If you are going to guide, you will want to use ASCOM and serial -- and if you do that forget about Explorestars. It is not ASCOM compatible and probably won't be for some time to come. But believe me, that is not a problem. You can use Stellarium to visually pick stars, and it is fantastic. You can load it on your Asus and play with it as a planetarium program and see what I mean. Then just imagine that all you have to do is click on an object and send your scope to it. Man. It's nice.
As you enter into this journey, please understand getting good astrophotographs is far from simple. You will have to acquire much skill and knowledge -- about many different aspects of the technology involved -- to achieve great results. This ain't no point and shoot hobby. Part and parcel to this is the mount and its peculiarities, all the software and their own quirks, and the workflow necessary to get everything just right. Not to mention the atmosphere, light pollution, and plain old weather. These are all things that will come about but in a step by step process, with each new piece of knowledge or skill leading to improved results.
As to guiding and focal lengths.
The rule of thumb for focal lengths being 4:1 is one of those things that is no better than free advice. I will give you my understanding of things, which as always is possibly flawed in which case I welcome comment.
The deal is sort of this. You will in general be able to guide your mount to an accuracy of about .3 or so pixels on your guide camera. That means the guider will hold the guide star within an area about .3 pixels. This is useful, but not adequate for understanding. What we want is to hold the guide star to a position which limits its motion on the imaging camera pixel. Soooo, we need to understand the relationship between the guider pixel and the imaging pixel size, but in image scale arc seconds) not in pixel size (microns).
The pixel on an imager covers and angular extent of the sky. The image scale of a pixel in arc seconds is given by the equation
Res = (P/F)*206.3
where Res is the arc second pixel scale or resolution, F is the scope focal length, and P is the pixel size in microns.
Your f4 guide scope has a focal length of 200mm. If the guide camera pixel size is about 3.1 microns this gives you a pixel scale for the guider of about 3.2 arc seconds. Thus you might expect the mount to guide to about 1 arc second.
Now the imaging camera is attached to a scope with a 750mm focal length. Assume the camera, DSLR, has a 5.2 micron pixel. Then the image scale for the DSLR and the 750mm scope is 1.43 arc seconds. Not bad. It would seem the guider will guide so as to keep the image within one pixel of the imaging camera.
Set the Res of the camera = .3*Res of the guide and solve for the ratio of the F...
Pi/Fi = .3*Pg/Fg
Where Pi is the pixel size of the imager, Fi is the imager scope focal length, and the g subscript denotes the guider. I have divided each side by the constant 206.3 factor.
3.3 (Fg/Fi) = Pg/Pi
and we want Pi/Pg to be about one. This happens when Fi/Fg is about 3.3:1. So that's where 4:1 comes from.
This is all wonderfullness but reality will set in. PHD2, the guide camera, and the mount constitute a closed loop control system with its own set of dynamics. This system is trying to control the position of a spot on the guide camera as the spot position is perturbed by many different effects...wind, refraction, atmospheric seeing, vibration, flexure, mount PA error, mount tracking rate error to name a few. This is why perfect guiding is impossible. The Dance is to get the things you can manage correct, so that the closed loop system will be able to handle the things you cannot manage. And like any dance, this takes time to get good at it.
So that is lesson 1. Why is 4:1 suggested. Its because things can only operationally track to a certain precision and we need to make that precision work with our imager.
I started out with a 50mm, f4 guide scope and a guide camera with 5.2 micron pixels. Didn't work as well as I needed, so I got a 60mm guide scope with a 280mm FL. Better but I want more performance so I am going to go to smaller pixels in the guider one of these days. You are starting out with a good guide camera pixel scale it seems so you are on the right track it appears. If you get an imaging camera with smaller pixels than in the example above where I assumed 5.2 microns, you might find you will want to increase the guider FL. Also, you may find you will want some more aperture to make more guide stars available.
ASI Air is something I would warn you off of if you are just getting started. All of the software you need to get up and running is basically free by going the ASCOM route with PHD2, POTH, Stellarium, and BYN. After you get good at using that, or at least schooled in its use, then consider spending the money.
One nice thing about going wired to ASCOM is that you can use SkySafari on your iPhone or I think Android to control the mount at the same time you use Stellarium or any other program on the PC. That is a subject I have posted on a lot, and will do so more in the future as needed.
You will have to get good a polar aligning the mount. The iEXOS100 needs a bit of help in that regard. You can use SharpCap I think with your Nikon to get polar aligned if you can see Polaris. If not you will need to get good at Drift Aligning or find software that will help you align without Polaris.
Finally, if your Nikon is old enough it will not allow shutter control in Bulb with the USB connection, you need a seperate shutter release cord. BYN manages it for you but you will have to buy the thing. It's easy to find out. Just plug your camera into BYN and try to take a bulb exposure. BYN will fuss if it cannot do so with the USB. If needed I can write an article about the shutter release cable.
You can test out scope control by Stellarium by using the scope simulator in POTH. Do this. It is just tricky enough to set up that you will want to do it when there is daylight and you have coffee, music, AC. You will need to load a helper program called StellariumScope. Configure Stellarium according to the instruction in StellariumScope and maybe in Stellarium's instructions.
Finally, put on your to-do list to figure out plate solving. It will make your life a lot easier when you start trying to image deep space objects. And you might look into Astro Photography Tool (APT) which integrates everything into one interface, and also integrates with Stellarium.
Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired