No barlow for rear cell of SCT?

[brancusi]

For prime focus astrophotography, we can use focal reducers for a wider field of view. For planetary imaging, it would be nice to get closer - but all the barlows I can find are for eyepiece use, not for screwing to the rear cell of the SCT. I am just failing to look hard enough, or is there something inherent in the optics that means adding a barlow to the rear cell simply doesn't work?

[Leveye]

I use a Baader 2" Quicklock on the back of my C8. Makes using a barlow and a camera or eyepieces a breeze. Very secure as well. Zero sag. Also many options at that point using either a DSLR or a CCD/CMOS camera Just need the proper adapters from 2" to 1.25" barell and a t-ring if using a DSLR. There are a few higher end barlows that use a 2" barrel like Tele Vue's powermates. 

[jambouk]

Or you could fit a crayford style focuser to the back and then add your barlow into that:

http://www.telescopehouse.com/acatalog/Revelation_SCT_Crayford_Focuser_Dual_Speed.html

There is no optical reason you can't use a barlow, there is plenty of mirror travel to allow you to alter where the inage comes to focus, unlike in reflectors and refractors where it can be harder to achieve focus with some additions.

James

[riklaunim]

Barlow at the SCT thread would end up far away from the planetary camera (which would increase magnification a lot). In general planetary imaging quite quickly will want motofocus for fine focusing so a moto-cryford for SCT. For mono cameras also a filter wheel would get into the path. Putting standard Barlow before the camera is just more flexible and easier.

[brancusi]

Thanks! Should I be worried about the length of the kit hanging off the rear? I have a pretty heavy SBIG ST-8E and the thought of a barlow in line makes me wonder about the mechanics. Should I use my diagonal for planetary imaging so that the kit gets stacked vertically or does this really not make much difference?

[JamesF]

I wouldn't use a diagonal for imaging personally.  Just make sure everything is secure.

James

[Vox45]

Sorry to highjack the thread but what would a diagonal do to the focal lenght ? I mean, would object be bigger or smaller compared to putting the camera at prime focus ?

[Zakalwe]

Thanks! Should I be worried about the length of the kit hanging off the rear? I have a pretty heavy SBIG ST-8E and the thought of a barlow in line makes me wonder about the mechanics. Should I use my diagonal for planetary imaging so that the kit gets stacked vertically or does this really not make much difference?

The SBIG isn't really the right camera for the job. You need a fast framerate camera that can record many tens of frame per second.

Weight isn't an issue and you can clamp the Barlow into the visual back. This does mean using the standard SCT focuser which will rapidly become a major obstacle, for two reasons:

1. It isn't really precise enough and it is difficult to get good focus
2. Every attempt to focus will move the scope. Planetary images subtend a very small arc and keeping them on the small sensor is difficult if the scope moves even a little.

The best option is to add a Crayford focuser on the rear and motorise it.

I use a Powermate, which can be threaded (via yet ANOTHER small expensive bit of aluminium)  to a T-thread, then onto my filterwheel and then threaded onto the camera. This is all held in a Moonlite focuser. Rock solid.

[JamesF]

Sorry to highjack the thread but what would a diagonal do to the focal lenght ? I mean, would object be bigger or smaller compared to putting the camera at prime focus ?

In the sense I think you're using the term, SCTs don't really have a "prime focus".  They focus pretty much everywhere, just not all at the same time (there's another thread relating to this running at the moment; I'll post the link when I find it).

Adding the diagonal will add about 60mm to the optical path compared with not having one and that will mean that the image will be slightly larger compared with not having one (on an SCT/Mak).  It's also one more optical surface to absorb light, and one more connection that has the potential to come loose or wobble or misaligned, which is why I don't use one for imaging.

James

[JamesF]

Here's the thread that includes some discussion of SCT focal lengths:

http://stargazerslounge.com/topic/238474-preventing-mirror-shift-on-an-sct/

James

[JamesF]

I use a Powermate, which can be threaded (via yet ANOTHER small expensive bit of aluminium)  to a T-thread, then onto my filterwheel and then threaded onto the camera. This is all held in a Moonlite focuser. Rock solid.

Woo!  Someone else with a Xagyl filter wheel, if I'm not mistaken

James

[Zakalwe]

Woo!  Someone else with a Xagyl filter wheel, if I'm not mistaken

James

Give that man a kit-spotters badge!

yep...it's a good little wheel for planetary and it integrates nicely into Firecapture via the ASCOM driver.

[Vox45]

In the sense I think you're using the term, SCTs don't really have a "prime focus".  They focus pretty much everywhere, just not all at the same time (there's another thread relating to this running at the moment; I'll post the link when I find it).

Adding the diagonal will add about 60mm to the optical path compared with not having one and that will mean that the image will be slightly larger compared with not having one (on an SCT/Mak).  It's also one more optical surface to absorb light, and one more connection that has the potential to come loose or wobble or misaligned, which is why I don't use one for imaging.

James

Yes, when I say prime focus I mean directly at the scope's visual back (not sure if there is a term for this) so adding a diagonal adds to the optical path therefore making things larger on the sensor ? I need to learn more on optics then as I thought that it was the other way around :/ ho well ...

thanks James!

[JamesF]

Yes, when I say prime focus I mean directly at the scope's visual back (not sure if there is a term for this) so adding a diagonal adds to the optical path therefore making things larger on the sensor ? I need to learn more on optics then as I thought that it was the other way around :/ ho well ...

thanks James!

There's more information in the thread I linked to, but basically an SCT or Mak can focus pretty much wherever the camera or eyepiece is (within reason).  The further from the back of the baffle tube the focal plane is however, the greater the effective focal length of the OTA (SCTs and Maks don't have a fixed focal length) and so the larger the image.  This is a specific "feature" of optical systems where there is more than one optical surface (so the flat of a newt doesn't count, for example) and the distance between them can be changed (thus the RC design is also discounted).

As an example, if I put my 450D on the back of my 127 Mak the Sun or Moon fit very neatly in the frame, almost filling it but with just enough wiggle room to allow for a little tracking error.  If I fit a 50mm extension between the camera and OTA then when the image of the Sun or Moon is in focus it is way too big to fit on the sensor.

James

[Vox45]

There's more information in the thread I linked to, but basically an SCT or Mak can focus pretty much wherever the camera or eyepiece is (within reason).  The further from the back of the baffle tube the focal plane is however, the greater the effective focal length of the OTA (SCTs and Maks don't have a fixed focal length) and so the larger the image.  This is a specific "feature" of optical systems where there is more than one optical surface (so the flat of a newt doesn't count, for example) and the distance between them can be changed (thus the RC design is also discounted).

As an example, if I put my 450D on the back of my 127 Mak the Sun or Moon fit very neatly in the frame, almost filling it but with just enough wiggle room to allow for a little tracking error.  If I fit a 50mm extension between the camera and OTA then when the image of the Sun or Moon is in focus it is way too big to fit on the sensor.

James

I read the thread you linked to. Very informative ! Thanks once more James for clarifying this topic... always a pleasure to read your clear and concise explanations on the matter

[riklaunim]

Planetary cameras are much lightweight than big DS cameras, and thus no problem with the imaging chain weight (although the shorter and lighter the better more or less).

Some of my past SCT planetary imaging chains:

[brancusi]

Thanks everyone, liking the advice about a planetary camera - making me think about a second scope as well now ... is there no end to astronomical spending ;-)

[JamesF]

is there no end to astronomical spending ;-)

Only when your wife finds out or you go to take your final place amongst the stars.  The two may not always be unconnected

James