[Planetary] A 4x Barlow is ideal for my system, but I can only get a 3x or a 5x, is the 5x going to be ok?

[JokubasJar]

I have a question. So according to the 5 or 6 times the pixel size rule for planetary. Something around a 4x Barlow would be ideal for my setup, allowing me to squeeze out the most detail out of my scope. My original plan was to get a 3x televue barlow, and use extension tubes to make it a 4x, I actually wouldn’t need tubes since the adc adds 30mm. Anyways, the 3x televue barlow is either sold out everywhere near me, or overpriced. The explore scientific focal extenders caught my eye, they are cheaper than anything by televue and as far as I know they work similarly to powermates and are of equal quality, however, they only offer 2x, 3x, and 5x, and as yall know, the extension tube thing doesn’t work with powermates/focal extenders, so my question is, would the 5x be ok? I know oversampling is encouraged in planetary but 5x puts me at almost f/30. I should be at around f/19 to f/26 depending on the seeing, so, what do you guys think I should do? Tracking isn’t a problem and using the fov calculator, the fov doesn’t look that bad. The thing is I really don’t want to get the 3x because it would put me below f/18.75 and I would waste resolution.
I have an 8" f/5.9 scope if yall are wondering.

[vlaiv]

What camera do you have?

[JokubasJar]

6 minutes ago, vlaiv said:

What camera do you have?

zwo 224 mc, pixel size of 3.75

[vlaiv]

And scope is 8" F/6 newtonian, right?

https://www.teleskop-express.de/shop/product_info.php/info/p5662_APM-Comacorrected-1-25--ED-Barlow-Element-2-7x---photo---visual.html

 

[JokubasJar]

7 minutes ago, vlaiv said:

And scope is 8" F/6 newtonian, right?

https://www.teleskop-express.de/shop/product_info.php/info/p5662_APM-Comacorrected-1-25--ED-Barlow-Element-2-7x---photo---visual.html

 

Yeah

[JokubasJar]

On 15/10/2022 at 00:33, vlaiv said:

And scope is 8" F/6 newtonian, right?

https://www.teleskop-express.de/shop/product_info.php/info/p5662_APM-Comacorrected-1-25--ED-Barlow-Element-2-7x---photo---visual.html

 

Soo, is it feasible or should I look further till I find a 3x televue barlow?

[vlaiv]

6 hours ago, JokubasJar said:

Soo, is it feasible or should I look further till I find a 3x televue barlow?

That barlow that I linked is excellent optically, designed for newtonians (corrects coma over larger field) and has x2.7 magnification (actual magnification varies with distance).

Optimum sampling is when F/ratio is x4-x5 pixel size, so in your case that will be 3.75 x 4 = 15 and 3.75 x 5 = 18.75.

If your scope is F/6 and you add x2.7 barlow - you get 16.2 - which is right in the middle of the range (in fact, closer to F/15 and I prefer lower of the two numbers for several reasons).

[JokubasJar]

Ok thats cool but I’ve seen on many many sites that when the seeing allows you can push it to 6x or in very rare cases 7x pixel size so I could miss out on those nights of good seeing. I don’t really need coma correcting since it’s planetary. 

[vlaiv]

32 minutes ago, JokubasJar said:

Ok thats cool but I’ve seen on many many sites that when the seeing allows you can push it to 6x or in very rare cases 7x pixel size so I could miss out on those nights of good seeing. I don’t really need coma correcting since it’s planetary. 

Telescope can't resolve past certain point and that point is calculated as:

f_ratio = 2 * pixel_size / wavelength

(https://en.wikipedia.org/wiki/Spatial_cutoff_frequency)

Wavelength for visible light is 400-700nm range (or 0.4-0.7um if we use micrometers for both wavelength and pixel size).

For 0.4um wavelength above formula transforms to

f_ratio = 2 * pixel_size / 0.4 = 5 * pixel_size. That is absolute maximum that aperture can resolve even in absence of atmosphere.

In any case - it is very decent barlow as far as price/performance metric goes. Coma will depend on speed of your scope, but "coma free" field is not very big - only absolute center is free from coma.

(source: https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm)

ASI224 has 6.2mm diagonal, or - corner is 3.2mm away from center. At F/6 - even at 2mm away from center you get tear drop spot diagram that exits airy disk circle.

[JokubasJar]

54 minutes ago, vlaiv said:

Telescope can't resolve past certain point and that point is calculated as:

f_ratio = 2 * pixel_size / wavelength

(https://en.wikipedia.org/wiki/Spatial_cutoff_frequency)

Wavelength for visible light is 400-700nm range (or 0.4-0.7um if we use micrometers for both wavelength and pixel size).

For 0.4um wavelength above formula transforms to

f_ratio = 2 * pixel_size / 0.4 = 5 * pixel_size. That is absolute maximum that aperture can resolve even in absence of atmosphere.

In any case - it is very decent barlow as far as price/performance metric goes. Coma will depend on speed of your scope, but "coma free" field is not very big - only absolute center is free from coma.

(source: https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm)

ASI224 has 6.2mm diagonal, or - corner is 3.2mm away from center. At F/6 - even at 2mm away from center you get tear drop spot diagram that exits airy disk circle.

Damn ok, thanks for the really detailed explanation. But I am currently finalising a big accessories order and the Polish shop I’m ordering from doesn’t have that specific barlow. Could I just get 3x focal extender? There wont be much coma at f/17.73.

[vlaiv]

2 hours ago, JokubasJar said:

Could I just get 3x focal extender? There wont be much coma at f/17.73.

Sure you can if you wish.

Coma is related to base F/ratio of instrument - not combined ratio (although when you "zoom in" - you are using smaller central portion of the field - just keep planet on optical axis).

For planets - you can completely avoid it if you have a good collimation of your scope and you keep the planet centered on the sensor. Use ROI and position ROI so that it is in center of the sensor rather than top left corner or whatever default location is (just check it and move it in center - in SharpCap you do it with mouse on ROI preview window on right side).

Barlow offers more flexibility than telecentric lens as you can dial in wanted magnification - F/ratio by varying distance of barlow element to sensor. With telecentric lens - it does not change much if at all with distance.

Telecentric lens is better if you have need for it visually (don't want to push eye relief further out) or if you plan to do some H-alpha solar imaging (for that purpose it is better than barlow).

Finally - with respect to F/ratio and planetary imaging.

Higher F/ratio won't help you capture more detail as telescope is limited by physics of light, but some people prefer to capture on higher F/ratio as it allegedly helps them with processing. Drawback of using higher F/ratio is that you get lower SNR per sub as light is spread over more surface. This makes it more difficult for stacking software to determine good and bad subs (too much noise will look like detail), it makes it more difficult for alignment points to do their thing (bigger alignment error) and finally it reduces total SNR of the stack for the same recording (need to capture and stack more good subs when too zoomed in).

 

 

[JokubasJar]

3 hours ago, vlaiv said:

Sure you can if you wish.

Coma is related to base F/ratio of instrument - not combined ratio (although when you "zoom in" - you are using smaller central portion of the field - just keep planet on optical axis).

For planets - you can completely avoid it if you have a good collimation of your scope and you keep the planet centered on the sensor. Use ROI and position ROI so that it is in center of the sensor rather than top left corner or whatever default location is (just check it and move it in center - in SharpCap you do it with mouse on ROI preview window on right side).

Barlow offers more flexibility than telecentric lens as you can dial in wanted magnification - F/ratio by varying distance of barlow element to sensor. With telecentric lens - it does not change much if at all with distance.

Telecentric lens is better if you have need for it visually (don't want to push eye relief further out) or if you plan to do some H-alpha solar imaging (for that purpose it is better than barlow).

Finally - with respect to F/ratio and planetary imaging.

Higher F/ratio won't help you capture more detail as telescope is limited by physics of light, but some people prefer to capture on higher F/ratio as it allegedly helps them with processing. Drawback of using higher F/ratio is that you get lower SNR per sub as light is spread over more surface. This makes it more difficult for stacking software to determine good and bad subs (too much noise will look like detail), it makes it more difficult for alignment points to do their thing (bigger alignment error) and finally it reduces total SNR of the stack for the same recording (need to capture and stack more good subs when too zoomed in).

 

 

I've just realized something, wouldn't the 462mc be perfect for my system? It's in general a better camera with great IR performance but it also has a pixel size of 2.9. 2.9x5=14.5, 14.5/5.91=2.45x barlow, and I guess it wouldn't hurt to just use a 3x barlow. Also the frame is a little wider so a bigger chance of catching some moons and stuff. Also, maybe off topic, but I am very interested in Venus imaging, with the 462mc and an 8 inch, what's the deepest IR filter I could effectively use on Venus, since it's the only planet that benefits from the very deep IR wavelengths. I know a lot of people say that if your below 10 inches you shouldn't use something like an 850, but if it's just for Venus, which is insanely bright, would it work?