NGC7023 with ASI1600mm at different focal lengths experiment

[halli]

There has been a lot of discussion about the performance of the ASI1600mm  - cool at different focal lengths.  

 

I decided to get my RC8 out recently as we have gone through a period of the galaxies outnumbering the nebula (At least where i live in Reading). Having been spoilt by using a TS71 using Ha on a mumber of emission nebula in the Autumn/early Winter, I decided to try and compare the results from the RC8 and the TS71 on the same object namely NGC7023 (Iris Nebula) which has just become visible at around 10 ish.  

 

The RC8 was using native focal length at @1600mm (F8) and the TS71 was around 348mm (f4.9).    

 

The expectations were that the TS71 was suited to the smallish pixels of the 1600mm and the RC8 would struggle  because of the longish focal length.

 

The data was as follows

TS71/ASI1600mm-c -  

Lum - 116x120s at Unity gain 1x1 Bin

RG and B aprox 35x120s each at unity gain 1x1 Bin

 

RC8/ASI1600mm- c

Lum approx 100x120s at unity gain 1x1

RG and B approx 50x120s each 1x1

 

So the data collected was of the same order for both scopes and from a light polluted part of Reading making LRGB imaging difficult -  Im awaiting some nice juicy emission nebula to appear in Cygnus soon !

Both were processed in PI as LRGB and applying noise reduction with ACDNR and TGVdenoise and cooled to -30deg C

The RC8 image was definitely noisier as you can see  than the TS71 as would be expected and I needed to turn the wick up on the TGV Denoise in PI (of which I am no expert!)  - it needs more data though.   

Also there is an opportunity to turn the gain up to 2 or 300 and with it the possibility of lower read noise which may help with the RC8

An interesting experiment and I am pleased that the RC8 and 1600mm-c seemed at least  to have the potential to enable a reasonable result with more data and a possible change of settings on the 1600 !

TS71

 

RC8

 

Comments or input more than welcome

 

 

 

 

 

[michaelmorris]

Nice images. I must admit I prefer the first one.

[halli]

Thanks for the comment tend to agree 

[Allinthehead]

Nice job halli. I'm imaging Iris at the moment and I'm using unity gain at 60 seconds. Any longer and my stars were over exposed. Looks like the stars on your ts are over exposed. This camera saturates bright stars very quickly.  The background also looks a bit blue. I reckon there's plenty of dusty stuff in the data with some careful processing. The rc8 image looks great, loads of detail. Just needs more data. Do you dither?

[halli]

Hi Allinthehead - ta for the comments.  

I totally agree with your thoughts.  

I have been using an Atik 383l previously and am still getting used to the variables re gain and exposure with the 1600.  I guess the tendency is to push for longer exposures to get more of the faint stuff and not properly considering the star exposure size.   I guess I need to try some star reduction processes in PI but probably the best is to  spend more time experimenting with  gain and exposure at the front end to get the stars right.

Ah yes dithering -  I must confess although I use APT and PHD2 and believe I have enabled dithering - it doesn't appear to work and have not yet looked into why.  i have used it in the past with a SLR but with Backyard EOS.

Certainly plenty of dust in this region and like you say I need some better processing to eke it out !  I think the correct depth of saturation and colour alignment may be key here.