I was wondering. Is there anyway I could use a mv ballast to strike and run run an HPS lamp? Is there a special mv to HPS retrofit ignitor I could buy? Could I run an mv bulb on an HPS ballast if I used it without an ignitor? Thanks.

If we put aside the "retrofit" lamps (specially designed to run on a ballast intended for other chemistry than is the lamp), the general answer would be no.
But there are some exceptions.

Generally the lamp could be operated on a ballast, if:
- The ballast delivers enough peak voltage for the lamp ignition (therefore pulse MH or HPS, won't start on any ballast without the ignitor)
- The ballast is able to feed the lamp with cold electrodes (higher voltage drop) just after ignition (therefore the MV runs well on a probe MH ballast, but the opposite is not that reliable)
- The maximum voltage the ballast generate is not greater than the lamp design can withstand, mainly during hot restart. (therefore you should never operate any probe start lamp on a ballast containing an ignitor generating anything above 750V, even when it may appear to work normally)
- And one of the most important, the load characteristic of the ballast should operate the lamp with it's characteristic in a stab;le manner on the desired current level. And vice versa, the lamp should not (by it's too high or too low) arc voltage be overloading the ballast. In another words, the rated LAMP (don't confuse that with line input rating) currents and voltages should match.

The simplest guaranteed way for all to match is obviously using the rated combinations (that includes operating the MV lamp on a probe MH ballast), but I know, that is not the question.

Otherwise in the US, only very few combinations are viable:
- You may operate some MV on some pulse MH HX ballasts, but the ignitor must be deactivated. With many of them it won't operate at full power, but it should be within the range the MV's still work well. Note, the opposite is not possible (even when you would add an ignitor), as the MH has lower arc voltage and it will load the MV ballast more than it is designed for. So the combinations:
  H46 on a MH35W, MV75W on M110, H38 on M98, MV125W (European) on a M90 and H39 on M102 or S56
- The S56 lamp could be operated directly on an M102, but the opposite won't (maybe only with deactivated internal ignitor and added extra superimposed type, plus ballast protection against rectifying lamp). The operation characteristics are the same, but the MH needs higher ignition voltage pulse and it may rectify at the end of it's life, so fry the ballast when not protected against this behavior (resettable thermal cut out,...)
The problem with ignitors in the US is, they are an integral part with the ballast (or better to say the ballast is an integral part of the ignitor functionality), so the parameters like generated pulse voltage depend on the ballast and not that much on the ignitor. The reason is, the ignitors use the winding in the ballast as it's step up pulse transformer, so without that it just generates about 100V or 200V pulses (between the tap and lamp end).
So if you want to use some nonstandard pulse voltage (or use remote ballast arrangement exceeding the maximum ballast to lamp length), the only option you have is the use of an European superimposed ignitor with the required parameters (with the remote ballast arrangement the ignitor is then placed inside of the lanterns). But these work only for lamps with arc voltagezs below ~130V (over the complete life; that is important mainly for HPS, where the arc voltage uses to nearly double for the same type at the end of life) and ballast OCV above 200V.
But be careful, in Europe are used even long range semiparallel ignitors, but those need special choke (with tap in 20% from the line input) and there is no way, how to make these working with the US HX ballasts.


In European area (and I would guess all 230V/50Hz markets) generally the MH's (there are practically just the pulse start types) are designed to operate on the HPS ballasts, but need higher voltage ignitors (unlike the US market, it is usually not integrated into the ballast, but sold as a separate component). These combinations are even officially rated.
And the usable "MV on MH ballast" combinations are practically the same as in the US (the MV's and pulse start MH's are of the same specifications on both sides of the "pond", just the HPS differ significantly)

I'll have to do some measurements on the HPS and mv ballasts when I get a voltmeter; they are both 100w. I just don't want to cook my little keystone ballast. It is about the size of an old leviton pulse rated mogul lampholder.

The 100W HPS uses just a series reactor ballast (a choke in series with the lamp, nothing else), the Neutral connection is just for the ignitor. Therefore ii has the OCV just the 120V mains, the arc voltage is about 55V (to stay below 50% of the OCV over the whole life).

The 100W MV (alike all MV's) is around 110V arc lamp, so need at least 200V OCV for the arc to operate steadily (again, the arc voltage can not be much more than 50% of the OCV; the MV does not increase it's OCV, so the 200V is still sufficient).

So when you connect the MV lamp to the HPS ballast, the lamp will either not ignite at all (when the ignitor is disconnected, the 120V is not enough) or start overdriven (the HPS ballast feeds ~1.9A into the cold burner, low voltage arc) the lamp. BUt as the arctube warms up and so the arc voltage rises above ~60..70V, the arc extinguish.

When you connect the HPS into an MV ballast, it will most likely not ignite (the usual 250V is usually not enough for a pulse start lamp), but if it ignites, the ballast current would be too low to warm it up properly, so the tube will likely remain in the low voltage state. But the problem would be with the ballast: As the load voltage won't rise to the full designed 100V, the ballast secondary would be stressed to higher current than originally designed for. And that may fry it.

Thank you for your help, Medved, I have gotten a HPS to Run on my advance Merc ballast when I warm it up with a HF fluoro ballast wired to 4x overdrive, sending 128 watts of power into the bulb. I have tried the merc on the HPS ballast, but it only works with an igniter, and it doesn't even reach ful bright (it glows as if it's greened out or cycling).

I don't think the 4xF32T8 (I guess that is the "128W") will overdrive the HPS, but can not exclude it. The F32 arc voltage is about 140V, while the HPS voltage is just about 55V, so who knows, what was the real current there... Definitely not anything really usable for real...

I don't think the 4xF32T8 (I guess that is the "128W") will overdrive the HPS, but can not exclude it. The F32 arc voltage is about 140V, while the HPS voltage is just about 55V, so who knows, what was the real current there... Definitely not anything really usable for real...

yeah, it didn't warm fully, it glowed Orange on the electrodes and just still had the glowing Mercury discharge in the center. I flipped the ballast switch a few times to give it a kick start and when I turned it on, it flashed briefly. It def took a lot of motivation for it to start. I wonder if the fact that I left the caps soldered to the IEC connector. I'd imagine that those caps have relatively low value (two small blue ceramic caps and one bigger plastic cased metalized Mylar cap).

Which caps do you mean and where they were connected? On the lamp output? Then they were for sure interfering with the ballast (the electronic has a capacitor parallel to the lamp, but it is part of the output resonant circuit and it's capacitance should be quite tight under control; connecting such extra means you have added who-know-what...).

With HPS, there should be really nothing, no extra capacitance at all (even on "magnetic" ballasts the ignition pulse is rather high frequency thing). Many times even just the parasitic capacitance formed by the cable between the lamp and ballast becomes way too high (therefore the length of the cable is limited to 6 foots or so; but better to keep it shorter, for longer cable you need special long range ignitor/ballast)

Which caps do you mean and where they were connected? On the lamp output? Then they were for sure interfering with the ballast (the electronic has a capacitor parallel to the lamp, but it is part of the output resonant circuit and it's capacitance should be quite tight under control; connecting such extra means you have added who-know-what...).

they are the power smoothing caps on the input plug of the unit. They were already there before I converted it from a computer PSU to a merc Ballast. 

If it is on the mains input, it should not harm.
Problem would be, if you intend to use that connector for the lamp output, then you should remove everything there (even fuses!).