M65 and M66 First light

[Carole]

First light with my Newtonian.  I need a lot more data clouds permitting, but thought I'd post it up.
Over two nights I have only managed to get 13 useable subs and even last night I didn;t get a lot done before it became very hazy as I was out all evening  

Total 13 x 5mins Modified DSLR 450D 800 ISO, CLS filter
Guided with Skywatcher finder guider PHD and dithering in APT
NEQ6, Skywatcher Explorer 200PDS
DSS and Photoshop CS3
I don't have a coma corrector as yet and did not have a Bahtinov mask big enough   so was forced to use FWHM which I've never really used before

Still glad to be able to try it out.  
Unfortunately I missed the 3rd galaxy in this set but decided not to mess about trying to find it as time was a factor with clouds etc.

Why have I used the DSLR, well I wanted to compare data, and I think on the basis of this the SNR is much improved on previous images.  
Also, my light box for the ED120 won't quite fit the 200P and AV DSLR flats are easy to do with the daylight sky.

I've included the famous diffraction spike as a sort of signature for my first light.

[MarkS]

Congrats on the new scope - it's quite a big one and very different to what you've been using.  

It's difficult to comment on the image because it is a bit small to see much detail.   I think you'll probably need the coma corrector.

By the way, I like the diffraction spikes!

Why have I used the DSLR, well I wanted to compare data

No need to make apologies for a DSLR

Mark

[Carole]

Thanks Mark, yes I will get a coma corrector when I have the money for it.  But the point of focus is only 11mm away from the closest inward travel, so I am wondering how much space the CC is going to take up. 

On the current experiment the DSLR/Newt noise seems pretty much the same/less than the QHY8L on the refractors, it will be interesting to see how much noise the QHY8L produces with the Newt.  Plus of course I need to compare noiuse during the summer months too. 

I think I am going to like this telescope, so long as I get the hang of collimating, I understand it all now, just need a method of adjusting the rear bolts without having to look through the cheshire as I can't reach the rear bolts.  The 2nd hand laser collimator I bought is not quite in collimation itself and so far not found a satisfactory way of keeping it steady enough in order to collimate the collimator - any ideas on this, I've tried sitting it in a sellotape holder and rotating it but it just doesn't rotate steadily? 

I've seen a Baader laser collimator that I might look into.

Carole

[MarkS]

On the current experiment the DSLR/Newt noise seems pretty much the same/less than the QHY8L on the refractors, it will be interesting to see how much noise the QHY8L produces with the Newt.  Plus of course I need to compare noiuse during the summer months too. 

That's exactly what I would expect given your imaging conditions.  A cooled one shot colour astrocam will not give results much different to a DSLR in light polluted places in cold weather using relatively fast optics.  By the way, where I live now is still light polluted enough for this to be true.

To explain further what is going on, there are 3 main sources of unwanted noise that will degrade your images:
1) Read Noise
2) Noise from the skyglow (or light pollution)
3) Thermal noise caused by the dark current

In light polluted situations where the skyglow noise is dominant over the read noise and thermal noise then changing from a DSLR to a cooled OSC will make no difference unless the cooled OSC has appreciably higher quantum efficiency (i.e. if it is more efficient at capturing photons).

On a warm Summer night or if you are using a narrowband filter (e.g. H-alpha which reduces the signal and the skyglow) or using very slow F-ratios (which again reduces the signal and the skyglow) then the thermal noise of the DSLR will become dominant and it is then definitely worthwhile using the cooled OSC.

However if, instead of long exposure imaging, you are taking short exposures then the skyglow noise and thermal noise in each exposure will be reduced which means the read noise becomes dominant.  In these situations a DSLR (used at a low read noise ISO) will usually give better results than a cooled OSC because, generally speaking, they usually have lower read noise.

Mark

[Carole]

Thanks Mark,

Carole