Also, how much magnification you actually get from the barlow depends on the separation between the barlow and the camera focal plane. When you compare the diameter of the planet (measured simply in units of imager pixels) with the barlow compared to without, is the planet's image 2X larger with the barlow, or significantly more ? The ratio of measured diameters will tell you how much of an increase you're actually getting from using the barlow (alone, with no eyepiece, correct?) with your imager setup.

Elsewhere in these forums I've read that for capturing the planets, an effective focal length (EFL) of around 5-meters is thought by many to be optimum for use with typical imaging cameras ... so as a sanity check, you might multiply your telescope's focal length by the planet-diameter-increase-ratio you measure with the barlow, and see if your setup with barlow is delivering an EFL of around 5m.

Lastly, it's worth keeping in mind that with the barlow, the total light received from the planet will be spread out over many more pixels (how many more is given by the planet-diameter-increase-ratio squared ... 4x more pixels if the size of the planet is doubled using the barlow), so therefore the image is fainter with the barlow vs. without. Are you using a longer exposure time (i.e., shooting with fewer frames per second of video) due to the fainter image ? If frame rate is too low, atmospheric seeing and/or image motion could explain some of the loss of sharpness you're seeing. If the frame rate is the same, then lower signal-to-noise ratio could be a factor (suggesting you might want to try capturing longer video sequences for doing your post-processing) [*].

Good luck,

-- Jim

[*] EDIT ... or as Borodog's reply (posted while I was busy composing mine) suggests, increasing the gain provided this doesn't increase the noise even more than the signal.

Regarding Borodog's formula, I can shed light on where the "206.3" factor comes from: the number of arcseconds in one radian is equal to 206,265 but since his formula uses mm for telescope focal length but microns for the pixel size (where 1 micron = 1/1000 mm), that factor of 1000 difference in the units reduces 206265 arcsec down to 206.3 ... so nothing mysterious there in the image scale calculation.

Regarding his "rule of thumb" for f/ratio = 5 x (pixel size in microns), note that because f/ratio = EFL / (Telescope Diameter), this is the same thing as saying the optimum EFL = 5 x (pixel size in microns) x (Telescope Diameter). So if one then takes this EFL and plugs it back into the first formula, one finds the "rule of thumb" target image scale is:

Target image scale (arcsec/pixel) = 206.3 / (5 x Telescope Diameter in mm)

... or equivalently --

**Target image scale = 0.2 arcsec/pixel x (206.3 mm / Telescope Diameter)**

... so for an 8-inch telescope, the target image scale is ~ 0.2 arcsec/pixel, and coarser sampling can be targeted for smaller aperture telescopes while finer sampling should be targeted for larger aperture telescopes (to take maximum advantage of the moments of very best seeing).

Lastly, it appears the 5m EFL target I quoted was most appropriate for 10-inch aperture telescopes, whereas per the calculations above starting with Borodog's formulae, a 4m EFL target might be more appropriate for 8-inch aperture telescopes ... assuming imager pixel size of ~ 4 microns in both cases. ((Larger pixel sizes require proportionally longer EFLs to maintain the same arcsec/pixel scale, so EFL ~ 5m might be optimum for an 8-inch aperture telescope in the case of ~ 5 micron pixels....))

Hope this helps.

**Edited by jkmccarthy, 15 September 2021 - 01:19 PM.**