Planetary imaging kit list for F4.5 Dobsonian

[simonharrison]

Hi all

I'm looking to try imaging the planets and moon. I have a 16" F4.5 Meade Lightbridge on an equatorial platform and just picked up a used ZWO 462 on eBay.

Firstly having read around about optimum pixel sizes and focal lengths for imaging, I believe I need something around a 3 or 4X Barlow. The 462's pixel size is 2.9 microns and according to the formula often quoted: pixel size * 5 (or 6 or maybe 7) gets me to a range of F14.5 to F20.3. So F14.5/4.5F = 3.333... and F20.3/4.5 = 4.51111... so essentially that means to image optimally I need a barlow of 3 or 4. Have I got that correct?

In terms of the barlow itself I'll probably try and get hold of a 4X Powermate as it's 2 inches (all my lenses are 2" so it can double up to be used with them for visual). I assume there's no real preference between 1.25" barlows/FEs and 2" ones? Apart from 2" ones being costlier!

Lastly I believe I need the following gear to go with the 462, 4X Powermate:

• IR cut filter
  • probably go ZWO again - not sure whether to get 1.25" or 2" though?
• ADC
  • (eg ZWO 125  - I assume it's OK to go 1.25" for this? Assume it will fit in the Powermate?)
• A long USB cable and a cheap laptop with USB 3 and a decent SSD to stream the images to
  • Or would an ASI AIR Pro be able to stream the images to my indoor PC?

Did I miss anything?

Thanks for any help,

Simon

 

[vlaiv]

I'd say that at most you need x2.7 barlow as F/12 is the most you'll need for 2.9µm pixel size.

APM x2.7 coma correcting barlow element?

[randomic]

I don't think you'll need a barlow at all. If I'm running the numbers correctly you're at ~1800mm focal length. With the 462 that puts you at an image scale of 0.33" per pixel which, if anything, is on the large side for planetary. I don't think you'd want the image any larger (at least, not for the drawback of putting more glass in the light path).

 

Might want to consider an IR-pass filter too.

The 462's bayer filter is almost transparent in IR and IR is less affected by seeing. Therefore, you can get some really high res mono shots of the planets which you can combine with colour data (using the IR-cut filter).

 

I've never used an ADC so can't really give my experience.

 

ASI AIR Pro could probably stream to an indoor PC, it depends what your wireless signal is like at the scope. Bear in mind that (in hardware terms) it's basically a Pi 4 so a laptop might be capable of getting better framerates from the camera.

 

Lastly, the 462's chip is very small, see how you get on but I find it very difficult to get the target on the chip.
To help, you could consider a flip-mirror so you could line up the object with an eyepiece (optionally one with a reticule).

Edited June 17, 2021 by randomic

[simonharrison]

Interesting, didn't expect that re: Barlow, as I'd read so much on forums about pixel size * 5 being the optimum F/number to image at. OK so for now I'll leave out getting a barlow, as I already do have an Altair 2x focal extender. Can see how it goes.

I just looked up my scope (Meade Lightbridge 16") and it's actually F5 and the focal length is 1829mm.

Thanks for the tip about IR pass filters. I'll get one of those as well. 

cheers

[randomic]

Here's a calculator which has a bunch of explanation about optimal image scale: https://astronomy.tools/calculators/ccd_suitability
It depends on focal length rather than focal ratio.

For long exposures you aim for 1-2" per pixel (as a rule of thumb, it depends on how still the atmosphere is). If your scale is too large it takes longer to collect the light, if it's too small then you're missing out on resolution.

For planetary work you're doing tens or hundreds of exposures per second and picking ones which come out best (where the atmosphere is stillest), a process called lucky imaging. Here you're operating much closer to the optical limit so you aim for somewhere between 0.1-0.5" per pixel.

Planets are bright so it's hard to run in to but as long as your exposure time isn't reducing your framerate you're golden!

[vlaiv]

6 minutes ago, simonharrison said:

Interesting, didn't expect that re: Barlow, as I'd read so much on forums about pixel size * 5 being the optimum F/number to image at.

Add Nyquist sampling theorem (two pixels per single wavelength) and take lambda to be either 400nm - lower end of spectrum or around 500nm as blue colors are more affected by seeing and you probably won't achieve full resolution in blue and you have formula for critical sampling.

At 475nm for 2.9µm that gives F/12.2