Atlair15 Jupiter ( late season ) under low Histgram

This year ,rain season is abnormal in some area too much heavy and some are no rain. Here south of Tokyo , continue to be fine days .
Jupiter is almost end of season , Altair 15 shot was done for Jupiter tonight in western sky.

There are 3 points below highlighted.
(1) only 353 framed got during  90sec
(2) FPS was only 4,,,,,,,
(3) Seems Histgram 27% => 150mm is notenough? West Sky too Low? Humid Atmospheric Air ? or Altair15 is too(poor maintained) old about mirror

Data:

Altair-15 D=150mm f1500 x2 barrow Asi120Mc Green 90sec cut .
AS!3 planet mode -> RegiStax5.1 wvlet 45 Dyadic PSE13 brighter .
PC: Microsoft Surface2Pro USB3.0
Camera=ZWO ASI120MC
Filter=G
CMI=121.5° CMII=203.5° CMIII=348.3°  (during mid of capture)
FocalLength=3400mmi
Date=130717
Start=192745.033
Duration=74.970s
Frames captured=353 <---(1)
ROI=496x448
FPS (avg.)=4 <---(2)
Shutter=37.00ms
Gain=38 (38%)
USBTraffic=57
Histogramm=27% <---(3)
Limit=90 Seconds
Sensor temperature=34.2 °C