There is a divide in the astrophotographer community and you find people using specialized CCD sensors and optional motorized filters -- this is the top-of-the-line approach and they often use very specialized software and $$$$ cameras.
Many others use a dSLR, and they divide between Nikon & Canon and I don't want to hear about which camera is better. You must choose the right software and be sure to check if the software really supports your particular model and what limitations it may have compared to the newer models & software.


General purpose tethering software
digicamcontrol.com is one tool which supports Nikon dSLRs. Open-source & free is another advantage. and now they've added support for Canon as well
breezesys.com/products.htm sells tethering software but you need to buy different product for Nikon & Canon.

Nikon & Canon also sell their own tethering software (Nikon Camera Control Pro 2 & Canon EOS Utility). The big image-editing packages from Adobe have some tethering functions and PhaseOne Capture One also offers such a feature. Here is a more extensive overview : tethertools.com/tethering-software
stark-labs.com/DSLR_Shutter.html serves a very specific purpose -- it allows a PC to let (mainly Nikon) dSLR take exposures longer than 30 seconds. These cameras limit exposures to 30s EXCEPT for MANUAL BULB mode and with a remote cable release and some adapter, a PC can mimic that. the SW has no image preview or other functions.

At the time (2011), the solution by Breeze was the one I liked best and especially the added feature of realtime green-screen overlay made it attractive for use in a demo photo-booth. It also allowed to adjust focus and later perform focus-shacked images. The license of this is problematic and even as a legitimate owner, I cannot re-install the software again. support told me I need to purchase a new $175 license.
Nowadays I would take a closer look at kuvacode.com/smart-shooter ($50, 30-day trial) and oxfordeye.co.uk/tetherPro ($50, no trial period)


Software specific for ASTROPHOTOGRAPHY
Why use a tool specially designed for astrophotography you want to know ?
You can do without -- but it makes life easier in a number of ways, especially when you connect a telescope mount and other equipment to your laptop as well.
-) control the telescope movement directly from the same GUI -- no need to switch between programs
-) send image data to other tools -- e.g. plate solver to determine the position of the telescope
-) help with tasks -- e.g. aid with mechanical polar-alignment
-) help with tasks -- e.g. provide feedback on focus & sharpness
-) help with tasks -- e.g. pause exposures when mount is moving (dithering, meridian flip, loss of auto-guiding tracking)


Astrophotography software to tether a CANON dSLR
One popular software is "Backyard EOS" and it has a reasonable price. For astrophotography, you will want the $50 premium version to be able to control your telescope mount and have the other features. The standard version costs $35USD.

My preferred tool isn't BYEOS -- I favor APT. ideiki.com The reason is more historic than the lower price of 12.70EUR (< $15USD). APT supports seamless interface with AstroTortilla (freeware plate-solver) while at the time BYEOS did not. (Now it has caught up). Furthermore, APT has good support for ASCOM-cameras in addition to dSLRs and BYEOS at the time was lacking that feature. APT supports my Starlight Express and I had no reason to switch.
Both, APT & BYEOS can control a mount but that doesn't mean, you cannot use the software without one.

Sequence Generator Pro has a much more complicated GUI than both APT & BYEOS : mainsequencesoftware.com $100USD. If you are willing to use a more complicated GUI while out in the field, it has some useful extras. Especially the $40 MOSAIC-wizard sounds like a very useful addition.
At $100 it costs as much as BYEOS plus BYNIKON together -- and SGP supports both camera families at no extra costs.
My biggest complaint about SGP are the large number of entries which makes this tools barely usable on the 1366x768 screen of small laptops

NEBULOSITY is another software to support both Canon + ASCOM-cameras and at $100 for the new V4, it also is cheap by astronomy standards. stark-labs.com/ nebulosity.html . Unlike APT & BYEOS, Nebulosity does little to integrate the whole image capturing process (slew, focus, guide, etc. ... ) and instead it has more sophisticated post-processing features.


Astrophotography software to connect to a NIKON dSLR
There now also is a BackyardNIKON -- and it has been approx a year since I have tried it. It is time I try again.

There is no version of APT or Nebulosity which supports Nikon dSLR and Sequence Generator Pro I discussed in the section covering Canon.

I've not seen any astrophotography software solely designed for Nikon without having a Canon counterpart. Smart-Shooter 3 and Tether-Pro possibly can do all the astro-photography image acquisition work and you can handle the mount with an optional, separate tool.



What else do I need to start with tethered astrophotography
The biggest investment is a dSLR -- preferably one with LifeView support other the USB connection. And many cameras can do that without the need of $$$ software. Here is a table : digicamcontrol.com/supported_cameras or check if you like APT (Canon only), Smart-Shooter or Tether-Pro better.
If you are looking for a new camera, consider a model with a flexible rear-LCD. While that is not a must and tethering can eliminate some inconveniences, flipping up a screen really can make observations much more comfortable. The basic up/down tilt is important while a screen with full tilt & rotate (articulated LCD) is a nice to have feature. You will be using the LCD during setup before you start connecting to your laptop -- my 2 cents.

The next accessory you need is a simple (6ft...10ft) USB-cable. As a precaution, I use TWO cables : a short cable which plugs into the camera plus a long one to connect to the laptop. If anyone stumbles over the cable, the two cables separate and (hopefully) there's no damage to the camera, connectors and laptop.
Without this fail-safe, such an accident yould damage the laptop or tip over the telescope + camera.

Additional batteries to power the dSLR are a must and I recommend using a battery-grip. The many long-exposures usually done during astro-photo sessions are taxing the built-in battery. Now add to this the power needed to stream live images to the PC and you can see the need for added power.
This can be a simple vertical grip battery pack or a dedicated 12V converter. 2...3 hours often can be done on a regular battery, and often a battery grip is sufficient for my ~5hr observations. Cold weather decreases battery life -- it also decreases my willingness to stay out there & freeze.
With a motorized mount, the need for battery power increases : ipernity.com/blog/stargazer95050/630825

While we're talking about battery power, your laptop also deserves some attention -- 2...4 hours usually isn't a problem but for a 8 hour night-shift, you usually need to add some booster. It can be a car battery + AC-inverter or a Li-Ion pack. And don't forget, the laptop often powers accessories via the USB ports.

Another important accessory beginners tend to forget is a place to put the laptop.The camera is on the tripod and you have the cables & accessories and now there's no way to place your laptop. In may cases you can improvise and bring a cardboard box, a folding folding-chair or camping table. A larger surface makes it easier to use a (wireless) mouse -- I'm using a trackball which requires only a small footprint. Others may use the trackpad and don't need extra space.

For casual astrophotography, you can use your tripod & (ball)head but when you want to aim accurately, you are better off with a geared or a 2-axis tripod head. And as your exposure times and focal length increase, you should be looking towards a motorized tracking EQ-mount and that will lead you the advantages of dedicated astro-photohraphy software tools.

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