DSO imager interested in planetary -- what kit is needed?

[Lee_P]

Bit of a long story here, but the short of it is that I'm a DSO imager that uses an Askar 130PHQ (focal length 1000mm) and ZWO 2600MC Pro camera. I'm currently having tech issues and as part of troubleshooting swapped out my original ASIAIR Plus for a new-style one (256G). That didn't fix it, but before I send it back to FLO I got to thinking about planetary imaging. This is something I've only very tentatively dabbled in, but I have some knowledge of the basics. The newer ASIAIR I currently have installed is supposed to be better for planetary work -- "Imaging efficiency at short exposures improves by 26%. FPS on video mode improves by 30%."

So what I'm wondering is, if I wanted to delve into planetary, what extra kit would I need?

* A new camera I guess, but what would the options be?
* 1000mm isn't so long; anything that can be done about that?
* Any filters or gadgets to open the door to solar work? (My 'scope is a Petzval-like design, which may be restrictive here).
* Anything else?

If it looks to be too expensive or difficult to try planetary then I'll just send the ASIAIR 256G back to FLO. 

Thanks for any advice!

 

 

[vlaiv]

5 minutes ago, Lee_P said:

"Imaging efficiency at short exposures improves by 26%. FPS on video mode improves by 30%."

Yeah, I would not really consider that seriously. Best it could read is: FPS potential increased by 30% (but not necessarily actual FPS), and for imaging efficiency - no comment there

You need to match pixel size of your planetary camera to F/ratio of the scope of choice. I would use simple telescope design for planetary imaging rather than telescope meant for DSO Imaging. There are some tradeoffs when you aim for good flat field - it usually scarifies diffraction limited performance in center.

Something like 6" F/8 newtonian will eat that Askar for lunch on planets.

Anyway:

- camera pixels must match F/ratio of setup (you can adjust F/ratio with barlow lens) - pixel size * 5 = F/ratio, so for say camera with 2.9um pixels you want your F/ratio to be around F/14.5

- get camera capable of high FPS (USB 3.0 connection and preferably computer capable of recording data at high rate so SSD / NVME drive)

- High QE

- As Low read noise as you can get

- for lunar and solar work, mono is better, for planets color/OSC is less hassle

- for white light solar - either get full aperture Baader solar foil filter ND3 version (photographic) for newtonians / maks (anything with mirror), while for refractor get Herschel prism. Get Baader Solar Continuum filter as well - here formula for F/ratio is pixel_size * 3.7 (if you use Baader Solar Continuum filter)

- for Solar Ha - that is whole new ballgame - get solar telescope and here F/ratio again different - with F/ratio = pixel_size * 3

 

[Cosmic Geoff]

You'd need a Barlow lens or equivalent to extend the focal length, and check the spec of your camera to see it you can get the high video frame rate and reduced ROI (region of interest) required for planetary imaging.  If not, you'll be looking at buying a dedicated planetary camera - various new models are available, some with 2.9um pixels. AFAIK, planetary imagers generally use a Windows laptop rather than an ASAIR to control the imaging.  You may also want an ADC (atmospheric dispersion corrector), a filter wheel, and a good-quality optical finder to help get the target on the camera chip.

You should be able to do solar imaging using a full-aperture solar filter, and some safe means of aiming the scope at the Sun.

[AstroMuni]

20 minutes ago, vlaiv said:

Something like 6" F/8 newtonian will eat that Askar for lunch on planets.

Could you explain the reasons for this please? I can understand that a 6" would do a better job than a 5" and all that glass wouldnt help either, but is there something I am missing?

[Elp]

Camera: 224mc/485mc are what I use, I think the 485/585 full resolution might be excessive considering how small planets appear on the sensor but keeping the planet in view is much easier if imaging at longer focal lengths, if you want to use one as a backup or secondary DSO camera the 585 will be great. If you've got the patience mono, but I don't think it's really necessary as I believe most use OSC cameras due to convenience (and you have to shoot fast due to planet rotations).

FL increase: add in at least a 2x maybe 3x Barlow, a good quality one. If you've got the funds get a TV Powermate, it's correction is just that much better especially when imaging. You simply add diagonal > Barlow/PM > camera (with eyepiece sized adaptor or thread directly onto Barlow/PM top). Any higher power focal extender and likely you won't get much use out of them due to the typically poor seeing to support their use.

Solar: simplest is white light solar film up front fitted to a custom made cell (made mine from double walled corrugated card, diameter of film made slightly larger to cover the whole objective). For safety I went with the higher ND grade film for visual, that way in future theres no guessing as to whether it's safe to use visually if you wanted to, standardise anything in life where possible. For locating the sun you can make a simple pin hole finder (pin hole section in front, a holder of some sort of semi transparent material further back for the pin hole to project the sun onto), or just buy the TV sol finder (3d print copies are also out there).

HA solar is another financial sink hole. With your current scope you'd have to find out if a Daystar Quark is compatible but with the amount of glass inside the scope I'd err caution as you might get excessive heat build up inside, an around f10 acro doublet might be the better option. With a diameter over 100mm in general it's safe practice to use a DERF filter over the objective which will reflect the majority of UV and IR radiation back and will reduced heat in the scope and thus on the Quark (the energy is magnitudes higher closer to focal point, remember how a magnifying glass focussed to a spot burns via sunlight). You don't need a DERF around below 90mm (the Quark has the basic filtering built into it safe for visual use barring the need for a DERF or not) but people still install a UV/IR filter or a wide bandpass HA filter in front of the diagonal (or as far away from the Quark as possible where the light energy is less focussed as most filters are very thin and any manufacturer will rightly warn you not to use such filters in this application at all).

Quarks have a quality lottery though due to being the bottom quality of the manufactured etalons in their use, higher ones going into their more expensive products, so buy from a reputable dealer with a solid quality check or returns policy.

Or just get a dedicated solar scope safely knowing your prized night asset won't get a solar battering. "Cheap" cost of entry is a Coronado PST, it's not built for photography, but eventually I managed it with a few additional bits, at 40mm aperture and 400mm FL you can view full disk too, very small compact and light. If buying used there are things to look out for (I wont go into it here). Lunt would also be the other probably better option though cost much more as do Coronado's Solarmaxes. Note Lunt are still operating from their founders (also ex Coronado employees), Coronado (the pioneer of affordable amatuer solar scopes) assets were bought by Meade.

For solar imaging: mono camera is best. The widely sought after one is the 174mm but I guess any one will do.

[happy-kat]

I'm just commenting initial thoughts as not experienced person but like quick gains. What is your guide camera, assume it might be the 120MM in signature, suspect that might be better then the big camera and use a barlow and have a go

[vlaiv]

13 minutes ago, AstroMuni said:

Could you explain the reasons for this please? I can understand that a 6" would do a better job than a 5" and all that glass wouldnt help either, but is there something I am missing?

Telescope needs to be at least diffraction limited for best planetary performance.

Telescopes that have built in field flattener / or have FF/FR added, are usually not diffraction limited telescopes - which means that their Airy pattern is larger than it should be for given aperture or that they behave as smaller aperture telescopes as far as resolving capability of telescope goes.

Look at spot diagram of Askar scope, and in particular RMS value on axis

it is 1.617 (which is very good value by the way, so this scope is really not that much hampered by being corrected for photography).

Now, let's do some math:

Diameter of Airy disk of diffraction limited F/7.7 scope is ~4.3um

Radius is therefore half that - or ~2.15um

There is relationship between Airy profile and corresponding Gaussian profile that goes like this:

So we are looking at about 1.46um of RMS (approximation) - we now have two values that we can compare:

~1.62 vs ~1.46

It is clear that Askar provides larger Airy pattern than diffraction limited telescope would - and is not diffraction limited. It blurs image more (much like smaller aperture).

 

[bosun21]

I also would leave out the diagonal and insert a flip mirror to enable you to easily position the planet in your camera. Most planetary cameras have a small chip size.

[AstroMuni]

5 hours ago, vlaiv said:

It is clear that Askar provides larger Airy pattern than diffraction limited telescope would - and is not diffraction limited. It blurs image more (much like smaller aperture).

Thanks. Does this mean that a Newt with a CC could have a similar disadvantage as compared to one without CC?

Edited October 12, 2023 by AstroMuni

[AstroMuni]

6 hours ago, Lee_P said:

A new camera I guess, but what would the options be?

If your current camera allows ROI image captures, the FPS might be acceptable for planetary.

[vlaiv]

4 hours ago, AstroMuni said:

Thanks. Does this mean that a Newt with a CC could have a similar disadvantage as compared to one without CC?

Yes.

It does depend on type of CC in question. Some are really terrible in that regard - like simple two element models. There is visible blurring/loss of resolution even in long exposure images where seeing mostly dominates, but in general - all will trade off some center of field sharpness for correction in outer part of the field.

There are some telescope designs that don't suffer from this - for example Mak-Newtonians are known to be excellent planetary performers (especially F/6 - so somewhat slower models) - while having good star definition over larger field.

 

[Lee_P]

Thanks for everyone's input here. I think I'll stick with deep sky, and maybe play about with lunar every once in a while. Meanwhile, the replacement ASIAIR will go back to FLO -- I really do love their no quibble returns policy!