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Hi all, here's my first attempt at M31.  I've used a Canon 600D (non astro modified) prime focus to a Celestron CPC800 (non edge HD) with a focal reducer f/6.3. I've collected ca. 40 light frames (30 s @ 1600 ISO) + 50 darks + 20 flats ( 0.25s @ 100 ISO from a tv lcd screen with a white image). Here are the raw data on dropbox. I'd be very grateful to anyone who will give me his impression and suggestions. Thanks! |
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The image is not appearing, and its not in your gallery. Carole |
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The image is not appearing, and its not in your gallery.Carole Thanks, should be ok now! |
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For a first attempt you have done very well, particularly in the light of such short exposures and with a DSLR. The core is not burnt out, but at 30sec subs this is not surprising. There is some colour there but its pretty hidden, i always find this a difficult one to bring out the colour. It's pretty noisy, but I think longer subs, 800 iso instead of 1600 iso will help to improve that. Not being able to see a larger version of the image it is difficult to tell, but most of the larger stars seem to have a small star at the 1 o'clock position (ish) and I think you might have a dodgy sub in there somewhere as this is probably some sort of "jump". I am presuming you are not guiding, and this is something to be recommended. Onwards and upwards. Carole |
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Hi Michele As Carole said this is an excellent result for a first attempt. It is very clearly the centre of M31. More importantly, the image looks to be in good focus - this is not always easy to achieve, especially if you are doing it manually. The dust lanes are well defined and look really quite nice. Carole is probably also right about there being a rogue sub in there somewhere. It would be worth going through them all individually, throwing out any duff ones and restacking. If you want more colour and less noise you are going to have to increase total exposure on the object. Longer subs and more of them is always the advice (so apologies for being a bore and bringing it up). However, this is where we are going to run into difficulties. If I understand correctly, the CPC 800 is an alt-az mount , correct? This is not ideal for imaging DSOs. Even though your scope will follow the target as the night progresses, the object will rotate as it moves across the sky. (Orion, for example, usually rises appearing to lie on his right side (our left as we look) and he sets lying on his other side. Your alt-az scope will follow him but in your subs you will see him rotating slowly.) Your stacking software will probably be able to register the individual subs, but it will have to rotate them to do so and, as a result, considerable cropping will be required. Because of this rotation, long subs are going to be impossible, because the image will rotate ever so slightly over the course of a 10-15 minute exposure. I have no idea how long an exposure you might get away with before this rotation becomes noticable (because I've never tried imaging with an alt-az). But it may be very problematic at this focal length. In terms of improving this image, I would suggest getting as many more subs as you are able to get. You have 20 mins total. Perhaps you could push this up to a couple of hours worth. You might need to gather subs at approximately the same time as the ones that you have presented, so that M31 is roughly orientated as per the original frames and less needs to be lost through cropping. Even with the reducer, imaging at this focal length will be a real challenge. Good luck, and congratulations yet again on a successful first image. |
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Well done Steve for noticing the mount as I was not familiar with this model. Yes agree with Steve that rotation will be a serious limitation with an Alt/AZ mount. Carole |
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Michele, Excellent first attempt. This is exactly how I started, with nearly the same equipment. My camera was different, but I used (and still use as the mount) a CPC800 XLT. The longest subs I could get were 30 seconds, usually much less time in order to avoid too much star trailing or rotation. One can do good work and learn quite a bit with an Alt/Az setup before getting an equatorial mount (I still don't have one). I took images this way for about 2 years before upgrading. Don’t worry too much about color, especially red, as this is very difficult to do at these exposure times. Once you upgrade to an astro-camera, the color will come. Instead, focus on processing technique, but don’t get discouraged. It’s relatively easy to process good data (think Hubble Legacy data), but I feel you learn quicker using data like this as it is much easier to see improvements in the image with different techniques. Eventually, you’ll need to get a wedge and guide system, then the sky’s the limit (no pun intended). I currently mount a StellarVue 90mm refractor on the CPC and now can image through either scope with exposure times up to 20 minutes. Great job!!! Ron |
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Hi Carole, Steve, and Ron , thank you so much for the very valuable feedback and the kind words! I know that my set up is not the best for astrophotography, but I'm trying to pull the best out of it! Yes Carole, I confess that I was a bit lazy about bad frames, I was more focused on getting as much signal to noise improvement as possible. I re-checked the light frames and you were right indeed, I found that 12-13 had visible star jumping (caused by what..that's a whole new story I guess!), and so with a "heavy heart", I removed them from the stack...I'm going to reprocess the remaining good ones. Steve, in order to focus properly I have used the live view of the camera on the polar star at 10x. When it's on focus the double star is nicely resolved, I guess one could use Castor, it would be even better! As for field rotation I was aware that it is a severe limitation to astrophotography with alt-az mounts, and so 30 seconds exposure was my conservative estimation to prevent noticeable field rotation. I actually found the exact formula to calculate the angular speed rotation (http://calgary.rasc.ca/field_rotation.htm). It depends on the azimuth and altitude of the object in the sky, on the latitude of the observer, and on the number of pixel in the sensor...it gets worse as the observer moves towards the poles and on objects high in the sky in the N ans S directions....so the rule of thumb is "stay away from north and south and point to an object low in the sky east or west". Depending on the position of the object you can image for as long as 1 or 2 minutes or even up to 4 before observing noticeable field rotation, perhaps next time I could do the actual math trying to extend my exposure. Ron, looking at your gallery it's really reassuring about the potential of the cpc800...and you just gave me a great idea....I confess I was already thinking about getting a decent equatorial mount with a small apo refractor, but with the wedge and a piggyback refractor I can get the same result at half the price, especially if I stay on the second hand market....that's genius...And I get to keep the cpc800 for visual and planetary photography! Thanks all again! |
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Hi Michele Sorry for not getting back to you sooner. Absolutely - try to get the best from what you have. Too many of us think that everything would be fine if only we had that extra bit of (usually expensive) gear. There is a cracking book for people just starting out in AP. It is written by a chap called Steve Richards and it is called 'Making Every Photon Count'. It is well worth getting hold of, if you have not already seen it. I certainly found it helpful. He will guide you through the pros and cons of an equatorial wedge vs an equatorial mount and a whole host of other topics. Personally, I would favour the mount over the wedge, but others might take a different view. I have found that sometimes, in an attempt to save money, I have ended up buying twice rathr than buying 'right' the first time. Good luck. |
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Hi Steve, thank you for the reply and for the book suggestion! |
