Space Tadpoles! (Am I expecting too much?)

[assouptro]

Hi Starpeople! 

Since I bought a second hand Atik 383l I have been trying to get the flattest field I can from my old 12” Meade Lx200 OTA on the bigger chip. (not big by today’s CMOS standards, but the biggest cooled chip I own) 

I have the spacing just about right, I’ve checked my image scale with astrometry.net and it came back with 1.17” per pixel which is close to the 1.16” per pixel predicted by astronomy tools fov calculator for the camera, scope, and Meade 6.3focal reducer/field flattener.

I have a tilt adjuster which I eyeballed on a neighbours chimney getting all 4 corners of the image as in focus as I could, 

The telescope was collimated without the image train, which leads to another question? Should I re-collimate once I have the oag, focal reducer, spacers and camera hanging off the end? I’ve found in the past trying this has made things worse! 
 

I have become quite critical when it comes to star shapes, I don’t own an 11” edge HD or a 12” ACF but I’d like the stars to be controlled as much as humanly possible in my old scope. 
this is 4 hours of 20 min ha subs binned 2x2 at 1.17”per pixel

 

The “tadpoles” are in IC410 in Auriga about 12000ly away and I believe they are sites for star formation 10ly in length!

I like the image, but I can’t help zooming in to the stars and cursing the coma.

There is some extra distortion to the star shapes causing triangular shapes on the right of this image that is caused by the oag encroaching into the fov which I have since retracted further but haven’t tested due to clouds. 

I have just bought an Alan Gee telecompressor mk11 which is in the post, I’m hoping that it could improve star shapes, although I cannot find a lot of data on the web for it’s use in imaging ( I will let you guys know if it works better than the Meade fr/ff)

There is another focal reducer made by Starizona that claims to “fix” these issues but it’s not cheap and I might be better putting the money towards a flatter field OTA? 

Rodd recently posted a similar image from his 11”edge HD with dedicated 0.7focal reducer with an asi 1600mm (I think) but he’s since taken it down so I can’t add a link here to compare, the fov was almost the same but obviously his stars were round!

Anyway, if you are still reading, thanks!

There’s an image, and a lot of questions! Thanks again  in advance for any response 

Bryan

[vlaiv]

Fact that you have collimated scope without gear on it and collimating the scope with imaging gear on it makes things worse, and that you have coma issues on one side of the image to me suggests that the problem might be some sort of sag because of the weight of imaging gear.

Is your primary mirror in any way attached to rear port of the scope? Can adding weight to the back port somehow tilt mirror a bit?

In lighter RC designs that I saw - this is the case and if primary lock screws are not tight - moving scope across the sky can impact collimation as gravity acts on imaging gear in different directions.

[assouptro]

3 hours ago, vlaiv said:

Fact that you have collimated scope without gear on it and collimating the scope with imaging gear on it makes things worse, and that you have coma issues on one side of the image to me suggests that the problem might be some sort of sag because of the weight of imaging gear.

Is your primary mirror in any way attached to rear port of the scope? Can adding weight to the back port somehow tilt mirror a bit?

In lighter RC designs that I saw - this is the case and if primary lock screws are not tight - moving scope across the sky can impact collimation as gravity acts on imaging gear in different directions.

Hi Vlaiv 

Thank you for taking the time to add your thoughts.

You could be right, I hadn’t considered the actual rear port sagging, it always seemed so solid and heavy that I dismissed that as a weak link! 

I guess the test will be attach the focal reducer and camera alone without oag etc and compare an image 

it will have to be a short exposure in a rich star field as I won’t be guiding. 
 

After I posted my question last night, I saw the clouds clear for an hour or 2 and so I uncovered the scope and checked the collimation, I needed to also check the OAG wasn’t visible in the image train and I could still see stars with it which I’m happy to report is all good 😊 

I was happy also to see my scope collimation looks good with all the gear on, at the centre of the image at least so I’m leaning towards tilt being the main culprit but I will test your theory first Vlaiv 

Cheers

Bryan 

[vlaiv]

4 hours ago, assouptro said:

I guess the test will be attach the focal reducer and camera alone without oag etc and compare an image 

Yes, simplest way would be to image 3 different star fields - one due east, one due south and one due west at about 45° altitude. That will let gravity act on camera / reducer combo from three different directions and then you can see if coma changes position in the image.

If it stays in the same part of image (say left side) - then you ruled out gravity, but there still could be focuser tilt.

Focuser tilt can be tested by rotating camera by 180° or 90° and shooting same part of the sky. Coma should do the same in opposite direction - change position in the image either 180° or 90° (depending how much you rotated camera).