ADC Question - What am I missing?
Posted 12 January 2018 - 06:36 PM
Posted 16 January 2018 - 12:05 PM
For the imaging trains of some folks the comments I made in my most recent thread about making sure you get enough distance between the ADC "output" & the camera sensor is very relevant...this is to ensure the dispersion from the ADC's prisms is wide enough to see good dispersion & correct for it - John Boudreau takes credit for this suggestion btw...
If you position the camera too close to the ADC output you are not going to get sufficient dispersion a lot of the time & it will seem to make your ADC appear useless!
For SCT users, operating their ADC's at f10/11 (ie, native scale) appears very adequate for good ADC performance from the ZWO unit...this means that placing the barlow after the ADC you may be able to control the resultant image scale a bit better - that and a little extra distance to the camera also ensuring that spread I mentioned first...better than having your barlow before the ADC (ie, nearer the focuser) & getting larger amplification because of the distance to the camera from the barlow, which usually means you jam the camera up too near to the ADC output.
Thanks Darryl, I had read that post and the image train was in this order - ADC, DSLR. No Barlow was employed as I was shooting the Moon in its entirety (utilizing a f/6.3 focal reducer also to fit the entire disk on the sensor). So I take it from what you say that maybe there wasn't enough distance from ADC to sensor to allow the ADC to correct the light. In which case, I shouldn't be using the ADC at all for full-disk moon shots....
I nearly tried Jupiter this morning at 6am but it was cloudy on and off, and bitterly cold, so reluctantly (and somewhat relieved) I was able to return to bed for another hour.
If I'm reading you correctly when you say
"this is to ensure the dispersion from the ADC's prisms is wide enough to see good dispersion & correct for it"
then it's just not possible to see the actual dispersion if the cam is too close to the ADC, and the image looks "OK",and it's for this reason that some distance has to be introduced to exaggerate the dispersion in order that it can be corrected visually (using FC ADC tuning tool, for ex). Right?
And sorry for this quick question - can the ADC can be used to correct the dispersion a star and not just planets? If so I intend practicing on a few at different altitudes.
Some of the points covered here over the last few days hit me as being a bit of déjà vu--- which is certainly not uncommon amongst Cloudy Nights threads :
With some added info brought up in Darryl's discussions with Kevin here, it does appear that Kevin's ADC issues are probably at least largely due to a need for added projection distance from the ADC for the reasons covered well here by Darryl. Kevin, at least in the case of imaging the Moon with a DSLR and ADC at f/6.3, you were no doubt operating with ADC dispersion limitations. And as I've mentioned in the past you're at an additional disadvantage compared to many of us because the planets over the next few years are particularly low in the sky for you, requiring additional projection distance from the ADC in most cases.
And while I agree that there are cases where a Barlow can be used after an ADC with slow focal ratio scopes like an SCT or Mak, I would still default to Barlow use before the ADC whenever possible. In some cases I'd also lean towards use of a telecentric-style amplifier (such as a TeleVue Powermate or ES Focal extender), as most such designs allow essentially the same magnification over a range of spacing distances: http://www.televue.c...werIncrease.jpg
BTW--- This webpage by Martin Lewis is an excellent analysis of ADC use and abberations in various systems that's always a good read:
- RedLionNJ, Pitter and kevinbreen like this