Just a quick shot last night testing out spacing with my reducer. four 30 second subs and two 120 second subs, darks applied and stacked in DSS. The spacing used here gives me about 1500mm FL at about F6.38.
I still have some coma but the vignetting is not as bad as with longer spacing so I will probably image in this configuration for a while.
The collimation is off by just a sniff, I did not have time to sort it but did have a chance to check it. This was really a lightning test before I went to bed last night with the gear set up, polar aligned (Thank god for Polemaster) a focus check between the imaging and guide cams (The purpose for setting up) the handful of subs shot and then everything packed up and a lightning process in DSS.
There is still some coma visible out in the corners on zooming in, but it is all going in similar directions rather than being radially outward from the centre of the field so hopefully a collimation tweak will reduce it.
A question for people a bit more experienced than me on this stuff.
The Blue PN is obviously a small object, I can zoom in but then get pixellation effects as I go closer. Will stacking more images improve the outcome if I use a fair few more subs and dithering between them? to gather/smooth data by pointing changes that are unlikely to be whole pixels in size.
Try it☺ Maybe bin it to the max then enlarge it later after processing.
Take it to photo shop and crop around your object and increase the size to as big as possible work on it then reduce it to normal size...or just go for a longer fl for this object☺.
Alex
Some time I shot NGC3918 with Rubinar 10/1000, Zenit MC TKA-2 tele-extender, camera was Canon 60D, and I even captured a bit of internal structure ( I need to find that image yet).
Yes, it is a small object.
There is still some coma visible out in the corners on zooming in, but it is all going in similar directions rather than being radially outward from the centre of the field so hopefully a collimation tweak will reduce it.
If what you are seeing is not all radiating out from centre, then it is not coma. Especially if it is all roughly in the same direction. More likely to be collimation. Coma radiates out from centre. Off course this is only by the way you've described things. If you had some pics of the four corners we could have a better idea.
Yeah, per my later post, the collimation is just a sniff off, it was not obvious previously but I have fiddled with the spacing and the effective focal length has gone from about 1340 to about 1500mm, presumably making it that little bit more critical.
I think there are two components to it, some coma that is more or less inevitable with this scope unless I spend a lot more than I want to on it (This really is my visual scope) and a tweak required on the collimation. Hopefully the clouds give me some opportunity to fiddle over this coming weekend so I can collimate a bit more precisely.
Bojan, as near as "Damn" is to swearing 1500mm FL, with 4.63um pixels. I am probably being harsh on it, this is an integration of only a couple of subs grabbed on the spot as opportunity knocked after I did something else and before I went to bed.
.... 1500mm FL, with 4.63um pixels. I am probably being harsh on it, this is an integration of only a couple of subs grabbed on the spot as opportunity knocked after I did something else and before I went to bed.
.. that means NGC 3918 is ~15 pixels in diameter on your image (8 arc", 0.57 arc"/pixl).. there is not much that can be done with available data.. you should see a hint of "ears" though...
I found the image taken with 2000mm FL (Rubinar 1000 + 2x telextender).
Perhaps a target at the native focal length of my scope (2350mm) sometime in future when I am happy with the setup using the reducer. The off axis coma is very evident but for a target that small I could crop it out.
I took this years ago with a 200mm f/5 Newt +2x Barlow making 2000mm f.l. For some reason it has come through HUGE; you will need to reduce screen to around 33-50%. I also greatly enlarged it.
raymo
Last edited by raymo; 08-02-2019 at 04:40 PM.
Reason: more text
You should be able to extract at least this much with wavelets (Registax) - C9.25 should be way better than 4" Rubinar 10/1000 with tele-extender (this is 30x30sec stack)...
I suppose that is the trick, I only have 6 subs to play with where you have 30. I might wait a month or two for it to be in a better position and have a decent crack at it at F10 over a night to see if I can get some surface detail.
I just re stacked it and stretched it differently in DSS and I can see a definite hint of the ears. The slightly off round stars is I think a resultant of some remaining coma plus the not quite collimation, three of the corners the fuzzy end of the stars point in broadly the same direction and the fourth more markedly different.
Is it just me or is Registax 6 a PITA? I have tried a couple of things in it and given up every time. When I get it to actually align and stack something I get a bright green image and generally after that it gets laggy and then goes unresponsive and I have to use task manager to kill it. My PC is not exactly the greatest on earth but no other program gives me so much grief.
It is looking like I might see some sky tonight, if that is the case I might jump in and get some more data. The question is if I do it at 1500mm or complicate my life by changing the setup to get the native FL at the price of starting from scratch
My first scope, a Celestron 4SE had this optimistically in the objects database in the hand controller. When I knew absolutely nothing, I couldn't work out why I couldn't see it.
