Using Barlows with Refractors for planetary imaging - Forums

I am seeking feedback on whether using Barlows (and/or powermates) with refractors (eg: between 61-91 mm) is possible for planetary imaging.

I currently own a William Optics GT-71 (focal length 420 mm) and I would like to take pictures of Saturn, Jupiter and Mars using a couple of Barlows in series (eg: total magnification x 4.5) with the TeleVeu power mate X 2.5 + Celestron X 2. Is this realistic?. Anyone with experience with this?. Is focus an issue?
Thank you and clear skies,
Alicia

Whether it can be done, it has been done. Whether it produces useful results with a 71mm refractor, I very much doubt it from my point of view. YMMV.

A Barlow will not bring out more detail. It will simply project the same detail in a larger size which suits the perceptual capacity of the destination (eye or sensor) better. For example if your telescope can resolve detail that is half a pixel wide, then you need a 3x Barlow to make it larger than your pixels. If your telescope can resolve coarser detail 2 pixels wide, then the Barlow will simply make the image fuzzier and fainter (like using resize to 300% in a program like Photoshop).

The actual detail that's there depends on aperture. 3" (71mm) is small for planets. 91mm is better. There is a group called Small Aperture Planetary imaging
https://www.astrobin.com/groups/149/small-aperture-planetary-imaging/

If you browse the pics there, you can see for yourself the massive difference between 70mm and 127mm.

Cheers,
Dimitris

Very helpful Dimitris, thank you
I think I need to keep trying to get the right focus with the Celestron SCT 8 and the ZWO 224MC with the Powermate 2.5. So far it has been impossible

Hi Alicia, you may be interested in this thread: My High Res. Planetary Imaging Guides - AstroBin and the links therein.

A lot of the sharpness of the final image is recovered in the development process since the individual image is almost always blurred by atmospheric distortions. Rouz shows some very instructive images how quality improves from step to step.

Clear skies
Wolfgang

There’s a simple rule of thumb that I’ve found to be helpful. To ensure the best match between the diffraction-limited resolution of your telescope and the sampling of your camera, you should aim for an F-ratio roughly equal to 5 times the size of your camera pixels in microns.

So for example if your camera has 3 micron pixels then you should target imaging at about F/15. If your scope is naturally F/5 then a 3x Barlow would be perfect.

My scope is an F/7 refractor and my camera has 2.4 micron pixels. Using a 2x Barlow gets me to F/14 which is close enough to the the target of 5 x 2.4 = 12.

Hope that makes sense!

Alicia Rossiter:
Very helpful Dimitris, thank you
I think I need to keep trying to get the right focus with the Celestron SCT 8 and the ZWO 224MC with the Powermate 2.5. So far it has been impossible

I would definitely focus on focus

. As others have mentioned, the general rule of thumb is to get a focal ratio of about 5x your camera's pixel size. Your 224MC has 3.75µm pixels, so that is f/18.75. The 2.5x Powemate will be a bit over-sampled, unless you have excellent skies. However, the bigger concern right now is getting the target onto the sensor and getting it into focus.

You didn't mention what kind of focuser you're using, so I'll assume the SCT is stock (i.e. you're turning a knob on the bottom of the scope). Try to ensure your final movement is a counterclockwise turn. Also, it might be helpful to try and obtain focus on a distant object during the day. Then once you've done that, try to get focus on the moon at night.

Thank you, I have no focuser installed yet in the SCT. I have recently purchased the ZWO EAF but have not installed it yet. Not sure if the EAF would make it easier in this case?. I did try to get focus during the day. But things get really complicated at night. The sensor of the 224MC is too small that I hardly see any stars. I am using the ASI AiR Pro and it is impossible to plate solve due to the lack of focus. I see a lot of people use the same equipment for planetary imaging with no issues. I don’t know what I am doing wrong 😕

You're not doing anything wrong; you're learning.

When I got my EdgeHD, I spent the first night doing nothing but trying to get it into focus. No luck. Getting to focus, especially with such a small sensor as the 224, is not an easy task. That's why I suggested you try it during the day. If you can get some distant object into focus during the day, then you've got the mirror relatively close to where it needs to be to focus on an object in space at night. The moon is a big, bright thing on which you can focus.

You're not doing anything wrong; you're learning.

When I got my EdgeHD, I spent the first night doing nothing but trying to get it into focus. No luck. Getting to focus, especially with such a small sensor as the 224, is not an easy task. That's why I suggested you try it during the day. If you can get some distant object into focus during the day, then you've got the mirror relatively close to where it needs to be to focus on an object in space at night. The moon is a big, bright thing on which you can focus.

I will try with the moon , I will let you know the results !

One other suggestion, if I may. I looked at your gallery and see you've done a few images with the 183MC and C8. Assuming you have one that came with the C8, I think the finder scope will come in very handy when trying to get your target planet centered on the 224MC's sensor.

What I did was with my 294MM, I slewed and plate solved on a bright star (Vega). Once it was centered, I then aligned the finder scope so it was perfectly centered in the crosshairs. Now, when I slew to a planet, I use the mount controls to ensure it is centered in the crosshairs of the finder scope, and that puts it squarely in the center of the sensor as well.

I've got my new planetary setup coming in this week - 662MC, ADC and Tele-Vue 2x Barlow. I'll be doing the same things I've mentioned here… getting focus during the day, then focusing on the moon at night. With my finder scope well aligned, I'll have a much easier time getting my target on the sensor. That's the plan, anyway

I've read some good suggestions and some not so good ones.

Using the Moon to find focus is a good suggestion. It is bright and large and even eye-balling it you should be able to point at it. For a SCT your best bet for all things planetary is to use a diagonal and a high power eye-piece for centring. In fact you could go one step further and make the EP parafocal to your camera and be done with it. As for the suggestion of focusing during daytime this is only meaningful if you have a very long baseline, several km away and even then it won't be in focus for object at infinity. In fact even focusing on the moon won't guarantee the focus is perfect for them planets.

I'd recommend using the finder and centre it against a bright star as seen through a high power EP (usually going through the intermediate step of centring against the some feature on the Moon as seen with a high-power EP). You most likely won't be able to use to land it on the chip when using a barlow lens but good enough to be seen in the EP (with the barlow), checking the seeing and centred well enough to be well within the confines of those tiny chips.

As for the suggestion that somehow the pitch of the sensor decides the focal length you should be using, this is balderdash. Worse still, it puts the carriage in front of the horse, in manner of speaking. Assuming that telescopic angular resolution in a wider sense is defined by the formula: lambda/D, with lambda the wavelength of light of choice (usually 550 nm) and D the effective diameter of the lens or mirror of the telescope the recommendation here is to sample that angular resolution with 4 pixels (e.g. between 3 and 5, with 3 being the bare minimum and 5 a bit over toward oversampling). For a 8" scope the above formula would yield 0.56" (in the Rayleigh sense the telescopic resolution would otherwise be 0.68"). If you want to sample this with ~4 pixels (for an intrinsically bright planet, i.e. Jupiter, Moon or Mars) then it yields 0.14"/px. For a C8 coupled with an ASI224 this can be achieved with a 2.5x barlow/powermate and a bit of spacing.

One other suggestion, if I may. I looked at your gallery and see you've done a few images with the 183MC and C8. Assuming you have one that came with the C8, I think the finder scope will come in very handy when trying to get your target planet centered on the 224MC's sensor.

What I did was with my 294MM, I slewed and plate solved on a bright star (Vega). Once it was centered, I then aligned the finder scope so it was perfectly centered in the crosshairs. Now, when I slew to a planet, I use the mount controls to ensure it is centered in the crosshairs of the finder scope, and that puts it squarely in the center of the sensor as well.

I've got my new planetary setup coming in this week - 662MC, ADC and Tele-Vue 2x Barlow. I'll be doing the same things I've mentioned here... getting focus during the day, then focusing on the moon at night. With my finder scope well aligned, I'll have a much easier time getting my target on the sensor. That's the plan, anyway .

Thank you, I have thought about using the finder scope and I will definitively try that resource as well. The reason I have pictures of Jupiter with the 183MC is because after a lot of frustration with the 224MC I was able to plate solve and get the planet in focus (using the ASIAIR pro) with the 183MC. I know is not ideal for planets, but after many lost hours with the 224MC the 183 was better than nothing.... Please have a look of my imaging train and let me know if it looks right?.. I don't have an ADC. By the way, focusing with the moon seems a good idea, although I wander whether it will be a challenge since the moon will be super big for the 224MC.. no?

Imaging train for the SCT8 (planets)

andrea tasselli:
I've read some good suggestions and some not so good ones.

Using the Moon to find focus is a good suggestion. It is bright and large and even eye-balling it you should be able to point at it. For a SCT your best bet for all things planetary is to use a diagonal and a high power eye-piece for centring. In fact you could go one step further and make the EP parafocal to your camera and be done with it. As for the suggestion of focusing during daytime this is only meaningful if you have a very long baseline, several km away and even then it won't be in focus for object at infinity. In fact even focusing on the moon won't guarantee the focus is perfect for them planets.

I'd recommend using the finder and centre it against a bright star as seen through a high power EP (usually going through the intermediate step of centring against the some feature on the Moon as seen with a high-power EP). You most likely won't be able to use to land it on the chip when using a barlow lens but good enough to be seen in the EP (with the barlow), checking the seeing and centred well enough to be well within the confines of those tiny chips.

As for the suggestion that somehow the pitch of the sensor decides the focal length you should be using, this is balderdash. Worse still, it puts the carriage in front of the horse, in manner of speaking. Assuming that telescopic angular resolution in a wider sense is defined by the formula: lambda/D, with lambda the wavelength of light of choice (usually 550 nm) and D the effective diameter of the lens or mirror of the telescope the recommendation here is to sample that angular resolution with 4 pixels (e.g. between 3 and 5, with 3 being the bare minimum and 5 a bit over toward oversampling). For a 8" scope the above formula would yield 0.56" (in the Rayleigh sense the telescopic resolution would otherwise be 0.68"). If you want to sample this with ~4 pixels (for an intrinsically bright planet, i.e. Jupiter, Moon or Mars) then it yields 0.14"/px. For a C8 coupled with an ASI224 this can be achieved with a 2.5x barlow/powermate and a bit of spacing.

I have a flip mirror. Will this help achieve focus and center the planet real time?.

andrea tasselli:
As for the suggestion that somehow the pitch of the sensor decides the focal length you should be using, this is balderdash. Worse still, it puts the carriage in front of the horse, in manner of speaking. Assuming that telescopic angular resolution in a wider sense is defined by the formula: lambda/D, with lambda the wavelength of light of choice (usually 550 nm) and D the effective diameter of the lens or mirror of the telescope the recommendation here is to sample that angular resolution with 4 pixels (e.g. between 3 and 5, with 3 being the bare minimum and 5 a bit over toward oversampling). For a 8" scope the above formula would yield 0.56" (in the Rayleigh sense the telescopic resolution would otherwise be 0.68"). If you want to sample this with ~4 pixels (for an intrinsically bright planet, i.e. Jupiter, Moon or Mars) then it yields 0.14"/px. For a C8 coupled with an ASI224 this can be achieved with a 2.5x barlow/powermate and a bit of spacing.

@andrea tasselli I think we end up with almost the same recommendation for Alicia, although we start from different places. As you say, the angular resolution of the telescope is given by lambda/D. At the focal plane, this corresponds to a length of F*lambda/D, where F is the focal length of the telescope. The number of sensor pixels that this covers (N) is then given by N = F/D*lambda/p where p is the pixel size. But F/D is just the focal ratio of the telescope (f, say), so we can rearrange the equation to get f = N * p/lambda. If according to your example we choose N = 4 and lambda = 550 nm (or 0.55 microns), then if p is in microns we find f = 7.3 p. Choosing N = 3 with the same wavelength gives f = 5.5 p, very similar to my recommendation.

So your recommendation that the barely-resolved disc should cover a certain number of pixels in the sensor is exactly the same condition as my recommendation that the F-ratio should equal a certain factor times the pixel size. Of course, one can argue about the number of pixels or the size of the scaling factor: if the seeing is good and telescope quality is high, then one can use more magnification than when the seeing or the optical quality is lower. But overall, I think the rule 'focal ratio equals five times the pixel size' is easier to apply and involves less calculations.

Alicia Rossiter:
I have a flip mirror. Will this help achieve focus and center the planet real time?.

Provided that you have the means to land it within the view circle of the EP yes. With a barlow/powermate of the amplification suggested (2.5x) wit would be kind of awkward but doable if you place the barlow/powermate ahead of the flip mirror. Doing so would most likely result in an increase in amplification w.r.t the nominal value though.

Know this: SCTs and the C8 in particular are meant to give their best image at ~ 150mm behind the visual back thread. You should therefore make provision that your original focal point is there or thereabout. When you add a barlow or similar the focal point would move outward so you shift the position accordingly. Most people would want to use an external focuser so as not to disturb the previously said best focus position.

Also, a SCT would not reach focus if your camera is too near to the visual back flange thread or too far away.

Alicia Rossiter:
Thank you, I have thought about using the finder scope and I will definitively try that resource as well. The reason I have pictures of Jupiter with the 183MC is because after a lot of frustration with the 224MC I was able to plate solve and get the planet in focus (using the ASIAIR pro) with the 183MC. I know is not ideal for planets, but after many lost hours with the 224MC the 183 was better than nothing.... Please have a look of my imaging train and let me know if it looks right?.. I don't have an ADC. By the way, focusing with the moon seems a good idea, although I wander whether it will be a challenge since the moon will be super big for the 224MC.. no?

Imaging train for the SCT8 (planets)

That looks awfully close. Not sure you'd be able to get to focus with the camera that close to the tube. Hopefully someone with more planetary imaging experience chimes in with a definitive answer. I suspect this is the reason you can't get focus. When you imaged with the 183, what did your imaging train look like then?

Because the moon is big and doesn't entirely fit onto the sensor is the reason I chose it. Relatively easy to get it on the sensor and it's big enough that you have plenty of object to work with even if it's not exactly centered... and plenty of craters/details to determine how well you're focused. Besides, it's going to be pretty prominently in the sky this week.

When using a flip mirror and focusing with an eyepiece on the moon it may be advisable to use an ND filter in front of the eyepiece since you're essentially looking into daylight with your C8.

Clear skies
Wolfgang

Alicia Rossiter:
Thank you, I have thought about using the finder scope and I will definitively try that resource as well. The reason I have pictures of Jupiter with the 183MC is because after a lot of frustration with the 224MC I was able to plate solve and get the planet in focus (using the ASIAIR pro) with the 183MC. I know is not ideal for planets, but after many lost hours with the 224MC the 183 was better than nothing.... Please have a look of my imaging train and let me know if it looks right?.. I don't have an ADC. By the way, focusing with the moon seems a good idea, although I wander whether it will be a challenge since the moon will be super big for the 224MC.. no?

Imaging train for the SCT8 (planets)

That looks awfully close. Not sure you'd be able to get to focus with the camera that close to the tube. Hopefully someone with more planetary imaging experience chimes in with a definitive answer. I suspect this is the reason you can't get focus. When you imaged with the 183, what did your imaging train look like then?

Because the moon is big and doesn't entirely fit onto the sensor is the reason I chose it. Relatively easy to get it on the sensor and it's big enough that you have plenty of object to work with even if it's not exactly centered... and plenty of craters/details to determine how well you're focused. Besides, it's going to be pretty prominently in the sky this week.

The size of the imaging train (from the lock ring of Visual Basic to end of camera is 5 mm). I suspect you are right. I think the 5 inches (127 mm) should be from the Visual Basic lock ring to the camera sensor which is basically in the area of the smaller red ring after the power mate. I think in need to increase the imaging train with an extensor. I took the 5 inch measurement from Celestron web site

When using a flip mirror and focusing with an eyepiece on the moon it may be advisable to use an ND filter in front of the eyepiece since you're essentially looking into daylight with your C8.

Clear skies
Wolfgang

Thank you!

Alicia Rossiter:
Thank you, I have thought about using the finder scope and I will definitively try that resource as well. The reason I have pictures of Jupiter with the 183MC is because after a lot of frustration with the 224MC I was able to plate solve and get the planet in focus (using the ASIAIR pro) with the 183MC. I know is not ideal for planets, but after many lost hours with the 224MC the 183 was better than nothing.... Please have a look of my imaging train and let me know if it looks right?.. I don't have an ADC. By the way, focusing with the moon seems a good idea, although I wander whether it will be a challenge since the moon will be super big for the 224MC.. no?

Imaging train for the SCT8 (planets)

That looks awfully close. Not sure you'd be able to get to focus with the camera that close to the tube. Hopefully someone with more planetary imaging experience chimes in with a definitive answer. I suspect this is the reason you can't get focus. When you imaged with the 183, what did your imaging train look like then?

Because the moon is big and doesn't entirely fit onto the sensor is the reason I chose it. Relatively easy to get it on the sensor and it's big enough that you have plenty of object to work with even if it's not exactly centered... and plenty of craters/details to determine how well you're focused. Besides, it's going to be pretty prominently in the sky this week.

Thank you for all the good advice. Hopefully tomorrow will be clear and I will try again