Exposure and stacking

[GrampyG]

I suppose the answer to this one is the same as "how long is a piece of string".

I have just taken the position of an SW 80 ED together with an AVX mount plus guide scope/camera. This combo is somewhat different from my CPC 800.

I got the kit to start AP but need some broad guidance on exposure lengths for, say, nebulae plus number for stacks to start with. I'm using a Canon 1200 D (modified). If you could point me in the right direction I would be most grateful.

One more point-could a 2x doubler be used to increase magnification on the camera - if reqquired?

Many thanks

[wxsatuser]

I like to get at least 2hours of 5minute subs, the more the better of course.

The best way to judge the exposure for your sky conditions is with the histogram.

For whatever settings you use get the peak of the histogram somewhere in the range of 20 to 40%.

Here is one I did for a ten minute exposure at f4.5 iso800 using a Canon 60Da EF 200mm L lens.

The white peak is approx 25%

[Marky1973]

A lower number of total subs will still give you results but I am learning, as Mike says above, that more is definitely better. In fact, I noticed that 50 minutes of 2 minutes subs from my old set-up isn't that much poorer than 50 minutes of 5 minute guided subs on my new mount - so I can easily begin to see that I need to be upping the number of subs. Need to start playing around with the histogram properly to make sure I am getting the best for each exposure. So get the exposure right, chose the length of sub, and then get as many as you can I think is the simple answer!

But be careful - you obviously don't want the same length exposure on a bright Globular Cluster as you might on a faint DSO....and then you start to get into discussions around multiple stacks of different exposure lengths, layered in Photoshop/GIMP to balance fainter detail with brighter cores... makes my brain ache....

[lensman57]

I suppose the answer to this one is the same as "how long is a piece of string".

I have just taken the position of an SW 80 ED together with an AVX mount plus guide scope/camera. This combo is somewhat different from my CPC 800.

I got the kit to start AP but need some broad guidance on exposure lengths for, say, nebulae plus number for stacks to start with. I'm using a Canon 1200 D (modified). If you could point me in the right direction I would be most grateful.

One more point-could a 2x doubler be used to increase magnification on the camera - if reqquired?

Many thanks

With a few exceptions, M42,M31 ( here you have to use multiple exposure for the cores and the surroundings ) and so on for the rest of the targets in the sky the longer the sub length the better provided that your sky back ground and local LP do not overwhelm the signal. The larger the number of the subs the better the end result as the stacking algorithm will have a chance to build signal and therefore give you a better S/N. With Dithering During capture ( mandatory for DSLR ) and Sigma Slipping rejection you will also be able to remove the odd satellite trail and other nasties provided that you have enough subs usually 20+.

A.G

[cathouse42]

I suppose the answer to this one is the same as "how long is a piece of string".

I have just taken the position of an SW 80 ED together with an AVX mount plus guide scope/camera. This combo is somewhat different from my CPC 800.

I got the kit to start AP but need some broad guidance on exposure lengths for, say, nebulae plus number for stacks to start with. I'm using a Canon 1200 D (modified). If you could point me in the right direction I would be most grateful.

One more point-could a 2x doubler be used to increase magnification on the camera - if reqquired?

Many thanks

Length of piece of string = distance from center to edge * 2.

[lensman57]

In my post there is a typo and it should read Sigma Clipping not Sigma Slipping. I thought that I had corrected  the error in the edit but obviously not.

A.G