First ever true H-a mono shot. 1 hr 5 min on NGC 7293, with Astronomik 6nm Ha filter with the SBIG ST-8XE and CFW-8A FW. Scope is the Vixen DED108SS f/5 astrograph refractor, on a GPD2 mount.
240 sec light frames. No binning. Focus seems a touch out. Taken from suburbia.
Appreciate the feedback. My journey has really just begun
UPDATE: new images to help isolate the problems in posts 11, 12 & 13
Good step forward Lewis, looks like the sensitivity is there. However, the stars are huge for 6nm Ha, as they were with your colour image. There is a pair of stars just outside of the main nebula at about 2 o'clock that should be well separated (see other Helix images on IIS) - the stars are merged in your image. Suggest that in Pixinsight, click on a few unsaturated stars in "dynamic PSF" and see what it reports as the FWHM.
Unless you had extremely bad seeing, first guess is that the focus is out - what procedure do you use and do you refocus as the temperature changes?
Second possibility is mount tracking noise - do you get tighter stars with very short exposures on bright stars.
Third possibility is an optics fault. If this image was taken with a focal reducer - is the spacing as specified. I would check without a focal reducer first off and would also try some other scope (an ED80 or even an achro if you have one) and see if you can get tighter Ha stars. Also, do a star test to see if the main scope has acceptable spherical aberration. If your main scope is faulty, the only real option is to get it factory refurbished - unless you are extermely lucky, all you could expect from trying to adjust a multielement refractor without alignment equipment would be to make it worse.
Do you have a Bahtinov mask you can use? This will tell you if you can actually achieve focus or whether there is an optical issue at play. Bahtinov Grabber is a great utility to use with the mask as it quantifies how far (in microns) the sensor is from the focal plane, and will report when the sensor is within the Critical Focus Zone for your particular optical setup.
If you're just going off FWHM numbers that wont give you the full picture, especially if there is variable seeing or an optical issue. If there is a fundamental problem with the scope, then even if you've supposedly minimised FWHM you may not actually be in focus.
My suggestion is to set up both of your scopes, and then focus each one in turn with a bahtinov mask. Then take a few images and use Dynamic PSF in PI as Ray suggested and compare the results (once you've converted the results to arcsecs) for each scope. That might help you to determine if the issue is with your focus procedure, or with seeing, or with the optics of the DED.
good idea to use a Bahtinov mask as Richard suggests.
also, you are pushing uphill using manual focusing at f5 - you have less than 100microns to play with if you want to get diffraction limited results. At f5, the tube length fluctuations with temperature will be significant - particularly if it is aluminium - will need regular refocus in varying temperatures.
Star testing is easy to do and will soon rule in/out any major optical problems.
You have what is likely to be a crackerjack scope - should be great when you get focusing sorted - everything else is looking good
I have given up chasing FWHM with this scope - fruitless exercise, since the refocus adjustments are so darned minute. I honestly almost touch the focuser knobs in or out - microscopic movements near prime focus! It really needs a dual speed knob - might see if the Vixen dual rate adapter can be converted.
I am running a focuser image routine in MaxIM - 20 second exposures seem the best for it. And then doing it BY EYE. Yeah, no figures, no Bahtinov (though tried the Bahtinov - it's still chasing the tail in many ways).
In the non-stacked lightly stretched images, the double star that Ray is discussing is in fact distinctly 2 stars. Stack and stretch, and they merge a little. I am also using the PI BatchPreProcess routine for stacking, so the light frame calibration is not exacting (as the disclaimer also says). DSS yields a similarish result though anyway!
Yes, I have decided to go with a robotic focuser of some description - have not decided fullly which one as yet, but most likely the Sharpsky.
The issues I had with the colour shots most assuredly were related to the faulty SXVR-M25C - it LOOKED like a collimation issue, but there is tilt in the image (NOT apparent with the SBIG - perfect across the field in Lum and Ha), not fully "filled" pixels, and NOT debayering corrctly (I can fully see red, green and white in a zoomed star image - the blue channel has gone to poop). The SXVR also failed cooling the other night. The issues began about 8 months ago, and I have been in contact with Terry since day 1 - initially a massive drop in resolution between frames, then the graininess despite the cooling working, and then the cooling failing almost. I sent it back yesterday to Terry to look at.
So, in summary, I think the scope is fine. Maybe a VERY small miscollimation (the star test shows an EXTREMELLY small asymmetry, but I can notice it). Focusing is the issue. Additionally, I need to get off my butt and do measurements for both the Sharpsky, as well as finish the dimensions for the light cone I am having made for it (I am getting a fair bit of vignetting, since the 4" drawtube ABRUPTLY steps down to the 2" adapter - I will have a cone made with rib baffling to eliminate a lot of this).
all sounds positive Lewis. just a suggestion, maybe you can measure the distance that the focuser travels for one revolution of the shaft - then check with Dave Trewren to make sure that the minimum focuser step angle is small enough to give you say 10 micron focus steps or smaller. It will probably be OK, but easy to check beforehand.
Nice going Lewis, impressive amount of outer halo in there with four minute subs at F5, no binning. Im only new to this sought of depth, so, maybe quite normal light gather. Its cool seeing other works like this, Top stuff !
