Takahashi 1.5x extender for 160ED on full frame, is it a good choice for remote imaging?

Bill McLaughlin:
IMHO:

1) There are three reasons to go remote...

    a) Clearer weather
    b) Darker skies
    c) Better seeing

"a" is not relevant to focal length and speed but the other two are.
"b" argues for a faster system to get the faint stuff.
"c" argues for a longer focal length to get better detail.

Unless you want to spend a huge amount of money (think PW Delta Rho), you are not going to get both in pone scope so you have to choose.  The ideal system therefore is two scopes, one short and fast and the other longer and slower. Just how long depends on the seeing at the site. Not all dark sites have great seeing so you need a realistic evaluation of that (preferably another user, not the owner). If you are under (about) 2 arcsec seeing you will want something 1.5 meters and up. The 160 with extender will not do that.

So I would go with the speed w/o the reducer. In fact, I plan to replace one of my two scopes at a remote site in June with a 130D because the present refractor is too slow (f 5.8). I do have two scopes there, the other being a CDK 14 - so high res is already covered.

Thanks bill. So I am primarily going to a remote hosting site for darker skies, and maybe slightly better weather. But I am confused by your point.  Why would being under darker skies encourage you to get a faster system? 1 hour of data from a Bortle 1 is equivalent to 21 hours under Bortl 7 for the same system. Shouldn't that mean that being under darker skies means sacrificing some speed for more focal length is less of an issue? 



I was hoping to get even slightly more details while sacrficing some speed. What I am trying to decide is if that is a good tradeoff or not in case of the E160ED.

Luka Poropat:

Bill McLaughlin:
If you are under (about) 2 arcsec seeing you will want something 1.5 meters and up.

If you are at 2" seeing and have a 3,76um sensor you are already sampling at 0,52"/px (at 1500mm focal lenght).
Even including the effect of square pixels and convolution I still dont see any benefit it it. 

You will be seeing limited anyways. 

Personally I usually shoot with 2" seeing  and I am already seeing limited at 550mm, let alone 1500mm.

I don't have a direct way of measuring my seeing conditions but I did find a significant improvement in FWHM values when I went from 900mm 5 inch refractor to an 11 inch SCT. 

I don't know how accurate this site is, or if I am misinterpreting what they are saying,  but supposadly my seeing condtions from my backyard are well below 2".

https://www.meteoblue.com/en/weather/outdoorsports/seeing/victoria_united-states_4739157

Bill, did you notice your FWHM values in arc seconds are lower with the extender, compared to the native corrector, or were they the same? 

Also how well did it perform on your full frame sensor?

andrea tasselli:

Ashraf AbuSara:
I don't have a direct way of measuring my seeing conditions but I did find a significant improvement in FWHM values when I went from 900mm 5 inch refractor to an 11 inch SCT.

The average FWHM (in ") from your single subs IS your seeing. The MeteoBlue stuff is rubbish.

... assuming you are not undersampled, and everything from focus, to tracking to optics to collimation is perfect and seeing is your only limitation.

What do you think of the extender idea Andrea? Yay or nay?

I have one of the 160ED extenders, but I rarely use it. But I image mostly from my home in Bortle6/7. If I recall, it did help with corner illumination on the full frame a bit, but not a ton. The 160ed still drops to about 65% in the corners of the full frame. With the extender it was a little less exaggerated.  (there's a whole post on full-frame imaging with the 160ed you should check on) In terms of image detail, I couldn't tell the difference in sharpness. 

If you are really going to use the AM5, I think you'd want to keep your focal length at the F3.3 and instead do shorter exposures. What sort of performance are you expecting to get on the AM5? Why not something equitorial if portability isn't a factor?

In the end, I think it's just preference. If you go with the asi2600 at F3.3, you can invest that extra 3k into the mount. Just an idea

Ashraf AbuSara:
I haven't been using my AM5 lately which is why I wanted to send it.

Ahhh! that makes a lot more sense then. I thought you were buying all equipment new. 

I've actually been considering my epsilon for moving remote and the filters are something I may want to adjust if I do. (they are Baader Highspeed, 3.5/4nm versions) I bought that mono duo camera asi just released. hopeful the guiding will be fine and I can stop with the guidescope on the epsilon. But might have to move to wider bandpass if I do.

Bob Lockwood:

Ashraf AbuSara:
Unfortunately my Chroma narrowband 36mm filters are 3nm so I may have to get a new set that can work at f/3.3.

You should try the 3nm filters first. I used 3nm filters for years with my E-210 f/3 and never had issues with them.

Thanks I will give it a shot. I just wonder realistically how much signal is lost due to band shift at that focal ratio.

Dan Brown:
I use the e160 at f5 and f3.3. My usual HFR at f3.3 is 1.5-1.65. With the extender my typical numbers are 1.6-1.8. 
Dan

Thanks Dan. I am presuming that HFR is in pixels? If it is in pixels that extender must be getting you slightly more details because the image scale would be about 30% smaller.

Dan H. M.:
I can’t speak for the E160 but the extender for the E130 has proven useful. There’s a clear gain in detail vs the stock corrector. 

Do keep in mind you’re going to have to be even more diligent with collimation though.

Interesting thanks for the feedback. I would have thought the slower focal ratio of the extender would have made collimation/ tilt less critical. Is it because of an improvement in spot sizes?

I’m a little bit confused by some of the advice in this discussion. The ɛ160D with 1.5x extender would be on paper is advertised at 792mm f/4.95, which at 3.76μm pixels would sample at 0.98"/pixel. Which isn’t demanding exceptional skies for sampling.

Recently I picked up (ah, financial indiscretion) an ɛ180D with the 1.5x extender, and it has been an absolute delight to use in both configurations, and my time so far with the has not left me dissatisfied with the sampling of my results. Say nothing for using a tool like BlurX and latitude afforded with some drizzle upsampling (which, for me so far, has been beneficial even in terms of sampling thanks to the generally moderately stable atmosphere where I image). The performance of the extender is exceptional.

At a remote imaging dark site, I would personally be more concerned with having something set up to run at the field of view I was interested in than having it be as fast as possible—at least in this case where both fall in sampling ranges that allow for benefit. Presumably the purpose of setting up at a remote site includes not just the dark skies, but also a cozy quantity of good imaging nights across the year, so plenty of time to work on projects. Especially at this rather fast focal ratio.

To my ears, the option of using an extender makes sense. Although either configuration sounds delightful. If I had this set up at a remote site I might run natively for a part of the year and with the extender for the remainder of the year.

James Peirce:
I’m a little bit confused by some of the advice in this discussion. The ɛ160D with 1.5x extender would be on paper is advertised at 792mm f/4.95, which at 3.76μm pixels would sample at 0.98"/pixel. Which isn’t demanding exceptional skies for sampling.

Recently I picked up (ah, financial indiscretion) an ɛ180D with the 1.5x extender, and it has been an absolute delight to use in both configurations, and my time so far with the has not left me dissatisfied with the sampling of my results. Say nothing for using a tool like BlurX and latitude afforded with some drizzle upsampling (which, for me so far, has been beneficial even in terms of sampling thanks to the generally moderately stable atmosphere where I image). The performance of the extender is exceptional.

At a remote imaging dark site, I would personally be more concerned with having something set up to run at the field of view I was interested in than having it be as fast as possible—at least in this case where both fall in sampling ranges that allow for benefit. Presumably the purpose of setting up at a remote site includes not just the dark skies, but also a cozy quantity of good imaging nights across the year, so plenty of time to work on projects. Especially at this rather fast focal ratio.

To my ears, the option of using an extender makes sense. Although either configuration sounds delightful. If I had this set up at a remote site I might run natively for a part of the year and with the extender for the remainder of the year.

That's exactly the kind of information I was trying to contend with and gather from this post. The extender is not exactly cheap at $1100.  What I am trying to determine, assuming we are not limited by seeing, is if the extender will indeed result in improved resolution in the smaller targets that maybe imaged with this configuration that it is worth the investment upfront. It's also not just added cost, but also lost speed and FOV. So the question is does it actually resolve better details to be worth those tradeoffs? 

Having a smaller theoretical image scale does not always translate to a better real life improvement in resolution, especially when we are talking about the same aperture. But it seems to me from your description that you have found the extender, combined with drizzle 2x and deconvolution, to improve the resolution of your images compared to say the native corrector with similar drizzle deconvolution?

Any objective measures in your experience comparing the extender vs the native corrector that perhaps can answer that question? Maybe the FWHM results you are getting compared in your subs and final raw stacks?