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PeterCPC

Deep Sky Images with an Alt/Az Mount and no wedge

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I use a CPC800 which I originally got for observing but am now really into astrophotography. I don't want the bother of using a wedge/polar aligning/guiding etc but still want to take some DSOs as well as planetary images. I have had some success with this such as my image below of the Orion Nebula and various other DSOs. What I would like to establish from other members of SGL is what other objects can be successfully captured using this method. I know that Globular or Open clusters are reasonably easy to get but I am particularly interested in Galaxies and Nebula. For example I have never been able to get a good capture of M31 and wondered if anyone else, using this type of set up, has and could offer any tips.

It would be great to see other members photos and tips on what objects are possible. I have taken other DSO such as the Dumbell and M82 with some success.

I know that it is not possible to get images like you see in magazines of feint nebula but some objects must lend themselves to getting acceptable images and it would be good to have a list of suggested targets and tips on getting them.

Peter

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That's a beautiful image if captured by an alt az mount... I don't think you need any advice :-)

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Great image and you dont need any help from me either, the problem with Andromeda is its huge size my personal opinion is that 150mm to 300mm focal lengths works best.

Alan

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But I know that some objects just can't be captured using an Alt/Az - what I want to know is what Galaxies and Nebula are worth trying e.g. I have tried to get the Rosette, which I read should be possible but I can't get it (yet). I have Michael Covingtons DSLR Astrophotography book but there's no list of DSO's worth trying with an Alt/Az.

I was hoping that some other Alt/Az users might say which ones they have been successful with.

Peter

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Peter,

You'll run out of targets very soon, better get wedge and guiding. As such you can image everything bright with short exposure.

You may also consider upgrading to Hyperstar. You'll get something like this with not much efforts:

http://www.flickr.com/photos/70135313@N02/6393115433/in/photostream/

With some more efforts you can image something fancy like this:

http://www.astrobin.com/13719/B/

Mark

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As mentioned above the problem you will have is the limiting factor of short exposures.  Now a huge number of subs can help to a degree but the fainter the target the longer the exposures need to be to capture. Andromeda will be way to large for your scope unless you want to get into the tricky business of mosaics. There are a few other galaxies that you could try. With such short exposures I would say the rule would be this, if you can see the object as a faint fuzzy through the scope visually then you may have some success photographically with lots of stacked subs. If not I doubt it is an option. You should have no problems with planetary stuff though. What camera are you using?

Great image by the way, fabulous detail in the core.

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Nice image, well done.

Assuming you keep going then there is some spending to be done..  however I would advise you get to the point where you know you've run out of headroom with your current gear before making the next step.

M31 has low surface brightness, you need very good signal to noise..  so that spells long subs, which spell a wedge.

What you can do is look up the brighter objects first, then maybe you won't buy a wedge, but you'll buy a completely different mount or scope..  because you'll know things by then that you don't at the moment.

here's a link to a list of messier galaxies including surface brightness, you'll notice M31 is at a dim 13.3, whilst there are many a full magnitude brighter which will mean grabbing those in roughly 1/6th the time.

http://www.astroleague.org/al/regional/Messier_Dim_Dimmer.pdf

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Essentially field rotation limits you to an exposure of around 20 seconds.  I haven't done AltAz photography for a while but recall getting the Eskimo nebula with the original DSI and M57 with a modified webcam.  My attempt at the Horsehead showed something but only just.

I would try any of the brighter DSOs, most of the Messier objects should be worth a go.  How about the Leo Trio?

You can, to some extent, make up for lack of signal by taking lots of images.  180 x 20 secs will have just as many image photons as a single 60 minute frame.  It will have more read noise but that's it.

Chris

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It's tough stuff because you are limited to such short subs to avoid field rotation...

Unfortunately 100 x 20s subs do not equal 10 x 200s subs in the real world.  There is an excellent explanation of why in Charles Bracken's The Deep-sky Imaging Primer (my latest hobby horse is recommending this excellent book!) but the bottom line as I understand it is that you need a very dark sky otherwise you'll struggle to drag the signal out of the background skyglow...

Ultimately you need to restrict yourself to the very bright stuff as that will give you reasonable SNR against the skyglow.

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It's tough stuff because you are limited to such short subs to avoid field rotation...

Unfortunately 100 x 20s subs do not equal 10 x 200s subs in the real world.  There is an excellent explanation of why in Charles Bracken's The Deep-sky Imaging Primer (my latest hobby horse is recommending this excellent book!) but the bottom line as I understand it is that you need a very dark sky otherwise you'll struggle to drag the signal out of the background skyglow...

Ultimately you need to restrict yourself to the very bright stuff as that will give you reasonable SNR against the skyglow.

Can you explain how this is so please because I don't see how longer exposures will change the total number of skyglow or signal photons arriving on the CCD.

For example, if skyglow photons are arriving at 10 per minute on average then 30 second exposures will have an average of 5 photons each.  120 fields like this will contain 120*5 = 600 photons.

One 60 minute exposure will contain 10*60 = 600 photons.  Where's the difference?

The same thing applies for the signal of course.  The same total acquisition time will acquire the same amount of data.

Chris

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With short frames everything comes down to readout noise.

Each frame has both slowly cumulative noise from the skyglow and camera where more time = more noise, and one off noise per frame: readout noise.

This is what the OP is limited by, so 100x20 subs include 100 lots of readout noise, where as the 20x100 subs has only 20 lots of readout noise.  Long subs are desirable so long as your telescope is pointin in the right direction the whole time!

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Thanks for all your comments and the compliments!

I will definitely follow up on the links given. I am not going to go for wedge and guiding - several reasons but there are issues of weight and what I can handle and I love my CPC so I will not get another scope, but also the time necessary to set up before being able to start captures.

Using my existing set up I can be ready to start captures in 15 mins and with our weather that's quite a consideration.

Peter

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Many thanks for posting this Peter.  I have just bought a second hand 9.25 PCP and was wandering the same things you asked about. There's some interesting information that has been contributed.  Clearly the way to go (without buying a wedge) is via a number of shorter exposure subs.  When we have some clear skies I'll be experimenting in the same direction. 

I also totally understand your reluctance to replace your scope or use a wedge.  I feel the same way and love the easy operation and convenience of a fork mounted scope.  Personally I think there is quite a lot that can be achieved using such a set up with the right technique.  I suspect the quality that will be attainable will be more than enough to satisfy any imaging ambitions I have - I'm primarily a visual observer of many years and am only willing to allocate a limited amount of my clear-sky time into imaging.

Thanks again for the post.

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Many thanks for posting this Peter.  I have just bought a second hand 9.25 PCP and was wandering the same things you asked about. There's some interesting information that has been contributed.  Clearly the way to go (without buying a wedge) is via a number of shorter exposure subs.  When we have some clear skies I'll be experimenting in the same direction. 

I also totally understand your reluctance to replace your scope or use a wedge.  I feel the same way and love the easy operation and convenience of a fork mounted scope.  Personally I think there is quite a lot that can be achieved using such a set up with the right technique.  I suspect the quality that will be attainable will be more than enough to satisfy any imaging ambitions I have - I'm primarily a visual observer of many years and am only willing to allocate a limited amount of my clear-sky time into imaging.

Thanks again for the post.

It's great to hear from someone with a like mind. I would love to see any images that you do and would be happy to compare notes. All we need is some clear skies now!!

Peter

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That is a super M42. Alas, to put it in context M42 is the only object I've yet imaged (in thousands of imaging hours) which absolutely does need very short sub exposures to avoid burning out the core. I used 10 seconds for my core region. It is by far the brightest nebula of which I'm aware and unlike any other.

However, globulars, yes and why not some small bright planetaries like the Blue Snowball? A bit of reseach might find more of these and the long FL of your scope plays well for them.

Olly

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Hello again Peter.  I had my first meaningful outing with my CPC in the observatory last night - an unexpected clear spell!  I took the opportunity to use my StarSense accessory with it, and as expected it worked like a dream.  I had already used it on an 8SE and it is great for us lazy astronomers who can find things unaided in the night sky, but perhaps need to find things more easily/quicker to take advantage of any all too brief clear skies that come our way.

When I've had the chance to work on it, I'll certainly share any results I get or lessons that I learn.  Be patient though, of late clear skies have been at a premium as they have been for most of us.  Regards, Paul

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That is a super M42. Alas, to put it in context M42 is the only object I've yet imaged (in thousands of imaging hours) which absolutely does need very short sub exposures to avoid burning out the core. I used 10 seconds for my core region. It is by far the brightest nebula of which I'm aware and unlike any other.

However, globulars, yes and why not some small bright planetaries like the Blue Snowball? A bit of reseach might find more of these and the long FL of your scope plays well for them.

Olly

I am an admirer of your work which you obviously put a lot of effort and dedication into. I know that I cannot get results like yours but am happy to dabble and see what I can achieve. For instance I took this image of M82 a little while ago. I know that it does not have the sort of detail that you could get but I am reasonably happy with it - its just fun to have a go. 

I am also getting some good results with my planetary images so there is lots to do.

Peter

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Hello again Peter.  I had my first meaningful outing with my CPC in the observatory last night - an unexpected clear spell!  I took the opportunity to use my StarSense accessory with it, and as expected it worked like a dream.  I had already used it on an 8SE and it is great for us lazy astronomers who can find things unaided in the night sky, but perhaps need to find things more easily/quicker to take advantage of any all too brief clear skies that come our way.

When I've had the chance to work on it, I'll certainly share any results I get or lessons that I learn.  Be patient though, of late clear skies have been at a premium as they have been for most of us.  Regards, Paul

Thanks Paul

I took mine out last night - it was crystal clear only to be completely overcast 30 mins later so I did not achieve anything. Interesting to hear what you say about the StarSence - do you think that it makes it much easier? I generally do AutoAlign with two stars and it only takes 5 mins or so. Does it improve the alignment? 

Regards

Peter

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Can I ask what camera you use?I have an 8se and only recently for the first time saw M42 which looked beautiful through the scope. I have a phillips 900 webcam and was wondering if this would be able to capture it at all?

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Hi Peter.

It's certainly easier to use the Starsense in that if you have a GPS scope like the CPC there is no user input apart from levelling the scope onto the index marks  and just hitting 'enter' and 'align' a couple of times.  Oh yes, you have to switch it all on first!  :grin: .  The very first time you use the Starsense you have to calibrate it on one star, but it's only a one off.  Having calibrated mine last night, that's it, you will never need to use your finder again, unless you want to just have a quick point at something without powering the scope up.

                     I agree that AutoAlign with two stars is very easy, and was my preferred method pre-Starsense.  The main advantages of Starsense for me, is that you don't have to bend down or stretch to look through the finder and you just can't make a mistake with Sarsense - it either doesn't work (you've broken it) or it does with no input on my behalf.  In semi-cloudy conditions or when rushing to get going or if I'm tired I have sometimes mid-identified a star and then wandered why my 2 star AutoAlign wont work!

                     Also, the Starsense has an uncanny knack of working in partial cloud cover and when the cloud cover is quick moving - conditions it may be challenging trying to align the scope using other methods - when the star disappears that you are trying to align on or you mis-identify a star because others are obscured.

                      Regarding the pointing accuracy, using the Starsense for visual observing it appears just as good to me.  There are ways of further enhancing the pointing which would probably be more accurate if I was bothered to do it - if for no other reason in that you don't have to keep doing it every time you set up when there may be observer input error or inconsistency.

It's a shame you're not near West Yorkshire, I would happily give you a demo and you could have a play and see for yourself.

Regards, Paul

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