Anthony,
All mounts have a periodic error, that stem from various fundamental mechanical factors. This errors are greater in some mounts than others. The general peak of this error will be based at the fundmental period of your main worm gear, there will be other smaller peaks based around the periods of other smaller gears in your gear train. These usually will not affect short exposure work like Planet imaging. But will affect longer exposure DSO work.
This is because the aim of the game in terms of imaging is to gather as much signal as possible so as to reduce the overal Signal to Noise ratio. That is each frame you capture of certain lenght t, will have some random noise. However by capturing more frames you, the common signal (the actual object) willbe boosted and random noise will be rejected.
A t second exposre at a FL of f, will prduce a signal+noise s+n.
If you double the FL to 2F, and wish to capture the same amount of signal strength then you must double the time for the exposure. So eventually your exposure length will be affected by the movment caused by the PE of your mount.
With Planet imaging long FL is a requirement to achive bigger image scale. However given there relative brightness of planets to DSO's (even at long FL's) they require much less in terms of exposure to cpature and using high frame rate camera's you can gather plenty of frames to improve signal quality. Even at long FL's the exposure time is still relatively much smaller than the usual PE of your mount, so its not as badly affected.
For DSO work, your often need much longer exposure times in order to get some signal to be recorded. The longest exposure time will be determined ofcourse by your light pollution limit. This is the point at which your signal will be washed out by the light pollution. At shorter FL's you can use lots of shorter exposures to gather signal and as such are going to be less affected by the PE of the mount. But at longer FL's your exposure time will need to be increased and you will start to see the PE affect your images. Hence guiding is a requirment at these FL's.
Essentially as you increase FL, your field of view will reduce, your exposure time will increases and the PE is going to become more noticable.
One form of correctio for PE Is PEC, where essentially you record the PE of yoru mount using a star. Your mount electronics then can reaplay this back while you are mount is running to negate the effect of the error. Imagine a Sine wave and you then add a sine way 180degrees out of phase to it. You esentially cancel it out. Essentially each gear/worm or gear/gear will have a period and a error around that period, if these are small then they can be remvoed by recording and replaying a inverse waveform. Two factors are critical here the magnitude of the erorr and the period.
The magnitude must not be high enough to move a star in your frame such that its recordable. The period should be such that its longer than your exposure lenght. If the magnitude is high and the period is shorter than your exposure time then even with correction you will see it. Because even the correction will cause an error, due to other factors like backlash etc. If the magnitude is small and the period is long then PEC and guiding will help get you longer exposures (up till your sky limit). However there is only so much this can do as the random error will not be repeatable and hence can not be removed. This requires guiding on a star while imaging to remove, so live corrections can be made to the mount while taking your exposure.
But even with guiding and or PEC some mounts are better than others. Infact some mounts do not come with PEC at all as there PE is very low. And can be more easily guided out. Hence the reccomendation to get the best mount you can afford.
You have in your post noted a very important aspect your location, this will determine your sky limit. You can use your DSLR and do a quick check of this by exposing a short Fl lense for various periods and see when the sky glow washes it out. And if you are not going to image anywhere but your primary location then this is important deciding factor. And even if you visit dark skies you need to establish how often so you dont buy equipment that is only going to be used some few days in year.
One other method to tackle bad PE is the use of AO, adaptive optics. This an expensive option but can provide very good results. But I find that the money you spend on this could be better invested in a better mount. However AO does have its place and I have seen amazing results from its users. A lot of the fork mounted scopes from Meade and Celestron suffered high PE magnitudes and AO is a great way to reduce this effect and make them viable imaging platforms. Check out this guy's page
http://www.astroanarchy.blogspot.com/
And here is his example, right one is using guiding via a second telescope and left one is using AO.
http://bp1.blogger.com/_9HcYqbvUc4s/.../AO-no+AOy.jpg
As others have suggested there is much to be learned ahead of you and you will greatly benfit from going to club nights and interacting and making friends with others. There information and help will be your gretest asset, not your equipment.
I leave it for others more experinced to correct any errros i have made. As I am no expert on these, only surmising what i have gleaned so far.
Regards
Fahim
