Star shape assistance please - refractor :) - asi6200mm

[rustigsmed]

Hi all,

I took the plunge and picked up a zwo asi6200mm to pair with my TS130mm apo. I have a .79 reducer/corrector on the scope. While I have had the scope for a while i haven't really used it much - so in reality i'm a fair refractor noob, coming across from newts.

I took a lot of LRGB last night of the horsehead, today i realised that my flats were too saturated - i didn't realise just how much the full-well reduced going from gain 0 to gain 100 (logarithmic). hopefully i can correct that tonight with duller flats (and shooting with gain 0). One issue sorted.

Anyway - I have a lot of strange shaped stars especially in the bottom left, bottom and bottom right.

Am i looking at;
- tilt /
corrector spacing /
OAG prism intrusion
/ all of the above - something else?

The spacing "should" have been correct - my corrector is screwed directly to the scope draw tube, and the imaging system screw straight onto the corrector, no clamping. I was fairly annoyed with zwo, as i purchased it based on their "telescope side M48 solution" unfortunately their M54-M48 0mm adapter is loose, there is no stop and not really a real solution at all. So I am going to have to find another connection option to increase rigidity.

Apart from that - camera seems quite good - files are about 119mb. I am very out of practice on processing, setting up etc which doesn't help :lol:

7x2mins Carina neb Ha - see star shapes lower left, lower right etc.
https://www.flickr.com/photos/80336656@N07/50812174883/in/dateposted-public/lightbox/

attached a super quick 20minute nb starless carina for no particular reason.

cheers

russ

[Camelopardalis]

Russ, looks like tilt...but I'm speculating. How do the left and right edges look about halfway down the image? The Flickr is too low res to really see.

I've got a similar issue with my 60mm scope, where two corners look like they've got wings and the other two corners are streaky, almost comatic towards the centre.

[Ryderscope]

Quite opportune this Russel as I'm setting up an ASI6200 myself at the moment so we can share some stories along the way :). In my case I have an FSQ106 that I have purchased as pre loved and initial tests show a combination of tilt and possibly collimation. I won't be able to diagnose any further until these pesky clouds go away.

Nevertheless, looking at your image on Flickr. It won't let me download a copy so difficult to look too closely. If you could provide a link to a raw fits file I can have a closer look.

It is clear though from what I can see on the Flickr image that there is radial elongation on the top left and to a slightly lesser extent bottom left. If it were incorrect spacing from the reducer to the image plane on the camera I would expect concentric or radial stars in each corner so maybe tilt is worth looking at.

Did you install the ZWO tilt adjuster at the camera end or the OAG end? I'm not using the ZWO tilt adjuster so cannot comment on its use but I have seen reports that it is difficult to adjust.

A question about the M54->M48 adapter. Does this mean that you are using the TSRED379-130 Reducer/Corrector with the M48 connection on the camera side? If yes, I see from the Teleskop site that the backfocus distance is 55mm which is exactly what the ASI6200 image train delivers with the tilt adjuster included. This means that you really don't have any room for tolerance in the backfocus from the reducer to camera.

If the M54-M48 0mm adapter that comes with the camera is not doing the job then the only solution that I can think off is to remove the ZWO tilt adjuster and have an adapter made up which is M54-M48 @ 5mm length. This would give you a more robust connection which is going to be required anyway even though you will be losing the capability to adjust for tilt. Alternatively, have the adapter made to a 3mm length and purchase a couple of 48mm spacers to allow for some trial and error in the backfocus.

So thats a few thoughts from my side Russel. I will provide an update on my setup when I get back under the stars with my new camera.

Clear skies,
Rodney

[rustigsmed]

Thanks for the replies guys I appreciate it :thumbsup:

Yes you're right - I've just rechecked and flickr struggles with the full res it seems. And it seems like tilt is the likely foe - thankyou.

I've uploaded a fit file here.
https://www42.zippyshare.com/v/2wBRgp2p/file.html - if you are using an ad block the download button is to the right of the link.

yes am using the TSRED-79 with the M48 connector. Well I should have an extra 0.6mm with the filters included :lol: Thanks for the thoughts there Rodney, yes I have the tilt adjuster on the telescope side - I think I will be going with the first suggestion, and if I still have tilt I will go for option B.

Good luck with your tak and 6200 I hope it works out alright and it isn't collimation.

i have noticed another comment on the zwo website for the camera and someone complaining about the same M54->M48 adapter - zwo replied saying they don't have a solution for it. Frustrating everything else feels quite rock solid.

I've tried to cut out a few small sections from most of the corners (attached). (from left to right - images start; bottom left, mid left, top left, top middle, top right, middle right, bottom right).

Russ

[Camelopardalis]

Nice and clean :thumbsup:

If normal refractor + flattener guidance applies, try to adjust the L-R tilt as the stars indicate the sensor is too close to the flattener on the left, and slightly off on the right. You'll need to experiment to get it more uniform.

Oh and it always helps to guide...looks like there's some drift in the stars too, which will make it harder to see the optical deformities.

[AnakChan]

I’ve got the same ASI6200MM & it definitely nitpicks any flaws in your optical train. I’m guessing quite likely it’s tilt -,l at least with my 3 scopes...some better than the others.

Best run your images through CCDInspector to confirm.

[gregbradley]

Definitely tilt and perhaps some spacing issue with the reducer.
Reducers are more fussy with regards to spacing than flatteners.
This hi res sensor will pick up more errors than other more forgiving sensors.

You should contact Joshua Bunn on this site and he can make you a custom adapter. I use him all the time, he's great and the main advantage for him over Precise Parts apart from he is cheaper is Ashley uses an auto adapter maker on his site and if you get it wrong too bad.

Josh really makes sure he understands what is needed before anything is made. He backs up his products.

Get a solid adapter made up as you are going nowhere until you have a solid square connection.

Then play with tilt. If you are able to insert shims to do the packing in your setup that will be easier than those 3 screw push pull systems which I have found difficult to use in the past.

By the way those images look terrific so its well worth the time and effort.

I am about to go through the same process but with a QHY600m. The QHY is known to be more precise with sensor levelling and there are several posts on Cloudy Nights complaining about ZWO poor levelling of sensors in the 6200. But its all correctable so don't fret.

Greg.

[rustigsmed]

Thanks for your comments and tips everyone!

I managed to setup again (although it was all a rush PA, capture etc) and got improved stars in the same edges (but not perfect). When I screw the camera on to the adapter/telescope it nearly always ends up at the same rotation angle.

Greg thanks for the idea i had actually just contacted Josh just before you send that suggestion. I have a part coming my way, so we'll see how that goes :thumbsup:

On another note I've had massive struggles with flats. My earlier attempts had severe brightening in the edges and my newer flats - which are way better but they still have concentric circle gradients :shrug: hoping it shows on the rosette (Ha, 4x5mins) attachment - have pushed up the levels to try and make it obvious. (histogram 40% (doesn't seem to make much difference whether it is 18, 25, 30, 35, 40 45, 50% of the histogram.

Any suggestions?

Cheers

[multiweb]

Regarding over correcting flats there could be two reasons. Either you have vignetting and noise can be perceived as brightening on the edges or your flats are not correctly scaled with corresponding dark flats. Bias frames shot at the same temperature and same duration. As for the concentric rings your light source might be too bright and bounce into your light path. So dim it and increase flat exposure.

[rustigsmed]

hi marc,

thanks for the suggestion, the flats applied to the image in my post below were 17.58 seconds - I have another set of flats which took 44 seconds, but that was for 50% histogram. I take you point though I could lower it further perhaps and try that - perhaps a 30-40 second for 25% for example.

OK - this is potentially something - I have applied Bias which i took (correct temp-0.32 seconds) in NINA and bias (a bit warmer - 0.32s) in SG PRO. The SG PRO was used in the image below - the ones from NINA result in washed out (and bad edge images). They must be doing something different under the hood. both were supposedly on GAIN 0.

:question:

[multiweb]

As a last resort you can pixelmath your existing flats and bias frames and iterate to find the sweet spot. Trial and error.

[Zuts]

Maybe I am doing it wrong, but my flats only take 36 milliseconds each to get a 50% histogram using my laptop as the light source. Not 17.58 full seconds as in your post above?

[rustigsmed]

this is with a Ha filter and on gain 0.

[Zuts]

Interesting, on my ASI 1600 MC Pro at unity gain and with a ZWO duoband filter I needed to go to 150 milliseconds to get to 50%

[gregbradley]

50% of saturation of the sensor seems way high.

I find 19000 ADU works well for flats on my CCD cameras. It should be the same for CMOS. Of course everything needs to be the same, temperature, gain, offset, mode (QHY).

Greg.