Rigol 1102E -- Digital Filters not available at slower time scales

[random13]

It's my first scope: Rigol DS1102E. Just playing around, trying to learn how all of this works.

I set up the "blink" example on my Arduino and hooked up the probes. Simple square wave at 0.5 Hz, but it looks kind of ... fuzzy.

I know! Low Pass Filter!

Except... at time scales < 50ms / division the Digital Filter option is greyed out.

I looked through the manual and googled, but I can't find any discussion of this ... feature?

Changed my "blink" to 100 Hz and so got to play around with the filters, but now I'm wondering:

What is the reason for the interaction between the time scale and the filters?

I found out that it's pretty easy to connect the scope to a computer and retrieve long time series of data. Would that be a reasonable way to work around the lack of filters at the slower scales?

Thanks for any insights!