Newtonian Star Question

[MattJenko]

I have started using a 250px for imaging purposes and managed to get everything working and produced a 5 min test image of M3 with a Red filter this week. I am naturally very happy with everything as a first pass, but I would like to garner some views from the more experienced imagers out there, about how to improve the star shapes, especially the brighter ones. They are not quite right and I would like some pointers as to what to check first please. I have a feeling my collimation attempts for visual are not really cutting it here, but there seems to also be some comet like features more pronounced in the lower right (coma/tilt)? I am using an Atik 414ex and there is no coma corrector used in this, as I hoped to be able to get away without using one with my little chip.

Many thanks, Matt

 

[Starlight 1]

3 hours ago, MattJenko said:

but I would like to garner some views from the more experienced imagers out there

You can count me out on this Mat. but I know you get there in the end  keep going  looking good . As you said it more on right look at focus tube may be moving under load of camera

is the best I can come up with.

[alan4908]

Hi Matt

I've recently started using CCDInspector to check the quality of my sub frames. 

If I put your image through it it tells me you have (based on looking at 228 stars) an aspect ratio of 32 and a tilt of 47%, Collimation of 27.7pixels and Curvature of 26.6%. 

For my set up, a very good aspect ratio result is 6 and I cannot tell if a star is non circular if the aspect ratio is below about 20. A tilt of 26.6% is also high. I haven't explored the collimation area, so I cannot comment. 

I'd suggest you download a trial version of CCD Inspector and then perform measurements after making any adjustments. I had a similar problem recently and found it was due to camera tilt, so maybe you want to start there.

Alan

[MattJenko]

Thanks Alan. Those are some interesting stats. I might have to have a closer look at that piece of software. Starlight1 - I'm glad you have confidence - at least someone does!

[ollypenrice]

We all have confidence in you, Matt! I think CCD Inspector has to be a good bet for reflector users. 

Olly

[MattJenko]

Thanks Olly

Looks like there are a lot more things to get right with big newt imaging than the plug and play ED80, so very glad I started with the frac. Shame the weather is not playing nice, as I am really looking forward to getting my hands dirty with all this.

[Tinker1947]

The Image looks  good, but you will need a Coma Corrector and a bit further down the road a Filter Wheel and bit further than that a Electronic Focuser, so your not lost as to how it all goes together here is my set-up, its simple and stable, Baader CC in the focus tube, this connects to a Baader Vari-Lock 29 (precise spacing), then the Atik EFW 2 Filter Wheel, finally the CCD a Atik 414EX Mono, one good reason for the Atik Filter Wheel you get the Atik Software free and its so easy to use and would be worth paying for, so here's a picture of what it looks like. 

http://www.firstlightoptics.com/adapters/baader-varilock-29-lockable-t-2-extension-tube.html

[theastrodragon]

On ‎18‎/‎03‎/‎2016 at 16:06, alan4908 said:

Hi Matt

I've recently started using CCDInspector to check the quality of my sub frames. 

If I put your image through it it tells me you have (based on looking at 228 stars) an aspect ratio of 32 and a tilt of 47%, Collimation of 27.7pixels and Curvature of 26.6%. 

For my set up, a very good aspect ratio result is 6 and I cannot tell if a star is non circular if the aspect ratio is below about 20. A tilt of 26.6% is also high. I haven't explored the collimation area, so I cannot comment. 

I'd suggest you download a trial version of CCD Inspector and then perform measurements after making any adjustments. I had a similar problem recently and found it was due to camera tilt, so maybe you want to start there.

Alan

Collimation seems poor. CCDI recommends getting this down under 5pixels, the lower the better.

Tilt is high too. Curvature is a little high, but Newtonians do suffer from this without a corrector; its not usually a big problem unless you are using a large CCD. I'm not actually sure how CCDI distinguishes between curvature and coma.

[MattJenko]

Thanks Tinker.  With a 414ex, EFW2 at 35mm total and then 20mm is putting you right on the edge of the CC sweet spot are you not? I already have a SX filter wheel with my 414ex, and that is fatter so thinking I would need a thinner T adaptor. I have a few bits of small tube lying about to try out first. Baader MPCC is on its way courtesy of a lovely SGL-er.

Most of the tilt is almost certainly down to my heavyish rig sitting in a 2"-1.25" adaptor with a 1.25" nosepiece on the SX FW at the moment. Arrival of the MPCC should help get that into a more solid config.

As for the collimation, I am not surprised it is off, having had me play about with it for visual. Itching to get cracking with all this now and to get into CCDInspector.

Shame I live in a cloud.

[Tinker1947]

I just took a look at the amount of travel left on the Varilock 29 about 1/2mm which appears to work..

NGC 891, 20 minute single exposure with the present set-up