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Guiding a Mak 180 for small DSO's...?


AndyUK
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Having spent a couple of hours last night on M57 with the 40D on the MN190 (the weather looked too dodgy to try out the new CCD camera :)), I should have realised that the FOV was way to wide - Okay, the star field's nice, but M57 itself is (of course) tiny...

I'm wondering if anyone's tried guiding a Mak 180 (or similar "slow" scope) for 20-30 odd minutes or so (and what results they got with what camera?).

Also, is there an equation for calculating the difference in exposures between different focal ratio scopes? Specifically, what would be the equivalent exposure on an f15 scope compared to 1 min @ f5.3?

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Each full f/stop doubles the exposure time required. f/15 is almost 3 stops down. So an f/5.6 equivalent 10 min exposure at f/15 is 80 min!!!!

Edited by johnrt
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IMO, the Skymax 180 is ideally suited for solar system imaging, it's too slow at f15 for DSO imaging. Plus with a focal length of nearly 3 metres, your guiding is going to have to spot on and have one of those rare nights when the conditions would allow you to image at that resolution and reap the benefits.

In terms of relative imaging times for extended objects (nebulae, galaxies), I've gone with the 'focal ratio squared' formula, ie: it takes 225 minutes for a Skymax 180 (15x15=2225) to get the same kind of image quality that an f5 scopes does in 25 minutes (5x5=25).

IIRC, SteveL used a barlow on his MN190 for M13 to great effect. That could work on M57 with your 314 I reckon :).

Tony..

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Cheers John - I had a feeling it was a case of "squares". I was anticipating that it might well need 30min subs... I wonder if anyone HAS ever gone for 80 mins subs?! :)

Tony - Yes, I really bought that scope solely for planetary work, but was just wondering if it might be feasible to put it to other uses as well. I did also think of using a barlow on the MN190, and at f10.6 (4 times exposure) I might get away with 20 mins as opposed to 30 mins, but I just wondered whether the addtional optics in the light path might be a trade-off against a longer exposure with a "straight" Mak 180 (and of course even narrower FOV), but I'd forgotten about the effects of seeing... :p

As you've also noted though, the additional sensitivity and smaller FOV of the 314 should help as well... I guess I can try the barlowed route first and see what I get. However, there's PLENTY of other objects up there without needing to try and make life even more complicated!

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As Tony says, on non point sources exposure time goes as the square of the f ratio. Not good news! However, going after those outer shells in narrowband would be worth a shout at f5. Then maybe you could Barlow it for the (L)RGB?

Olly

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Hi Uranium - You're probably right... but although I know the 314 will (sort of) halve the FOV, how do I get closer in than this?

EDIT: Ah... thanks for the input Olly... I haven't got the SII/OIII filters yet (or indeed really given the 314 a proper first light), so perhaps I should put M57 on the back burner for now and concentrate on something a little more "appropriate"...

post-18819-133877607969_thumb.jpg

Edited by AndyUK
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Thanks Olly - The start of those shells beginning to show is the reason for wondering how to get in closer... As you said the 314 will help, but I was really after more FL (without having to buy yet another new scope of course!). Using CCDCalc, in theory the Mak 180 would do a pretty good job, but I now acknowledge that the seeing would have to be amazing to use it on this... (and knowing my luck, I'd be away that evening!)

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I use a skymax 180, guided with an ed80. I have imaged m57 with this setup using a eos450. Whilst it's not the ideal imaging setup, I have been able to get a fairly good image, at least for me anyway. I tend to use upto 3min exposures and plenty of them. I would suggest trying it and see how you get on.

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Hi Keith - Thanks for that... To be honest, I feel quite bad that the Mak 180 has only made it out once in the last 2+ months - I missed Saturn (it skimmed across the roof of the house and is now in the streetlight zone) and with Jupiter not about for another few months, I'm beginning to wonder why I bought it(!), hence wondering if I could use it to get in close on any of these tiny DSO's with the small FOV the 285 chip will provide. When I've got used to the camera a bit more, I might well give it whirl just to see what I can get...

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It's definitley worth a try. I have the 180 and also the ed80. For small objects, the long focal length of the mak makes the images a better size on my canon.

If you are interested, take a look at my website www.imagingexplained.co.uk, there are number of images in the gallery section showing what I can achieve with this scope. Note however that I am a relative beginner so I have a long way to go still.

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Hi Keith - Thanks again - I've just had a quick look... That's great what you've managed to do with the Mak 180! Obviously it's really a planetary scope, but you've certainly made it work on more than a few DSO's... It's cerrtainly given me a target to aim for certainly...

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OAG + good guiding (usually very short exposures - quick correction but of very low strenghts) and it should be doable on HEQ5 or better. I did guide C8 on f/6.3, and also C11. On around f/10 for C11 and HEQ5 the guiding is at it limits for HEQ5 - motors just move more than needed, like it's over it motor-resolution.

surface-bright very small planetary nebulae usually could be imaged without guiding using planetary cameras and alike - by gathering a lot of short exposed frames.

http://www.rkastrofoto.appspot.com/site_media/astro/orig/sombrer-22-04-2011/fixsomb1.jpg - 2800 mm :)

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Cheers Rik - I think the OAG bit would probably kill me at the moment (something else to learn :)!). However, you Galaxy image is excellent and again certainly shows that it IS possible... Thank you!

Admittedly, I hadn't thought about trying the small planetary nebs with a planetary camera as I don't think my Phillips web cam is really up to it (or sensitive enough?)

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DMK or something similar. It's not about sensitivity (that also), but about gain levels, which in such cameras are high, so even at very short exposures you will see PN on the frame, and as O-III sometimes may be very bright - you can take the advantage of fast shutter doing exposures below 1 sec. And small frame is small in size - which is easier to stack than a lot of bigger frames.

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