QuestHandler: incompatibility of minVal, maxVal, and range?


I am programming an experiment in which participants need to identify the tilt of a target line.

I defined my staircase in the following way:

#create the staircase handler QUEST
staircase = data.QuestHandler(startVal=5, startValSd =20, pThreshold = 0.75, beta=3.5, delta=.01, gamma = 0.5, nTrials=trial_no, minVal=1, maxVal=75, range=140)

I want the first orientation to be tested to be with 5 degrees tilt (startVal=5), the minimum offset from the vertical to be 1 degree (minVal=1) and the maximum to be 75 (maxVal=75). However, without defining a range, I encouter the problem as documented earlier here, with limited intensity values that are tested:

So, I decided to add a range (range=140), which is centered on the startVal. However, when doing the task, with all correct responses, thresholds go far below zero, which I did not expect to happen, since perfect performance should go closer and closer to 1.

Am I using these parameters wrongly? Any ideas on how to solve this issue?

Any help is greatly appreciated.

Best and thank you,