A trip round some winter nebulae

[GavStar]

This year the clear nights when I have been able to observe seem to have fallen around the full moon. So I’m eager to take every opportunity I get when the moon is not around to observe. 

I’ve also today received my astrophotography atlas today and looking at their 93 must image objects, I can see that soon many of the good winter nebulae will start to disappear.

I had about an hour before the clouds came in tonight, so I quickly set up my tec160 and just focussed on NV with my 55mm plossl.

Rather than describe what I saw, I’ll let the unprocessed iPhone photos do the talking as they are a good indication of the eyepiece views. It was interesting to see which objects showed up well and which were a bit disappointing.

In order

1) California 

2) Flaming Star 

3) Medusa 

4) Jellyfish 

5) Rosette

6) Crab

7) Monkeyhead

8) Seagull

 

[Stu]

Great shots Gavin.

I confess I still haven't stopped chuckling about the Monkeyhead Nebula, it looks just like, well, a monkey head! Brilliant!  

Jellyfish looks pretty good too. California will be spectacular once you've got the wider field.

[Doc]

What is the exposure time using your iphone?

Nice photo's btw, as you say very similar to the views one gets at dark sites.

[GavStar]

13 minutes ago, Doc said:

What is the exposure time using your iphone?

Nice photo's btw, as you say very similar to the views one gets at dark sites.

Thanks Doc. 15 seconds exposure. 

I think I have the gain up too high on my NV for the iPhone photos - the California photo was lower gain and I think it looks less washed out

[GavStar]

27 minutes ago, Stu said:

Great shots Gavin.

I confess I still haven't stopped chuckling about the Monkeyhead Nebula, it looks just like, well, a monkey head! Brilliant!  

Jellyfish looks pretty good too. California will be spectacular once you've got the wider field.

Yes, the monkeyhead has become one of my all time favourite objects - I’d never heard of it a week ago. Amazingly bright considering.

[PeterW]

Try ced214 in Cassiopeia before it heads off till autumn. You also missed the Pac-Man, good shots. The jellyfish looks very impressive, even a little bit “shrimpy”, we need to try with faster optics under better skies. 

Peter

[GavStar]

2 minutes ago, PeterW said:

Try ced214 in Cassiopeia before it heads off till autumn. You also missed the Pac-Man, good shots. The jellyfish looks very impressive, even a little bit “shrimpy”, we need to try with faster optics under better skies. 

Peter

How fast do you think is ideal, Peter? These were at f3.5.

[PeterW]

As fast as she’ll go.. NV objectives and cat lenses can run at f1.5. I wonder what would happen with the 55mm on my f4 newt. You’ve truly joined the “dark side”.

Peter

[jetstream]

4 hours ago, PeterW said:

we need to try with faster optics under better skies. 

What exactly do the faster optics provide for the NV? does aperture matter?

[jetstream]

5 hours ago, GavStar said:

So I’m eager to take every opportunity I get when the moon is not around to observe. 

Great report Gavin!

[PeterW]

NV is like a camera, faster produces brighter images for a given exposure time... for diffuse objects. Aperture only matters for point sources. For the biggest objects and to get an overview the aperture in only about 25mm. NV is a definite cure for (raw) aperture fever.

 

PEter

[Stu]

1 minute ago, PeterW said:

NV is like a camera, faster produces brighter images for a given exposure time... for diffuse objects. Aperture only matters for point sources. For the biggest objects and to get an overview the aperture in only about 25mm. NV is a definite cure for (raw) aperture fever.

 

PEter

Presumably aperture will be important for smaller objects though Peter, so you can combine a fast focal ratio with longer focal length for larger image scale?

[GavStar]

I’m not a photographer so this is a learning experience for me. It certainly seems that in NV world generally you need to think in photography terms rather than the normal visual astro approach.

I’m getting a photo visual telecompressor (aka reducer) for my refractors today with the aim to increase the speed of my system still further. 

For my 160 refractor the speed will be reduced from f3.5 to around f2.5, which I think means that around twice the light will get to the sensor and result in evern brighter images of the dim objects. 

Unlike photography, when observing in NV you can’t change the exposure time (clearly) so you need to speed the system up to get more light in.

However, the downside of increasing the speed is that the magnification of my system (using my 55mm plossl) will reduce from 20x to 15x (and fov will increase) which will make smaller features such as the horsehead and the cone more difficult to see. For big objects, smaller scopes are good because you can get fast speed and big fov. You can’t change the fov and keep the speed without changing the scope since the NV monoculars are a fixed 40 degree fov eyepiece.

But for smaller objects, eg galaxies, planetary nebula, ideally you do want image scale as Stu says. In that case you still generally want fast speed so the answer is to buy a bigger scope - aperture fever again.

In practice, what seems to happen is that NV users in the US have 2-3 scopes to use with NV (for very big, big and smaller objects) and just the one eyepiece, which is for US users the monocular itself, for me it’s a 55mm plossl attached to my NV monocular. (US tend to use the monocular direct into the diagonal which gives a 27mm eyepiece)

I use this different approach because in Europe we are restricted to what NV monoculars we can buy. US users prefer direct because they say having an eyepiece and the NV together in the diagonal is unwieldy- however I find it fine, the NV eyepiece is really light and not that big compared to some of the heavyweight eyepieces. 

NV is a bit strange - you want a change in magnification and image scale - don’t change the eyepiece, change the scope!

[alanjgreen]

2 hours ago, Stu said:

Presumably aperture will be important for smaller objects though Peter, so you can combine a fast focal ratio with longer focal length for larger image scale?

For the smaller stuff, focal length combined with speed is what you need. I think a c11 with 2800mm focal length and a 0.63 focal reducer to add more speed should be good with this kit.

An sct does not care about the 40 degree fov limit either. 

The problem on a big dob will be all the nudging at 40 degree fov unless it's driven but then it's big an unwieldy,

A cpc1100 or even cpc800 (compact) would be worth a go with this kit. The focal length means you can get some magnification which for most objects you actually need.

A 55mm plossl will be under f4 in an sct. Wide field nebula sorted.

[GavStar]

Now this is a fast SCT

https://www.firstlightoptics.com/optical-tube-assemblies/celestron-rowe-ackermann-schmidt-astrograph-telescope.html

No idea whether it would be possible to stick a diagonal on the back rather than a camera though??

I have read that there is an issue with using a 2inch diagonal together with a focal reducer on SCTs so it may mean that a 55mm plossl cannot be used and only a 40mm one could be used which would limit  the speed to about 4.2

[alanjgreen]

12 minutes ago, GavStar said:

Now this is a fast SCT

https://www.firstlightoptics.com/optical-tube-assemblies/celestron-rowe-ackermann-schmidt-astrograph-telescope.html

No idea whether it would be possible to stick a diagonal on the back rather than a camera though??

I have read that there is an issue with using a 2inch diagonal together with a focal reducer on SCTs so it may mean that a 55mm plossl cannot be used and only a 40mm one could be used which would limit  the speed to about 4.2

It only has a 620mm focal length plus it's HUGE. It dwarfs a c11 in physical size!

Its as if you need a x4 secondary mirror (primary mirror in sct is f2 native with the secondary being a x5 magnifier that drops the speed to f10) on the sct rather than the standard x5. You would trade off one fifth focal length to get an f8 scope. 

The RASA must have a fundamentally different secondary mirror to a standard sct as it only changes the speed by x1.1. F2 to f2.2.

[GavStar]

Alan, it’s aperture that matters not focal length. To get the required speed for a sct you use a focal reducer which effectively reduces the focal length of the sct.

For example, using a 6.3 focal reducer with the 800 sct and then the 55mm plossl results in a magnification of 23x, no real difference to the 55mm in my 160 frac of 20x. The f ratios of both this setups are broadly the same (3.1ish vs 3.5). 

For a 1100 sct using a focal reducer of 6.3 gives a focal length of 1760 and using the plossl on this gives 32x, better but not really the jump we are after.

The guys in the states use eg 20 inch f3 dobs (see the link)

 http://www.loptics.com/articles/nightvision/nightvision.html

Then they don’t need the 55mm plossl, maybe 27mm is ok due to the speed, which would give 60x magnification. At f4.5, they could use an 18mm eyepiece and get 90x.

[alanjgreen]

20 minutes ago, GavStar said:

Alan, it’s aperture that matters not focal length. To get the required speed for a sct you use a focal reducer which effectively reduces the focal length of the sct.

For example, using a 6.3 focal reducer with the 800 sct and then the 55mm plossl results in a magnification of 23x, no real difference to the 55mm in my 160 frac of 20x. The f ratios of both this setups are broadly the same (3.1ish vs 3.5). 

For a 1100 sct using a focal reducer of 6.3 gives a focal length of 1760 and using the plossl on this gives 32x, better but not really the jump we are after.

The guys in the states use eg 20 inch f3 dobs (see the link)

 http://www.loptics.com/articles/nightvision/nightvision.html

Then they don’t need the 55mm plossl, maybe 27mm is ok due to the speed, which would give 60x magnification. At f4.5, they could use an 18mm eyepiece and get 90x.

That may be correct for large nebula. BUT you need the focal length to get the magnification (for other objects). Not with >mm plossls but with small <mm delites.

I was thinking of using delites down to 7 or 9mm in a c11.

 

    As the NV on its own gives (1760/27=x65 native magnification @f6.3)

    a 9mm eyepiece gives (27/9*65 = x195)  27/9*6.3 = f18.9

    a 13mm eyepiece gives (27/13*65 = x135) 27/13*6.3 = f13

    a 18.2mm eyepiece gives (27/18.2*65 = x96) 27/18.2*6.3 = f9.3

 

1) Globular clusters can take nearly any f-ratio, from f1.2 to f24 or higher.  Stars hold up really well with NV tech.

2) Galaxies - edge-on's hold up better than face-ons.  I'd say up to f12 works well on the brighter edge-on's e.g. M104, NGC 4564, M82.  Go faster with face-on's.

3) Planetaries - small bright ones hold up well (e.g. NGC 6210) even at f24, others appear better at faster than f10.

4) Emission nebulae - with a narrow band H-alpha filter, faster is always better.  Slower than f10 takes away alot of structure.  Without the H-alpha filter, as slow as f10 still works on some targets.

 

 

[GavStar]

As Peter says earlier, faster is always better for NV, no matter what the object.  Some objects withstand higher f ratios better than others but it will always have an impact. The link I posted to earlier has 6 photos of the same object taken at increasing f ratios and it’s clear how the quality drops off as the f ratio increases.

To get the higher magnifications , the f ratio is increased significantly. I can get use a 7mm delite on my setup to get 160x but the f ratio becomes 27 so the objects are too faint. To keep the f ratio lower I need bigger aperture, not longer focal length.

The info you posted above from an NV poster on cloudynights regarding the different categories of object and which f ratios work is a personal view from him. I will certainly be doing my own experiments on globulars, galaxies, and planetaries to see what works for me. But I think I’m likely to max out at f10 (60x) with my 18mm delite and not want to go to any higher f ratio. It’s going to be trial and error and seeing what I like.  I’m not expecting to be able to see spiral arms on the face on galaxies since I think face ons will need a low f ratio to see.

I know for nebulae (apart from m42) I need to keep the f ratio below 4 to get the nebula structure visible. In fact my main aim is to get even lower f ratio, around 2, to see more of the fainter stuff even though the magnification will be lower.

So there is a compromise between going faster and getting better views or getting image scale but with fainter views and less detail. If you want to keep faster for good views of fainter stuff and still have image scale then you need bigger aperture.

For galaxies, although with f10 you may be able to still see brighter galaxies like m81,m82, the view will have less clarity than using f4 or less. 

From my experiences so far, I think NV excels on nebula, particularly large nebula, not so much on galaxies.

[Stu]

1 hour ago, alanjgreen said:

That may be correct for large nebula. BUT you need the focal length to get the magnification (for other objects). Not with >mm plossls but with small <mm delites.

I was thinking of using delites down to 7 or 9mm in a c11.

 

    As the NV on its own gives (1760/27=x65 native magnification @f6.3)

    a 9mm eyepiece gives (27/9*65 = x195)  27/9*6.3 = f18.9

    a 13mm eyepiece gives (27/13*65 = x135) 27/13*6.3 = f13

    a 18.2mm eyepiece gives (27/18.2*65 = x96) 27/18.2*6.3 = f9.3

 

1) Globular clusters can take nearly any f-ratio, from f1.2 to f24 or higher.  Stars hold up really well with NV tech.

2) Galaxies - edge-on's hold up better than face-ons.  I'd say up to f12 works well on the brighter edge-on's e.g. M104, NGC 4564, M82.  Go faster with face-on's.

3) Planetaries - small bright ones hold up well (e.g. NGC 6210) even at f24, others appear better at faster than f10.

4) Emission nebulae - with a narrow band H-alpha filter, faster is always better.  Slower than f10 takes away alot of structure.  Without the H-alpha filter, as slow as f10 still works on some targets.

 

 

Hi Alan,

Can you let us know how you derived the above info? Which scopes have you used NV gear in? Would be interesting to hear.

Thanks

[alanjgreen]

18 minutes ago, Stu said:

Hi Alan,

Can you let us know how you derived the above info? Which scopes have you used NV gear in? Would be interesting to hear.

Thanks

I got the above f ratio stuff from @GavStar who pulled it off cloudynights.

I have not used NV as yet, still saving up...

I am researching what scope I would need, if I go for it (probably need price to come down before I jump in). From what I have read, SCTs seem a good match for galaxy viewing.

 

[Stu]

2 hours ago, alanjgreen said:

2 hours ago, Stu said:

 

I got the above f ratio stuff from @GavStar who pulled it off cloudynights.

 

Ah, ok. Seemed strange quoting back to him stuff which he had told you, that's where I got confused, I thought it was info you knew 

Gavin's emphasis has been around nebulae mainly so SCTs less relevant for this I guess.

[Alan White]

Monkey Head? 

Looks far more like a Hedgehog face to me!

[PeterW]

Always thought of it as a lion... bit like Legends of Chima http://legolegendsofchima.wikia.com/wiki/Laval

Peter

[Stu]

7 minutes ago, Alan White said:

Monkey Head? 

Looks far more like a Hedgehog face to me!

It's funny what different people see in it. I was laughing at it once I saw the monkey's face, and showed Mrs Stu and she couldn't see what I was laughing at. Took her a little while to 'get it'. Of course I might be seeing something completely different to the official version but I'm happy with my version ?