BrendanC

Textbook stuff!

Sorry, I didn't mean a less usable image, I meant less image - as in, the more the image is dithered, it effectively crops out more of the edges of the image. Anyway, looks like this is something I don't need to worry my little head about (yet). Thanks.

Hi all, So, dithering distance when using APT dithering... I'm not guiding (yet), but I am using APT's built-in dithering feature because it helps me prevent walking noise. However, I'm slightly confused about the dithering distance setting. From the user guide, I understand the setting is specified in pixels. However, the number of pixels covered, will equal a different number of arcseconds covered, depending on my focal length and camera. For example, my 130PDS has a focal length of 650mm, while my EOS1000D has a pixel size of 5.71 microns. According to this page (https://astronomy.tools/calculators/ccd), the number of arcseconds covered by one pixel is (5.71/650)*206.265=1.8 arcseconds per pixel. So, if I specify, say, 10 pixels for the dithering distance, that would cover 1.8x10=18 arcseconds within the image. Let's say I then use my 0.9x coma corrector, which reduces my focal length to 590mm. Now, each pixel accounts for (5.71/590)*206.265=2 arcseconds per pixel, so now 10 pixels of dithering distance covers 2x10=20 arcseconds of image. Then, if I use my 3x Barlow, with no coma corrector, my focal length becomes 1950mm, which radically changes things. Now, each pixel is (5.71/1950)*206.265=0.6 arcseconds per pixel. So, if I specify 10 pixels in dithering distance, this is now 0.6x10=6 arcseconds of image. Quite a difference! So, could someone clarify please: when I specify the pixel distance, am I right about the difference in arcseconds of image that would be covered depending on my optical train? If so, does APT auto-adjust to compensate for this in some way? Or, is the pixel distance constant, because what really matters is in fact the actual number of pixels of dithering, regardless of the arcseconds covered? Or am I not making sense here? Should I just carry on and stop worrying? Oh, and while I'm at it, what would people recommend for the optimal distance? I've seen references to at least 12 pixels (see https://stargazerslounge.com/topic/253335-how-much-dithering/ so I'm currently at 13 pixels to be on the safe side, but I'd like at least to understand why I'm doing this, and whether I could go lower, because higher dithering distances will mean less usable image after stacking. Thanks, Brendan

Thanks for the kind words everyone. This was really my first time with multiple exposures across several nights, at 60 secs, including auto meridian flips, full calibration frames, and piecing together a mosaic. So, lots of new things came together pretty well in the end.

Actually, two - one of the galaxy itself, and another mosaic to show more of the surrounding space which includes M32 and M110. Details are: Sky Bortle 4 Scope Sky-Watcher 130PDS Mount Sky-Watcher NEQ6 Camera Canon EOS1000D (modded) with Sky-Watcher 0.9x coma corrector Capture and control software APT No guiding APT dithering every third image Approx 9.5 hours total integration time ISO800 60 second exposures Flats 25 Dark Flats 25 Bias 50 Darks 50 Stacking in DSS Processing StarTools and Photoshop CS2 Mosaic assembled in Microsoft ICE

I'm getting the dreaded error 99 on my EOS1000D. It's happened several times, and each time I've noticed the mirror seems to be stuck in the up position. This video shows the same problem (I realise it's a Nikon but the problem is the same): https://www.youtube.com/watch?v=ACWamsXdIvM Eventually after turning it on and off, taking the memory card and battery out, randomly it seems to be ok again for a while. I can immediately tell when it's working again: the sound of the mirror going up and then back down again is recognisable. The camera is currently working but it's an intermittent fault and it's concerning me. Is there any fix I could be looking at to avoid this happening? If not, then any recommendations for good, reliable repairs in the Bucks/Herts/Beds region of the UK? And does anyone have any rough idea how much this might cost to fix?

Good point, I'll take a look at some point over the next few days after I've broken the rig down (I want to keep it all as is over the next few days because, shock horror, we might be getting some good nights!) I still think that, once I've figured out the best exposure time for the 'laptop against the scope' method, it's not such a hardship to stay in Manual. But it would be good to fix it, as you say, just because, well, it's better when things work the way they should. Having said which, at pretty much every step along this torturous AP path, hardly anything has worked the way it should... Having said which, I just realised, if it was turned up/down, then that wouldn't account for why the t-shirt and screen methods had the same exposure time.

Yep, ISO is definitely the same (800 - I shoot everything at that, makes calibration library so much easier and it's supposed to be good for signal/noise ratio for my EOS1000D), and I'm using a 130PDS scope, so no lens to factor in here. I just think that it's 'one of those things' - AV mode isn't quite cutting it. It's no real problem just to do this manually, I've already set up a plan in APT to take a series of shots at different exposures, then I'll see which are best for the histogram, and iterate until I've hit the sweet spot.

Thank you! I think I might just abandon the AV exposure technique and get more hands on with the histogram. Really appreciate the help here.

OK, well, in that case my entire flats approach is wrong! Which is totally why I need help with this. APT tells me to set my camera to 'AV' mode. AstroBackyard also says AV is the perfect solution to taking flats. So, that's what I've done. Canon's AV algorithm just selected the same exposure time, it would seem. In other words, don't blame me, blame 'the algorithm'! All I need is for someone, anyone, to take a look at either flat and tell me if they're ok.

Thanks for sharing. I can see that mine are probably similar to yours. But if anyone could check mine I'd really appreciate it.

Hmm, I hope I've shared them ok using OneDrive... I used the AV exposure setting on the camera because that's what APT uses (and I've seen it recommended elsewhere too, not least by AstroBackyard). My understanding was that this means the correct exposure time is automatically calculated without needing to manually check the histogram. The exposure time was 1/500s for each. I've only just started taking flats. The past few times I've used them, they've just produced weird results in StarTools. So, I think I must be doing something wrong, but I'm 'flying blind' because I don't know what a 'good' flat or master flat should look like!

Hi all, I'm not even remotely sure I'm doing the right thing with my flats. I've tried two methods: one with the laptop white screen against the scope, using a Flats plan in APT; and one with the same method, but using a white t-shirt between the laptop and scope. However, I have zero idea whether either method has 'worked', or how I would even check this. So, would anyone who even slightly remotely has an inkling of what they're doing, please take a look at an example of each one and let me know what they think? They're here: https://1drv.ms/u/s!AqovBuVZMwj3iZl0sdWvAf1KkFaXEA?e=uN3cAA - one RAW sub taken directly from the screen (.CR2 format, from a Canon EOS1000D), and one using the t-shirt method, and the master TIF for each method, created in DSS as a result of stacking 50 of each.. The t-shirt versions seem quite dark to me, but I know that the actual colour of a flat isn't that important. Still, it would be good to know whether one, or both, or neither, are any good! Thanks!

Yep, already ordered one. I could either get one for about £1.50 on Ebay delivered in a month, or pay £6.50 on Amazon and get it delivered tomorrow. So, that's hardly going to break the bank. Hopefully it'll fix this problem, which will be a huge relief. There are so many available online, I bet it's a common thing to break, either that or the hinge, they're both a bit on the weak side. I did look around for the 'extrusion' bit but it'll just be a small piece of black plastic that could be anywhere. I've since realised through a process of Sherlock Holmes-like deduction that it must have been that small bit that was causing problems getting batteries in and out - it must have fallen into the compartment, and I must have got it out when I cleaned it. Now I know what I'm looking for, I can actually see where it's snapped off the door. Another lesson learned. I cannot express how relieved I am. Again, thank you so much for your help.

HOLY MOLY! It worked! So I can press the micros switch in, and the camera comes on. However, it still doesn't work when I close the battery door. So it seems to me that the bit on the door, that pushes the micro switch, has come off. This is encouraging. I either need to figure out how to make my own 'protrusion', or source an alternative EOS1000D battery compartment door. Thank you so much, you've really helped me out here.

Argh! Earlier today, when I opened the battery compartment (very carefully) on my EOS1000D, the lid came off. I really didn't force it or anything. I just clicked it back into place, thought nothing of it. Then I couldn't insert a battery for some reason. After blowing into the compartment and cleaning it, the battery went in fine, but now it won't switch on. I've tried it with several different, charged batteries, and with my power pack. Nothing. I know that the battery compartment door needs to be fully closed for it to work, so I'm wondering whether this is the problem? Any ideas? Getting a bit desperate now...

Thanks.

Argh. So close, and yet so far... If anyone else knows of flexure issues with the Svbony, please shout!

@AdamJ Focal length would be 590mm with coma corrector, 650 without (Skywatcher 130PDS). I would be using a ZWO ASI120MC for the guide camera (not the mono, I know mono is better for guiding but I want to use it for planetary imaging as well), and for DSOs I would be using my modded EOS1000D. I'm sure they will both work for what I need, but like @Louis D, I'm very interested in the flexure that you mention. If that's an issue with the Svbony, I'll go with the Orion.

Interesting. That was kind of my take on this all too, before I came across the Svbony. The Orion is more likely to work, focuser not needed, less flexure too. BUT it's more expensive! Having said which: did the Svbony actually do the job for you? Or, when you mention flexure, did you actually have a problem with that?

Yes - but I'll be using them for guiding!

Excellent response, thank you so much!

Thanks - any ideas why the Orion is therefore 33% more expensive? Is it just brand value?

These are for guiding, not finding.,

Hi, Both of these models do what I want/need (I've checked regarding my OTA's focal length, pixel size, camera resolution etc): Svbony at https://www.ebay.co.uk/itm/50mm-CCD-Imaging-Guide-Scope-Finderscope-w-Bracket-For-Astronmical-TelescopeNEW/322140242550?hash=item4b010e0276:g:aS8AAOSwcntcmJTB Orion at https://eu.telescope.com/Orion-Mini-50mm-Guide-Scope/p/103604.uts The Orion is highly regarded - but the Svbony is 33% cheaper! Can anyone give me a good reason to go for the Orion over the Svbony? Does anyone have experience with both? Thanks, Brendan