f/8 enough?

LewisM · #1 04-07-2015, 05:04 PM

Is f/8 at least starting to be sufficient for galaxy imaging? I have an FSQ-106 with all the extras - reducer and extender, so I run the gammut of f/3.65, f/5 to f/8.

I did image some GX with the FS78 at native f/8 and the detail, scale and resolution seemed OK.

Opinions appreciated. Of course, I cannot bolt on ANOTHER extender to make it f/12.8....

RickS (Rick) · #2 04-07-2015, 05:46 PM

Lewis,

I'd be looking at your image scale rather than f/ratio to decide if you've got the right set up for imaging galaxies. Of course, aperture will determine how long it takes to capture decent SNR

A FSQ-106 @ f/8 would be OK for the larger galaxy targets. With drizzle you'll get halfway decent resolution. It will need a fair amount of capture time to get a quality image.

If you want to go for more exotic targets then larger aperture is needed unless you have a lot of good sky time and patience.

Cheers,
Rick.

ZeroID (Brent) · #3 04-07-2015, 07:55 PM

Quote:

Originally Posted by RickS

Lewis,

I'd be looking at your image scale rather than f/ratio to decide if you've got the right set up for imaging galaxies. Of course, aperture will determine how long it takes to capture decent SNR

A FSQ-106 @ f/8 would be OK for the larger galaxy targets. With drizzle you'll get halfway decent resolution. It will need a fair amount of capture time to get a quality image.

If you want to go for more exotic targets then larger aperture is needed unless you have a lot of good sky time and patience.

Cheers,
Rick.

Which is why I built the RN 8F8 Newt. 8" at F8. Weather has been extremely uncooperative for testing and I am build a guiding solution for it but some unguided 20 sec frames @ 1600 ISO on Eta C gave me a much better SNR in a fraction of the time. Enough for me to pull quite good results off a single frame. It seems to handle a 2" Barlow well ( F16 !! ) so the combo should give me some capability with smaller targets, planetary and DSO. F8 also gives me an almost coma free FOV and it's proved to be a very good mirror.

It's a sod in the Ob though, being 1600 long it nearly touches the walls

Shiraz (Ray) · #4 06-07-2015, 04:02 AM

depends on the camera Lewis. If you have small pixels, 800mm fl will give OK pixel scale, but it is not suitable (for galaxies) with large pixels. The 100mm aperture is small enough that it will slightly degrade the resolution in very good seeing and CA will probably also limit the resolution a bit.

at f8 and with small pixels, the system will be fairly insensitive and that will mean extended imaging times.

As Rick suggested, you should still be able to get good images of the bigger, brighter galaxies in acceptable time.

multiweb (Marc) · #5 06-07-2015, 08:03 AM

Got the FSQ106N at F/15 a couple of times and I could do 2min subs in Ha with good SNR. They are incredibly sharp scopes with no scattering and very good light gathering. I don't anticipate you'll see any major difference at F/8 from prime focus if you use a quality barlow or the Tak extender.

LewisM · #6 06-07-2015, 08:24 AM

Will be using the Tak Q extender.

Wanna trade your 106N for my 106ED? (I'll include the reducer and extender)

Appreciate the input all.

multiweb (Marc) · #7 06-07-2015, 08:31 AM

Quote:

Originally Posted by LewisM

Will be using the Tak Q extender.

Wanna trade your 106N for my 106ED? (I'll include the reducer and extender)

Appreciate the input all.

I've finally got all my adapters done.

Fat chance now. I'm not going through it all again. I'll stick with it

Atmos (Colin) · #8 06-07-2015, 08:43 AM

I personally try not to get stuck on f/ratio. You're always going to be sacrificing resolution (image scale) imaging speed unless you're willing to spend the money buying large fast scope.

Even if you need to increase it to f/20 and its corrected, go for it! It'll take longer to get your SNR up but you'll get what you want. Most of my research over the years has been done on an f/10 with no issues and I've even considered pushing that to ~f/40 for planetary imaging.

LewisM · #9 06-07-2015, 08:47 AM

Quote:

Originally Posted by multiweb

I've finally got all my adapters done.

Fat chance now. I'm not going through it all again. I'll stick with it

Pas probleme - I would include a buzzilion adapters (tubes, clamps etc). You would need to acquire nothing.

Just a thought....

LewisM · #10 06-07-2015, 08:48 AM

Quote:

Originally Posted by Atmos

I personally try not to get stuck on f/ratio. You're always going to be sacrificing resolution (image scale) imaging speed unless you're willing to spend the money buying large fast scope.

Even if you need to increase it to f/20 and its corrected, go for it! It'll take longer to get your SNR up but you'll get what you want. Most of my research over the years has been done on an f/10 with no issues and I've even considered pushing that to ~f/40 for planetary imaging.

WOnder - with the FSQ extended to f/8 and then use a 2x barlow, making it F/16.... nah, wouldn't come to focus I am sure anyway

multiweb (Marc) · #11 06-07-2015, 09:00 AM

Quote:

Originally Posted by LewisM

Pas probleme - I would include a buzzilion adapters (tubes, clamps etc). You would need to acquire nothing.

Just a thought....

What's wrong with the ED? You have more back focus and you can use the reducer. win-win.

multiweb (Marc) · #12 06-07-2015, 09:09 AM

Quote:

Originally Posted by LewisM

WOnder - with the FSQ extended to f/8 and then use a 2x barlow, making it F/16.... nah, wouldn't come to focus I am sure anyway

Get one of these and never look back. People say it's an expensive barlow but my argument is the following.

1_ It's a fluorite quadruplet with zero light scattering.
2_ It's light weight, super short and gives you a 90mm imaging circle.
3_ You get magnification from x3 to x8 by just adding tubes and it doesn't change your back-focus.

Now if you think about the money, how much would an assortment of Barlows/Powermates to cover the same range of magnification cost you? Would it be the same quality as well? Easier to use? How heavy is a x5 powermate?

The decision for me was a no brainer. Even if at the time I bought it, it was four times the price of the scope I used it on. But hey, I still got it, I use it all the time and it just works. And I saved a lot of money.

I always say measure twice, cut once.

LewisM · #13 06-07-2015, 04:20 PM

I had been contemplating that Marc (it's $300 cheaper to buy it from Teleskop Express than from AP).

I may do it one day... after we recover from the European vacation the wife and Daughter No1 are taking right now.

LewisM · #14 06-07-2015, 04:21 PM

Quote:

Originally Posted by multiweb

What's wrong with the ED? You have more back focus and you can use the reducer. win-win.

Nothing wrong with the ED... you know me and fluorite though

pluto (Hugh) · #15 06-07-2015, 04:38 PM

Lewis I have a similar image scale to you, 900mm with an 8300 chip (you use an 8300 don't you?). I've only imaged a few larger galaxies - M83, Sculptor, Cen A, NGC300, Leo triplet - but I think it's pretty good for the occasions I'm under dark skies and can image something other than emission nebulae through NB filters