Anyone mono imaging with an achro?

rustigsmed (Russell) · #1 16-11-2016, 02:08 PM

Not that I've spent more than one night imaging this year due to weather and other issues. I've been pondering getting a large achro for imaging. of course we all know they are no good for OSC imaging as all colours don't come to focus - or if you are capturing Luminance with a mono cam. I am fairly certain there is no issue with NB imaging. so in short I suspect if most of the imaging is done in NB it should be fine for that purpose.

I'm not so sure on RGB imaging (mostly because I haven't done it properly yet - just dslr and NB). of course with RGB obtaining L is very important but how well does a synthetic L work from RGB data is it comparable? does it yield a result 90%75%50% as good?
Is an R or G or B filter narrow enough for the achro colour/focus issues?

Most of the time will be NB but curious as to know the situation with RGB. Of course the plan would be to go large ~152mm f6.5(or do we go in inches now its bigger aperture territory?

)

LightningNZ (Cam) · #2 16-11-2016, 02:39 PM

You'll still have to refocus for each colour, so that could be a PITA without a repeatable electronic focuser?

rustigsmed (Russell) · #3 16-11-2016, 03:49 PM

Quote:

Originally Posted by LightningNZ

You'll still have to refocus for each colour, so that could be a PITA without a repeatable electronic focuser?

Hi Cam,

I find I have to refocus every hour anyway currently. and it can be automated with SGpro etc. but yep a moonlite or similar would be needed. perhaps getting a reputable focuser to fit the OTA may be an issue

glend (Glen) · #4 16-11-2016, 03:58 PM

Yes i am doing it. I use a Bresser 152mm f5 (Flatfield achromat with modified Petzval design). Importantly it has a very strong Hexfocuser that can support an imaging train. Purchased from Teleskop-Express at a rediculously low price, unavailable from suppliers outside europe. BTW Teleskop-Express remove the 19% Euro VAT when they sell out side the EU, this offset pays for the shipping.

https://www.teleskop-express.de/shop...0-mm--OTA.html

No CA issues obviously as i am narrowband only with this scope. Surprisingly there turned out to be little focus difference, especially with Ha and Sii filters which are very close in the spectrum. Yes Oiii requires a focus check. I use a bahtinov mask to check focus when i switch filters. Importantly my Baader NB filters are parafocal and that helps. I tend to shoot Ha and Sii back to back on the same night and do Oiii another night, this cuts the need to refocus. You don't need a motor focuser if your prepared to do it manually, and the Bresser focuser easily supports my ASI1600MM-C and the QHY filter wheel - no sag, no tilt.
IMHO you would need to invest in proper narrowband filters. You can build a synthetic luminance layer with NB filters, particularly Ha and Oiii, but i find using Ha as luminance works pretty well.
I would not recommend trying LRGB imaging through any achro, but others might.

RickS (Rick) · #5 16-11-2016, 04:17 PM

Quote:

Originally Posted by rustigsmed

I'm not so sure on RGB imaging (mostly because I haven't done it properly yet - just dslr and NB). of course with RGB obtaining L is very important but how well does a synthetic L work from RGB data is it comparable? does it yield a result 90%75%50% as good?

I've done a few RGB only images with data from our automated/shared scope at SRO. In terms of detail there's a benefit as we get better FHWM in the colour subs that we do in Lum. For bright objects it works well. I did a very decent M31 with about 6 hours on each colour. The downside is that it does take substantially longer to get good SNR with dim targets.

Cheers,
Rick.

RickS (Rick) · #6 16-11-2016, 05:16 PM

Quote:

Originally Posted by RickS

The downside is that it does take substantially longer to get good SNR with dim targets.

Just happened to have some data that I'm working on right now. The target is NGC7497, a small galaxy visible through lots of dust. I extracted luminance from just over 31 hours of RGB data and compared it to just over 9 hours of real Lum. The SNR estimated by PI/SubframeSelector is about the same and eyeballing them they look very similar.

Of course, the luminance I'm actually working with is a combination of both

Cheers,
Rick.

mountainjoo (Jerome) · #7 17-11-2016, 12:59 AM

Quote:

Originally Posted by RickS

Just happened to have some data that I'm working on right now. The target is NGC7497, a small galaxy visible through lots of dust. I extracted luminance from just over 31 hours of RGB data and compared it to just over 9 hours of real Lum. The SNR estimated by PI/SubframeSelector is about the same and eyeballing them they look very similar.

Of course, the luminance I'm actually working with is a combination of both

Cheers,
Rick.

Hi Rick. What weighting scheme do you use when generating a luminance channel from multiple filters?

RickS (Rick) · #8 17-11-2016, 07:02 AM

Quote:

Originally Posted by mountainjoo

Hi Rick. What weighting scheme do you use when generating a luminance channel from multiple filters?

Hi Jerome. My default is to do a noise-weighted integration (no rejection, of course.) That gives the best result in terms of SNR. If the weights end up skewed so much that an important channel isn't contributing then I might switch to equal weights instead.

Cheers,
Rick.

rustigsmed (Russell) · #9 17-11-2016, 08:03 AM

Quote:

Originally Posted by glend

Yes i am doing it. I use a Bresser 152mm f5 (Flatfield achromat with modified Petzval design). Importantly it has a very strong Hexfocuser that can support an imaging train. Purchased from Teleskop-Express at a rediculously low price, unavailable from suppliers outside europe. BTW Teleskop-Express remove the 19% Euro VAT when they sell out side the EU, this offset pays for the shipping.

https://www.teleskop-express.de/shop...0-mm--OTA.html

No CA issues obviously as i am narrowband only with this scope. Surprisingly there turned out to be little focus difference, especially with Ha and Sii filters which are very close in the spectrum. Yes Oiii requires a focus check. I use a bahtinov mask to check focus when i switch filters. Importantly my Baader NB filters are parafocal and that helps. I tend to shoot Ha and Sii back to back on the same night and do Oiii another night, this cuts the need to refocus. You don't need a motor focuser if your prepared to do it manually, and the Bresser focuser easily supports my ASI1600MM-C and the QHY filter wheel - no sag, no tilt.
IMHO you would need to invest in proper narrowband filters. You can build a synthetic luminance layer with NB filters, particularly Ha and Oiii, but i find using Ha as luminance works pretty well.
I would not recommend trying LRGB imaging through any achro, but others might.

thanks glen looks like an interesting scope quite fast for a refractor as well. from what I read the focuser and options for upgrading can be limiting so good to hear your experience with that model.

I think L-RGB imaging is definitely out, really interested to hear about any RGB experiences.

Quote:

Originally Posted by RickS

I've done a few RGB only images with data from our automated/shared scope at SRO. In terms of detail there's a benefit as we get better FHWM in the colour subs that we do in Lum. For bright objects it works well. I did a very decent M31 with about 6 hours on each colour. The downside is that it does take substantially longer to get good SNR with dim targets.

Cheers,
Rick.

Quote:

Originally Posted by RickS

Just happened to have some data that I'm working on right now. The target is NGC7497, a small galaxy visible through lots of dust. I extracted luminance from just over 31 hours of RGB data and compared it to just over 9 hours of real Lum. The SNR estimated by PI/SubframeSelector is about the same and eyeballing them they look very similar.

Of course, the luminance I'm actually working with is a combination of both

Cheers,
Rick.

awesome timing - thanks for the info Rick - it sounds a bit like OSC capturing as far as total integration times go

mountainjoo (Jerome) · #10 17-11-2016, 08:28 AM

Quote:

Originally Posted by RickS

Hi Jerome. My default is to do a noise-weighted integration (no rejection, of course.) That gives the best result in terms of SNR. If the weights end up skewed so much that an important channel isn't contributing then I might switch to equal weights instead.

Cheers,
Rick.

Great, thanks!

glend (Glen) · #11 17-11-2016, 08:43 AM

Quote:

Originally Posted by rustigsmed

thanks glen looks like an interesting scope quite fast for a refractor as well. from what I read the focuser and options for upgrading can be limiting so good to hear your experience with that model.

Russell, Moonlight have a focuser adaptor for the Bresser 152mm f5, which is the same one they sell for the ES AR152S. The Bresser 152mm and the ES AR152S have the same tube diametre, and focuser mounting arrangement, and are both made by JOC in China. So you would be able to replace the focuser if you wished, and motorise it for remote control. However, I have found no need for a focuser replacement with my imaging gear.