M42 advice

[ScubaMike]

I would like to attempt to capture M42.

I will be using an ST-150 (f5) with a 0.5x reducer, onto a DMK21A_04.AS. Per 12Dstring website, I should get a fair portion of the object.

As the DMK is mono, I will be using LRGB filters (kindly donated by Parallax on this site). I'm hoping this will overcome the inherent CA of this scope.

I am unguided, but have reasonable polar alignment (tracks Jupiter at F20 for hours without moving off the DMK sensor).

Can someone suggest exposures I should use for each channel?

This will be my first attempt at a DSO, hence the choice of target.

Any comments on the approach welcomed.

[Catanonia]

Jeez, F5 with reducer = a very fast scope.

Your collimation had better be bang on, trust me on this one.

My thread in this section on Orion was 3 min subs on a 200mm Newt running at F2.8 to give you some idea.

[rocketandroll]

As Cat says..... I'd be careful with trying to use such a massive reduction on an already fast scope!

The ST150 is the 6" achromatic refractor SW do isn't it? It has a native focal length of 750mm so you probably don't need a reducer to fit all of M42 in... I got a shot that fitted the whole main nebula, plus the running man nebula next to it in at 950mm focal length.... admittedly with a big chip DSLR... but even with a smaller chip CCD I'd think you'd get it all in at 750mm fl?

Trying to drop a frac down to f2.5 is likely to introduce all manner of wierd optical effects, not to mention making focus almost impossible.

Stick with it 'as-is' at f5 and go for subs of about 1min or so across the board, though RGB isn't something I've tried yet. Is the CGEM mount you have an alt-az or an EQ? If it's Alt-Az without a wedge I think you'll be limited to a minute or two anyway or field rotation will creep in.

M42 is quite strong in red and the running man has a lot of blue in it (as do the stars around the sword). The green channel will probably be the weakest.

Have a play.... collect as much data as you can with as long exposures as you can get away with without star trailing and see what you get.... oh, and if you can shoot a set of short subs too for the core at about 10 - 15 seconds that always helps bring out the detail in the brightest bit.

Ben

[jgs001]

Mike, you'll also need to check and refocus on changing filters, yes it will sort the CA, but you'll have to work for it.

[ScubaMike]

Thanks,

The CGEM is an EQ mount.

I was trying to scale the image so that it would fit on the sensor in a single image (avoiding mosaic for the first attempt).

The 0.5x reducer did resize to the sensor almost exactly.

Without the reducer, I am only going to get a small part of the target.

(I am relying on 12Dstring website for my sizings, as it has been reliable before).

[jgs001]

Try it and see Mike... if it turns out to cause problems, or you can't get focused... then remove it and try a mosaic ?? If it does work, that'll help a lot on the sub length

[ScubaMike]

Thanks John,

I will refocus at each filter change.

Nice to know I can work round the limitations of the scope.

Maybe I need to rethink my target if using a reducer will cause more problems than it solves. I would like to capture a nebula of some kind for my first DSO... suggestions?

[ScubaMike]

Try it and see Mike... if it turns out to cause problems, or you can't get focused... then remove it and try a mosaic ?? If it does work, that'll help a lot on the sub length

There is a lot of "suck it and see" in this, isn't there

[jgs001]

There is at that

M1 - The Crab looks to be a nice fit, but I'm not sure that'll be an easy one without longer exposures. I suspect the Bubble would also be a good one, similar problem though.

[rocketandroll]

M42's your best bet... I'd say aim it at the trapezium and see what you get... even a nice detailed image of the core would be pretty impressive if the whole thing won't fit.

I just ran it through 12D string (which was having some serious issues, though that may be due to my useless connection speed)... I just realised your chip is teeeensy, tiny and 640 X 480... so yes, it will only grab a tiny bit of the field of view.

Assuming you don't have another larger chip digital camera to use to try it with... not sure what to suggest... I'd think even if you did get the whole nebula in the chip, you'd be screaming for more resolution.

M27 or M1 are both about a perfect fit on that chip through that scope... though I think 27 is just about out of reach now until later in the spring.

Likewise, M20 is about right too... but that's a June/July and 'pray you have a good view to the south west' object :-)

Or M16... again, a summer object but looks like you could get your own low-res 'pillars of creation' on that chip quite nicely.

Maybe, try centering the image on M43 instead of M42? People often forget it's a seperate Messier object and I've not seen any nice detailed close-ups of it other than as part of M42?

Or go for a globular cluster of a bright galaxy?

M51 appears to be 'made for' that combo in terms of size!

Ben

[ollypenrice]

An ST150 is already fast and full of CA. If you reduce it further...??

I don't follow the filter argument. The blue filter will simply select the CA and remove the rest, to my mind. And the L filter will pass the lot. To diminish the CA of a poorly corrected scope you need to move towards monochromatic light, ie a narrowband filter. Then, since the focal length will change between colours, you really need a stacking software that will resize each layer. I'd use Registar for this because that's what I have.

Your best bet is in the red channel, I'd say.

But all this is theory and the best bet, as above, is just to have a go and prove the theory right or wrong. In my own work I always say that I don't hang the guide trace on the wall...

Have fun and if you only get a bit of clear sky start with the red.

Olly