard to see what you might find to reproach in this image.

Olly

A long time ago I bought a damaged TV Genesis, taking a risk. After checking the time zones, I rang them for advice. Within 30 seconds I was talking to a friendly optical engineer who told me what to do to fix the scope. The instructions were clear and the result was perfect.

Don't expect to hear a bad word about TeleVue from me....

Olly

22 hours ago, magnahrl said:

Sounds like a good idea with a 10" f/8 "shorttube" SCT on a really good alt-az, like Rowan AZ100 for observing DS. Instead of a traditional dob. Especially if you have to walk down and up a couple of stairs.

You did not consider a standard C11 + 6,3 focal reducer? I have a C8 and I often use it with the FR, like it   -two scopes in one👍

Cheers,

Magnus A.

For visual you can open up the FOV as far as the baffle tube will allow without a focal reducer: you just need a longer FL eyepiece, I think?

Olly

Strewth, you weren't messing about when you built that. Very impressive.

Olly

14 hours ago, tomato said:

The most stressed astronomers I have ever encountered were some visual aficionados who had their carefully cultivated dark adaptation ruined by a stray light at a star party. I’ve never got that incensed no matter how many times my camera fails to connect.😉

Hell hath no fury like an observer illuminated...

lly

9 hours ago, Giles_B said:

Thanks Olly.

So, my main object since about February has been to take images of galaxies. I've been doing this at 0.8x because my understanding was that f7.8 would make my targets too dim, even though the FOV would be better for objects of the size of M101 etc.

However from what you say, I've been misled and as a rule should always be guided by the FOV and not get hung up on the focal ratio - ultimately I'll get the same photons and a better image at the higher f ratios.

Scope is getting a clean at Optrep at the mo (filthy objective , long story) - but it sounds like I have a new project when it comes back!

 

Yes, this is my view. A given size of objective collects a given amount of light and a given amount of information in a certain time. To make a large image of a galaxy requires more information than is needed for a small image of a galaxy, so it needs more time. There are various ways of controlling the image size, a focal reducer just being one of them.

You'll soon find that, when imaging close to the limit of the seeing, you'll need a lot more time to make your image presentable at full size than at 66% of full size. For a given objective size, the thing to remember is that there is no short cut to get round this, even if focal reducers seem to suggest that there is.

Olly

50 minutes ago, Giles_B said:

That's a helpful exchange for me. Both points of view seem to make sense, even if you disagree. My primary objective is to increase the FOV, but it's interesting to think about increasing the SNR - I'll be interested to see if I get less noisy images as a result, although I'm unlikely to spend enough time looking at the same object with different set ups to prove this one way or another.

If your primary objective is to increase FOV then you would be making proper use of a focal reducer.

If you want a better SNR at a given focal length then you can either increase the aperture (by changing the scope, since Aperture Increasers do not exist ) or you can put the object photons you have onto fewer 'Effective Pixels' by binning or resampling.  If you are over sampled, doing the latter comes at zero cost in terms of resolution.

57 minutes ago, Elp said:

From my experience imaging Thor's Helmet at three different focal lengths I get the impression a shorter focal length (hence more signal concentrated across fewer pixels/area) gives the perception of better (brighter) signal given exposure times are equal. My Z61 (370mm) data was definitely brighter than the Starfield 102 (714mm) or C6 at 1045mm in the strong signal parts of the nebula.

But try taking your 3 focal length images at the linear stage and resample the two longer focal lengths till Thor's Helmet is the same size as it is in the shortest, then compare them as you stretch them.  This is a meaningful comparison and cuts to the heart of the matter.

The reason that talk of F ratio doesn't dissolve into nonsense in the daytime photography world is that, in that world, the pixel size and focal length are constant and only the aperture varies. 

Olly

28 minutes ago, Adam J said:

I disagree you are exchanging Resolution for both FOV and Reduced time to reach a given SNR. As a Rasa owner its exactly the same thing as your scope is doing only it is preconfigured to do that.

Resampling is worse because you exchange resolution for speed without also gaining FOV.  Can only do both with a reducer. 

Adam, 

I think it's all about maximizing the useful object photons.  If you want object photons from a wider object then the focal reducer makes perfect sense because it captures them when the narrower FOV doesn't and would require a time-consuming mosaic.

But if the object fits on the chip with and without the reducer, the reducer brings in precisely zero new photons. What matters now is what you do with them.

Olly

The only reason to use a reducer is to increase workable field of view. If it's not going to do that, it's not worth bothering.

If you would like to swap more signal for less resolution you can resample the image downwards before processing. Beware the F ratio myth when it comes to reducers.

Olly

On 09/04/2024 at 20:12, Louis D said:

The only way I would ever become an imager would be to setup a permanent remote observatory at a dark sky location (perhaps a vacation home) that I would get working well onsite and then switch to preprogrammed imaging sessions, weather allowing.

This is how I image and I don't think I'd do it on any other basis, though I admire those who do and, indeed, I cater for them. Initial observatory and rig setup does take a while but, when it's done, it's done and getting started is very quick. Using fast systems makes it even quicker. On this basis I find imaging incredibly rewarding and open ended, since your skills develop all the time.

Olly

I bought a used one years ago for galaxy imaging, a project which was hi-jacked by the arrival of a TEC 140, so I've only ever used it visually. What can I say? It's very good on small targets like galaxies. I can't fault it.

Olly

A great success. I do like your high black point and gentle processing.

Olly

I can't help on the kit side since I'm 'all observatory' but how nice to see you back!

Olly

What I would do (what I do do!) is make a basic stretch from linear till my background sky is just a little darker than I'd like it in the end. I de-star that.

I take the starless and open Curves (This is Ps but I guess GIMP is the same). I place a fixing point on the curve at the brightest point in the background sky. I place a second fixing point below that. If I now lift this curve from just above the two fixing points I'm stretching only the galaxy, which is what I want to do.

Once I've done this, I can do the opposite for the background sky. Again in Curves, I pick the brightest point in the background sky and put in a fixing point. I'll then put a series of fixing points in above that, this time, so all the galaxy stretching will be unaffected. Now I can pick a point below the brightest background fixing point and lift it, so reducing background sky noise. Don't eliminate background noise, it will look oily and false.

Olly

Let's not forget that RGB or OSC does have Ha signal but it only has it in a proportion which is about the same as seen by our eyes. For me, galaxy Ha is not compulsory.

Olly