Question, RA/Dec Motored EQ3-2 + Dovetail + B/S head

[RichM63]

I have a basic starter question.

Having the mostly unused above mount sitting forlornly in the corner and wanting to be a bit more ambitious, than just using my static tripod.

I picked up a 2 week old 50mm 1.8 mk2 and have just ordered an 8kg Ball Head + Q/R plate (from reading set up advice here).

My simple'ish' question, with clear, dark, no LP skies, fully motorized but unguided.

What settings would be a good starter guide for widefield with Orion roughly in the centre of FOV. This is clearly visible naked eye with the milkyway showing nicely.

Camera, 700D APS-C;

Lens, 50mm 1.8 mk 11

ISO, ??

f stop, ??

Shutter, ??

Length of each exposure, ??

Number of exposures, ??

I kow that factors such as PA, seeing, etc play a role, but let's take these out of the mix, the PA is down to me, and I can't do much about 'seeing'.

Thanks in advance.

[Knight of Clear Skies]

The 50mm lens is probably best at f4.0 ior f4.5, f3.5 is useable but there is some visible coma. Below that the corners start to suffer badly although on some targets cropping may be an option.

PA is very forgiving at 50mm, I was consistently getting 2 minute subs before I bought my polar scope. I have more trouble with periodic error than PA with my EQ3-2 but have managed 4 minute subs at 50mm. Exposure wise, the advice I've read is that you should tune the ISO level and exposure time so that the subs are peaking no more than 40% across the histogram. This makes it easier to check subs on the camera and helps prevents any bright details from over-exposing and blowing out.

Last time I used it I chose ISO 1600 and took six 120 second subs to get the second image (of Cygnus) in this thread, but that was from a very dark site. Hope that's some help.

[wxsatuser]

I tend to use ISO 1600 with my 60Da, seems to work the best at that.

The exposure needs to be long enough to get the faintest data out of the read noise.

Aim to get the hump of the histogram up near at least 25%.

So for any ISO and lens setting just expose so the histogram is around 20>40%

Depends how good an image you want but I would look for a minimum of an hour to 2hours of exposure, more if you can get it.

[RichM63]

Right ho, so f4 -f4.5, 20 - 40% on the histogram (ISO 800 - 1600) for 25% minimum, 2 - 4 min subs, and as long as possible on target.

That image of Cygnus was what sent me down this Crevasse in the first place! LOL

[Knight of Clear Skies]

Yes, I remembered that after I posted.

[Alien 13]

I agree with the posters above the newer Canon cameras do seem happy at ISO 1600 but as said if you are in a light poluted area then go for ISO 800 the maximum exposure time you can achieve is more critical.

I am one of the few who doesn`t rate the nifty fifty that well (sorry) maybe my sample was a rougue with very bad coma and was almost impossible to focus I much prefer the kit lens well the IS version which with mine is very happy wide open with very little if any coma it is a bit slow though.

Alan

[ollypenrice]

One good trick with most lenses is to focus on a star on one of the four points where the I/3 lines cross. (The four lines parallel with the edges and a third of the way to the opposite side.) This gives the best compromise and reduces stellar disortions across the image at any F ratio. It can allow you to open up by a stop or even more than that sometimes.

As a beginner it may not be obvious to you how critical is perfect focus. To find it zoom right in as far as you can (into pixelation if poss) to get the star as small as possible. Take plenty of time on this.

However long your subs are you'll find an improvement till you get into the thirties of subs after which it does tail off. Usng a Sigma Clip (AKA Sigma reject) is prodigiously powerful once you have a large number of subs.

If you choose Orion here's another good trick in processing; in Photoshop go to Image, Adjustments, Selective Colour and, in the reds, push the top slider to the left. It makes the Ha pop. Barnard's Loop here you come! http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-gfQmRVJ/0/X3/ORION%202014%20reprocessWEB-X3.jpg

Olly 

[RichM63]

Cheers, Olly. With DSS would this be KSC or MKSC and am I correct in thinking that 'large number' = 50+?

Paragraph 1, = imaginary noughts n crosses board and the 4 points denoting the central square?

I understand the critical nature of focus, thanks for the tip of how to get there, presumably on a high res large laptop screen is going to be better than on the back of the camera.

Cool processing tip, duly written into my 'notes' file. Thanks. (just tried it on a Sunrise to see the effect, certainly adds drama).

[Alien 13]

Funnily enough i prefer the camera live view screen to a laptop (if it is a flip out screen) focus is easy if you use zoomed in liveview in combination with ISO to bring a faint star to the point where its only just visible then the focus action becomes more like an on/off switch the alternative is to go for a bright star and use ISO/shutter speed so get the same effect.

This method removes any guesswork in judging size/brightness.

Alan

[Knight of Clear Skies]

Funnily enough i prefer the camera live view screen to a laptop (if it is a flip out screen) focus is easy if you use zoomed in liveview in combination with ISO to bring a faint star to the point where its only just visible then the focus action becomes more like an on/off switch the alternative is to go for a bright star and use ISO/shutter speed so get the same effect.

That's a good tip thanks. I was forced to focus on a dim star the other night and seemed to get a better focus as a result.

[RichM63]

I'll give that a go as well, I got used to focussing the old webcam on Jupiter's moons by varying the gain etc to obtain a pinpoint. So understand what is behind the method.

[Alien 13]

That's a good tip thanks. I was forced to focus on a dim star the other night and seemed to get a better focus as a result.

It does work i discovered it by accident but it does ensure that all the very faint stars in an image are pretty much a single or a few pixels wide it also helps with CA on the brighter objects I do also make sure the target star is off center as described in Olly`s post. 

Alan

[wxsatuser]

Cheers, Olly. With DSS would this be KSC or MKSC and am I correct in thinking that 'large number' = 50+?

Paragraph 1, = imaginary noughts n crosses board and the 4 points denoting the central square?

I understand the critical nature of focus, thanks for the tip of how to get there, presumably on a high res large laptop screen is going to be better than on the back of the camera.

Cool processing tip, duly written into my 'notes' file. Thanks. (just tried it on a Sunrise to see the effect, certainly adds drama).

Think it's anything over 12 subs and KSC is very useful.

I use it because of the many aircraft that fly over here, plus it gets the rogue satellites as well.