General Ignorance about AltAz and EQ.

[RichardA]

I'm looking for a new scope but I have visual problems - only 1 mediocre eye and that needs lots of contrast or I just don't see things. I have problems finding all but the brightest objects and even aligning the scope. I'm looking for some substantial automation. I would like to take photographs. I have settled on an 8" SCT - probably celestron. I am going to buy Goto but am also intending to go for Starsense to assist with alignment. I am down to a choice between NEXSTAR SE 8" AltAz or Adanced VX EQ.

Now I know that StarSense will not work well on poor nights, and there is less problem for me manual aligning AltAz than EQ.

Hiowever I was told today that AltAz is not good for Astrophotography  -I was told I would end up with tracks rather than spots for stars and don't quite understand why. OK I understand the difference with non-computerised mounts-you have two axes to worry about not one. BUt if both mounts are driven by motors and the motors are controlled by computers, surely they both follow the star. Again I can understand with stepper motors the star could end up as a small block the size of which would relate to the size of the motors "steps" but I can't understand why there would be a track. Both AltAz  and EQ with be rotating about an origin, one driven along a plane with one motor, the other driven by its two motors. 

[Mr Spock]

An EQ mount follows stars in an arc whereas an AltAz doesn't. What this means is for an Alt-Az mount the stars rotate. So although this kind of mount can track stars for visusl use, the field rotation causes stars to 'streak' on images.

[RichardA]

Sorry if I'm being thick, I just don't get it.

Surely EQ motion has the telescope tube simply rotating in a plane with its single motor keeping the star over the same spot in the eyepiece or CCD.  And surely the point of the computer with AltAz systemis is to use its two motors to achieve the same thing as near as possible,ie to have the image of the star over exactly the same place in the eyepiece/CCD. If there are errors in the way that works won't you be left wth a slightly larger square spot about the size of the steps of the motor rather than a track across it?

Can you point me to something that explains the error in my thinking (something with diagrams would help)?

[Stu]

Try this for some explanation:

http://www.astrotx.com/Field%20Rotation.htm

The EQ actually rotates the OTA whilst tracking, whereas the alt Az doesn't so suffers from field rotation

Stu

[nucleardwarf]

Try this for some explanation:

http://www.astrotx.com/Field%20Rotation.htm

The EQ actually rotates the OTA whilst tracking, whereas the alt Az doesn't so suffers from field rotation

Stu

Exactamundo.

[RichardA]

Thanks to the both of you. I was just focussed on the star in the middle. Never thought about the ones off centre. Still leaves me with a real quandary though. Anyone used this Starsense add-on to Celestron scopes successfully??

[tekkydave]

Hi, I have a Starsense which I use with my Celestron 127 slt. I have only managed to get out and use it a few times so I can't speak for how it is in lots of different conditions. When I have used it I have found it works very well, seeing dozens if not hundreds of stars not visible to the naked eye. I did a short review on this forum:

http://stargazerslounge.com/topic/200930-celestron-starsense-on-nexstar-127-mak/

I suspect that if the conditions are too poor for SS to align your scope then you will struggle to align it manually using the normal hand-control methods. There are a few others using SS here, hopefully they will chime in with their experiences.

I struggled to understand the field rotation 'thing' until I looked at the link that BigMakStutov posted. Its not obvious until you think from the point of what the camera 'sees' not what path the scope is following across the sky. Good luck with your new scope. I have seen good reviews of the AVX mount and I might upgrade to one at some point.