Advice on mosaic plan for Rosette

[powerlord]

Hi all,

Just looking for some advice as I plan my first attempt at a mosaic for the next clear night.

I'm planning a 4 part Rosette. planned in telescopia with coords ready to input into a plan in asiair pro.

I've got two choices to make:

scope:

either my 72ED + 2x ED barlow (840mm or so with the flattener)

or my SW200P (1000mm)

Obviously the newtonian is faster, but it is also a PiTA to use and means I've got diffraction spikes.

camera:

either I shoot with my asi1600 +EFW and do NB there

or I shoot with the asi533 and L-extreme, with maybe an extra pass at the the with just an Sii filter over it.

I'm thinking, best quality wise, probably SW200p and asi1600 is the way to go, but it does mean lots of heavy stuff on my EQ6-R, lots of faff setting up and taking apart over however many nights it needs, etc.

Compromising and using the 72ED+ 2x ED barlow makes it much more usable, but I'm wondering it if will push me down the asi533 road if I hope to get results without having to spend 20+ hours on it.

Obviously 'try it and see' is the default approach, but as this will take ages, I'm for once inclined to ask for some advice first.

FL wise... SW200 is F5, 72ED with barlow is gonna be, F11ish.

so 25 vs 121. Or about 5 time more light into camera with the SW200 as the 72ED.. which I imagine... all other thing being equal, would mean I need 5x as much integration to get the same results with the 72EDx2... ?

stu

[vlaiv]

Out of interest, you seem to want to do 2x2 mosaic and plan on doing that with 72ED and x2 barlow?

How about simpler approach? Loose the barlow and do regular image?

[powerlord]

done that - trying to get a higher resolution job, especially for more detail around carnival of animals.

[vlaiv]

Odds are - you are not going to get more detail with 70mm scope.

What is your average guide RMS on EQ6R?

In average / good conditions with 2" FWHM seeing and 0.8" RMS guiding, you can expect about 3.2" FWHM stars - that corresponds to 2"/px sampling rate.

With both cameras that is about 380mm of FL (unless you bin). That is about best in terms of detail that you can achieve give those conditions and 70mm scope. 840mm FL is of course overkill.

200mm scope, given same conditions will allow for 1.75"/px.

ASI1600 bin x2 will give you close enough - 1.57"/px

I'd go with that combination.

If you feel that you'll get wanted detail, don't let my above post discourage you.

[powerlord]

With the 72ed, I get about 0.3" rms. Seeing on a good night here I think is better than 2".but to be honest, one reason I want to try it is to find out for myself - are such mosaics worth it for me here. I suppose I'm likely to find that out quickest with the sw100, but it is far more tricky get get the guiding rms optimised for that. With the 72ed it's easy.

If I'm binning it's failed IMHO. I'm gaining  nothing with the mosaic really.

Which is what your saying.

With the long nights it's an easy enough experiment so I'll update with progress 👍

[newbie alert]

200p will give a better resolution out of the 2.. or if you want to make things easier on yourself then use the 72

Can't see the point of a barlow for deepsky on any set up myself

[powerlord]

2 minutes ago, newbie alert said:

200p will give a better resolution out of the 2.. or if you want to make things easier on yourself then use the 72

Can't see the point of a barlow for deepsky on any set up myself

drat.. you had to say that last line.. now I feel I need to go and prove you wrong. well right probably.. but still.. it's a challenge ! 😉

[newbie alert]

2 hours ago, powerlord said:

drat.. you had to say that last line.. now I feel I need to go and prove you wrong. well right probably.. but still.. it's a challenge ! 😉

You only have to look through a ep, add a Barlow does the image get brighter or dimmer? The 72 should cover the majority of the rosette, especially with the 4/3rds sensor that I think?? The 1600 has??

[powerlord]

It doesn't matter that it's dimmer - just means more integration time. It's not visual. The 72ED and 1600 covers it fine, but I get 12mp. A 4 panel mosaic gets me 48MP (well overlap, etc more like 40mp). the brightness of the image has nowt to do with the resolution of the image. It comes down to seeing as Vlaiv says - But I'd like to think I can eek a little bit more details from a mosaic over my 12mp asi1600 that's all. I could be wrong, only know if I give it a go really.

[david_taurus83]

I can't say it will work out well with the barlow. Your doubling your f ratio and will need much longer subs to get above the noise. With the newt you have aperture and f5 so that's what I'd be using if you want resolution.

[newbie alert]

3 hours ago, powerlord said:

It doesn't matter that it's dimmer - just means more integration time. It's not visual. The 72ED and 1600 covers it fine, but I get 12mp. A 4 panel mosaic gets me 48MP (well overlap, etc more like 40mp). the brightness of the image has nowt to do with the resolution of the image. It comes down to seeing as Vlaiv says - But I'd like to think I can eek a little bit more details from a mosaic over my 12mp asi1600 that's all. I could be wrong, only know if I give it a go really.

Yes I agree that the brightness comes from the aperture,  but you doubled your fl and kept the aperture the same. We're all seeing limited

[vlaiv]

55 minutes ago, newbie alert said:

We're all seeing limited

It's not just the seeing. It is combination of seeing, mount performance and aperture used.

If we compare 72mm aperture and say 200mm aperture in Airy disk diameter - you'll see quite a difference there.

First has 3.56" while second - 1.28" Airy disk diameter.

First one is comparable to 1.5" seeing alone, while second corresponds to 0.5" seeing.

[The Lazy Astronomer]

It's another vote for the 200p from me. By all means do a comparison on one panel between the 2 scopes though, but the theory says the 200p would be the clear winner. 

Just looking at optimal sub exposure time for the barlowed 72 actually:

sub exp time = C * read noise^2 ÷ sky background in e-/px/s

For a 1.5e- read noise mono cmos with 3.8um pixels and 60% qe, in fairly typical suburban bortle 5 skies with a 7nm nb filter you'd be looking at almost 20 minute subs for the sky background to swamp the read noise (C factor = 10). That puts quite a lot of pressure on the mount.

In contrast, the with the 200p, you'd be looking at subs of just under 4 mins in length to swamp the read noise by the same factor.

Then there's also the resolving power of the scope (maybe not the correct terminology here) - a barlowed small scope will certainly give you a larger image, but will it record twice as much detail than at native focal length? I doubt it.

[powerlord]

Thanks all. 200P it is. sigh.. big bl00dy thing.

anyway, forecasts are so bad it would appear there is not danger of this happening soon 😞