A very quick QHY8 bubble

[darditti]

I am posting this not because it is a great bubble image (I have taken better this year), but because I am so amazed by the results from the QHY8 camera and my new homebuilt 245mm f4.8 photographic Newtonian (as detailed on the DIY board).

This is just a single 2 minute broadband exposure without guiding on the night I first tested this setup.

I would say this result is comparable to at least 10 times that exposure with the unmodified EOS350D. This is using the standard QHY IR/UV block nosepiece plus an Orion Skyglow filter. I have a feeling that the QHY nosepiece filter is not that transparent in the far red, because results on the Horsehead were poor (as with the unmodified EOS). I will test it with a clear filter some time (but Bernard is out of stock of those currently).

It is a really sensitive camera, and I haven't had the trouble with window misting I had with my Artemis 285, as it generates enough heat to keep the window clear. It will, however, frost up instantly if the window is removed, or if it is moved further out than its preset position.

Capture using Nebulosity, processing using DeepSkystacker, Photoshop CS4 and GradientXterminator.

David

[Tim]

Interesting, the point about the ir window. In my opinion a modified 350D is more sensitive to Ha light than the qhy8, but thats just a gut feeling, and not based on empirical evidence. I suppose I could do a side by side with the two cameras and see.

Im glad you havent had problems with misting. It has plagued me so far with my qhy8, although using a hair dryer to warm the air and filter at the time of fitting seems to work.

Funny, there seems to be quite a bit of noise in that pic David. Is it just the processing that has emphasized it do you think?

For me, the biggest difference between the dslrs and the qhy8 etc is the bit depth of the image. The 16bit cameras capture a whole world of different colours, and in doing so enable stars to be captured with much more colour and definition, rather than the saturated white blobs you get with a dslr. Also, you can stretch a 16bit image far more than a 12bit pic.

Looks like a great job on the DIY!! Do you plan to add a coma corrector into the image path?

Cheers

TJ

[darditti]

In my opinion a modified 350D is more sensitive to Ha light than the qhy8, but thats just a gut feeling, and not based on empirical evidence.

Could be. There is the issue of how transparent to H alpha the stock QHY IR/UV block filter is, which you have to have on the camera (I have found), unless you replace it with another piece of glass at the same distance from the chip.

Funny, there seems to be quite a bit of noise in that pic David. Is it just the processing that has emphasized it do you think?

I'm sure that's because it is not an image stack. It's only a single exposure which has been stretched a long way.

For me, the biggest difference between the dslrs and the qhy8 etc is the bit depth of the image. The 16bit cameras capture a whole world of different colours, and in doing so enable stars to be captured with much more colour and definition, rather than the saturated white blobs you get with a dslr. Also, you can stretch a 16bit image far more than a 12bit pic.

Yes, all that is very noticeable. If you make a stack in DeepSkyStacker (or other program) you can save as a 32 bit file, which I presume is even better for stretching, though I haven't tested to see really how much difference it makes.

Do you plan to add a coma corrector into the image path?

Yes, but I want to keep off-axis guiding, which I have found (with other scopes) to be far better than using a guidescope. To combine a coma corrector with an OAG within the available light path of a Newtonian is a tricky problem, which, a search of the web indicates, has often defeated people. If I manage to solve it, I will post up details of how.

David

[661-pete]

Nice capture. 2 minutes in London :shock: :shock: :shock:!!!

Interested in the diffraction spikes on the bright star. I suppose that's a consequence of your rather unusually shaped spider - I'm not sure it really matters, but have you had some thoughts as to how to make the spikes more symmetrical?

Is this still without the MPCC?

[Psychobilly]

Showing great promise

It must be a marvellous feeling using a scope that you have built yourself Dave

Billy...

[darditti]

You are eagle-eyed, Pete. You could be right that the shape of the spikes is due to the (deliberately) unsymmetrical spider, though there is no coma corrector in the system yet, so coma may be having an effect, in fact, I think it is. My objective with the curved and rotationally non-symmetric spider was to minimise diffraction effects due to the spider to maximise resolution, as opposed to producing "pictureseque" diffraction spikes as some people actually try to do. (I was amused to read in AN about 3 issues back about about Greg Parker putting string in front of his C-11 aperture in order to deliberately create the Newt-type diffraction spikes which you don't get with an SCT - when it's supposed to be an advantage of the SCT design that you don't get them!!) I'll see how future pictures look anyway to see if the pattern of spikes is annoying or not. A 3 vane spider is expected to produce 6 diffraction spikes, as this one is doing, but they should be much weaker than they otherwise would be, because of the curved design.

It must be a marvellous feeling using a scope that you have built yourself Dave

Yes, Billy, it is good. I have gone through phases in thinking about ready-made and DIY telescopes. When I wrote my book I was quite pro amateur telescope making, but then I spent a period imaging the planets with a C11 and then a C14, and I found these so good, better than any telescopes I had used before, I started to feel there was no point attempting to make telescopes, as mass-produced ones have become so affordable and so good. I am changing my mind again now, to thinking that if you want something that is exactly right, you have to make it yourself (or commission someone else to make it to your design). You can get quite close with ready-made telescopes, but if you have the time and inclination, you can always do better.

David

[661-pete]

...as opposed to producing "pictureseque" diffraction spikes as some people actually try to do. (I was amused to read in AN about 3 issues back about about Greg Parker putting string in front of his C-11 aperture in order to deliberately create the Newt-type diffraction spikes which you don't get with an SCT - when it's supposed to be an advantage of the SCT design that you don't get them!!)

In my early imaging days, when I really didn't know two plus two, one or two guys back on UKAI helpfully processed my image attempts on-forum to help me out (as we still do to each other now and then - one of the good things about forums). One (who I'd perhaps best not name) added 4-vane spikes to my image using Noel's tools, to make the picture look more 'natural'. Since I have the original single-strut Orion 'spider', I get just two 'natural' spikes on my brighter stars but they're not very prominent: I must admit I don't like them much.

And there was a lot of discussion on UKAI a few years ago about putting string in front of catadioptrics to generate spikes. I think this was when scopes like the LX90 were coming into fashion and folks used to newtonians were suffering withdrawal symptoms perhaps...

But .... but it's all coming back to me now. My dimly remembered student days doing the astronomy course as part of the physics degree. Part of our coursework was astrometric stuff on prints of the Palomar Sky Survey. We were taught to use the spikes to get an accurate fix on the centre of a bright star. So the spikes have their uses.

Come to think of it - the Palomar Sky Survey was done on a 48" Schmidt - a catadioptric telescope. So it wouldn't have had a spider would it? So how come the Palomar plates had spikes (if I'm remembering this right)? Maybe I'm wrong, maybe these were plates off a different 'scope...

[661-pete]

Correction to above: I've just googled the Samuel Oschin Schmidt on which the Palomar survey was done, and it does indeed have a 4-vane spider - to support the camera at prime focus halfway up the OTA, and not in the plane of the corrector plate (look at diagrams). Seems a strange instrument to use but it brought stunning widefield results in its day and performed a real service to astronomy - albeit somewhat eclipsed by its more famous brother the 200" Hale...

[darditti]

Yes, this is the classical Schmidt design. The corrector plate is placed two focal lengths from the primary, the detector at prime focus. This design corrects for aberrations better than the design where the corrector is inside prime focus (which is like the Celestrons operate in Hyperstar mode) at the expense of being twice as long. It is part of the "secret of success" of the few true Schmidt telescopes.

David

[Starjumper]

Nice image.

John