First time getting EEVA results - M3

[Peter_D]

Hi,

Last night, it was my first time using my Nextar 6SE for EEVA and getting something for a result! I have a Zwo asi224mc which narrowed down the targets a bit. M3 was an ideal candidate so after allignment, the scope successfully went to the location: what a thrill seeing the glob visually first. It is in the eastern sky for me so ideally placed out of Cologne's skyglow to the west. Then I began to do live stacking: I have to say, I probably overdid the number of captures I did (350) - how many would people recommend for globulars? I could probably also invest in a bahtinov mask and maybe a better 0.5 Reducer - any recommendations on the latter? I had to crop quite a bit from my capture because of an elongated star near the periphery. Would a better reducer improve things or was it due to the number of captures? All a bit new to me but great to give it a go and get a EEVA DSO image from my Bortle 6-ish skies (I generally focus on the Moon and planets). But lots of room for improvement I know.

Clear skies

Peter

 

Edited May 8, 2021 by Peter_D

[Mike JW]

Hi Peter

The adventure begins and a nice first target to choose.

Stacking improves the signal to noise ratio but after about 30 in the stack the improvement in the signal to noise ratio (that is the reduction in noise) significantly tails off. I rarely see much if any improvement beyond about 40 in a stack. The only time I might go beyond this is to bring out seriously faint galaxy features. Also assuming your mount does not track perfectly the longer the stack the less sharp the stars will be.

When going for stars I will tend to use between 1 and 3 seconds for each sub and stop the stacking when I feel the stars are starting to loose sharpness or in the case of globulars you end up over exposing the core.

Bahtinov Mask - absolutely essential.

Why have you got elongated stars (coma) at the edges of your shot (before cropping)? I assume collimation is correct. For myself I use a C11 (use to use a C9.25) and if I attempt to push the focal reduction to F5.0 I start to get coma around the edges. I always use the Celestron f6.3 reducer and from experimenting with the distance from it to the chip sensor I can push it to f6.0 but faster than that coma starts to be noticed. Also you do need to have 100% focus (go get that Bahtinov Mask and the Celestron f6.3 focal reducer).

As to the best 0.5 focal reducer - my feeling is they are all very similar (other folk with more expertise may be able to advise).

Try  2 seconds and a stack of 30 !!!!!!!

Have fun,

Mike

[Peter_D]

Thanks Mike! Yes, don't know why I did 350 captures, it makes no sense for a GC..thanks  for the pointers. A Bahtinov mask will be on my shopping list as well as the Celestron corrector / reducer.

Peter

Edited May 8, 2021 by Peter_D

[Mike JW]

Just ask when/if you are uncertain of procedures, techniques....always helpful on this forum.

Mike

[Martin Meredith]

Hi Peter

As Mike says, short exposures work well for bright globulars. I'd say with my 8" f4 Newt, something around 5s is more than enough. If you have the choice in the software you're using, choose a nonlinear stretch of some kind (I find log worked well with globs).

A Bahtinov mask was one of my most valuable purchases. 

My scope/sensor combination provides a slightly larger field of view but there are 1000s of interesting targets for your FOV so long as you can get enough stars for alignment and can place the target on or near the sensor with your goto (plus local platesolving perhaps).

Re the elongated star, it would interesting to see the full field. I sometimes see the odd oval star but it turns out to be a close double! Always worth checking. Stacking actually covers up a range of sins (eg tracking issues) so it is worth looking an individual subs too.

Cheers

Martin

[Peter_D]

Hi,

Many thanks for all the feedback so far!

Attached is the original frame: you can see very noticeable coma on two stars. 

Here are details of my optical train:

6SE OTA => 1.25" Visual Back => Svbony 0.5 reducer => 1.25" nose piece => Baader UV-IR cut filter => Zwo asi224mc camera

Better to switch the positions of the reducer and the filter next time?

The C6's colllimation is spot on - hopefully getting the  Bahtinov Mask  will help things next time.

Clear skies

Peter

 

[Martin Meredith]

Hi Peter

Thanks for providing the sub. That doesn't look like coma to me, and neither does it look like tracking. Is it possible that the camera has rotated slightly during the exposure? The doubled star images suggest that it made a sudden rotation.

Martin