Getting Better Data - Help Needed.

[brenski]

Equipment/Software etc
♦ Zwo ASI 183MC Pro (cooled) ♦ Skywatcher Evostar 72ED DS-Pro ♦ Skywatcher Star Adventurer Pro 2i Wifi ♦ Optolong UHC Light Pollution Filter 2" ♦ Stellamira Field Flattener / 0.8 Reducer ♦ Bahtinov

So, The above setup is unguided - that perhaps a venture for when I have more cash. So my data (so far) has been ok-ish, but not what I'd call good.

Initially I was taking lots of images at 17s exp, then 22s. the gain settings are around the 170-200 mark. Nico Carver very kindly looked at my unguided raw data and this was the best he could get out of it:



I've now manged to get 35s exps unguided, but the data itself isn't improving much. where am I going wrong? yes, to darks, flats.bias. what other SC settings can I tweak to help things along?

[admin]

Hi,

I'm just going to throw something else in there... How confident are you on having good focus? To me, all the stars look a bit big and blobby, and if the focus is a bit out then that will not just give you blobby stars, but also destroy a lot of fine detail in the galaxy itself.

cheers,

Robin

[brenski]

hi Robin
thanks for the reply. I'm using a Bahtinov and the "spike" looks pretty sharp (at 100%) and aligned as per this online image that I've used as a guide:

[Menno555]

HI

The stars in the center are sharp(er) but the more out of the center, the less focused they are.
I think the cause is the distance between reducer/flattener and your camera. The reducer/flattener has a certain needed backfocus, in this case 55mm from the reducer to camera sensor. So you could check if that is the case?

If it is 55mm but you still get this, it's possible that the reducer and scope do not work well together.
You can partly "fix" it by placing your focus star out of the middle and then use the Bahtinov to focus. How far out of the middle you have to see yourself.
This way you are middling the focus. It's not perfect but it's something

Menno

[brenski]

thanks for the advice. Having just measured the backfocus there's definitely (at least) 55mm from reducer to Zwo sensor.

my setup (front to back) is:

Scope > Optolong LP Filter > Stellamira 0.8 Reducer/Flattener > Zwo tubes > Zwo 183 MC Pro

[Menno555]

You could try to add a bit more distance between reducer and camera but I doubt that will help: I just noticed that the stars are not elongated and most of the times just that is a sign of too little or too much reducer/camera backfocus. See image below showing how it would look like.
The camera and reducer combination itself is not the problem I think: the sensor size is not that big, so it should be fully illuminated.

One thing for sure, it's sadly not something that's SharpCap related, it really is somewhere in your equipment.

Menno

Clipboard01.jpg (114.87 KiB) Viewed 1335 times

[brenski]

You could try to add a bit more distance between reducer and camera but I doubt that will help: I just noticed that the stars are not elongated and most of the times just that is a sign of too little or too much reducer/camera backfocus. See image below showing how it would look like.
The camera and reducer combination itself is not the problem I think: the sensor size is not that big, so it should be fully illuminated.

One thing for sure, it's sadly not something that's SharpCap related, it really is somewhere in your equipment.

Menno


Clipboard01.jpg

good to know that backfocus isn;t an issue.
re: fully illuminated. is that something I do in Sharpcap - via "Gain" or elsewhere?

[Menno555]

Ah no. With fully illuminated I mean the "diameter" of the the light that is coming from the reducer.
There is only a certain "diameter" where everything is equal in brightness. The brightness will get less the more you you travel outwards from the center of that "diameter". The result of that is vignetting but that isn't showing in your example picture.

Oh, and never say never. You could try and add or reduce the distance to see if it has any effect.

Menno