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Help Me Think This Through

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#1 tazer

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Posted 05 September 2015 - 04:56 PM

My ES MN is long, tough to balance, hard to collimate, has odd diffraction spikes, has a mediocre focuser, and the collimation shifts depending on where the scope is pointing. It's excellent visually but has been the source of a lot of frustration when imaging. In an attempt to simply things I've picked up a WO GTF81. It's light, no collimation needed, has a great focuser, is well corrected, and has great glass.

 

With my NEX-5 on the MN I'm imaging at about 1.44" ppx @ f/4.8. With my a6000 on the WO I'm imaging at about 1.50" ppx @ f/6.6. Now, the a6000 has a QE of 61% whereas the NEX-5 has a QE of 32%. On the surface it looks like I'll be imaging at about the same resolution with a very similar exposure length. The resolving power of the of the WO seems to be about 1.43" so I shouldn't be losing any detail.

 

The only unaccounted for variable is that the NEX-5 is modified while the a6000 isn't. Considering I'd be imaging through a refractor, it should be irrelevant (as I'd have to use a UV/IR filter on the NEX-5 anyway.) 

 

Am I overlooking anything in my calculations? Since my seeing is usually about average I'm not too worried about loosing out on the 0.76" resolving power of the MN. 

 

Mark



#2 GaryCurran

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Posted 05 September 2015 - 05:25 PM

Why would you need to use a UV/IR filter on a refractor and not a MN?



#3 tazer

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Posted 05 September 2015 - 05:33 PM

I haven't seen much benefit in using the UV/IR filter on the MN. It doesn't look like the corrector imparts too much CA in IR whereas I expect the refractor to impart major CA in IR.

 

Mark



#4 GaryCurran

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Posted 05 September 2015 - 06:28 PM

You'll have to excuse me, Mark.  I'm still just learning about broadband imaging, and considering getting a modified camera.  My understanding of this is that removal of the UV/IR filter is to allow more of the H alpha wavelengths, which are close to the IR spectrum, and OIII, which is closer to the UV spectrum, as well as SII, HeII and, I think, IIRC, Nitrogen.  It isn't about chromatic aberration at all, insofar as I'm aware.  Since most UV sits outside of the visible spectrum anyway, except near infrared (which would be used in remote controls as such), are you going to see it?  I mean, is the sensor going to see it, other than NIR?

 

I would think that a camera modified to remove the UV/IR filter would be preferred for nebula and other emission type DSOs, and you'd want to use that on any kind of telescope, if you're imaging those types of objects.

 

EDIT:  I was just looking at Gary Honis' web page for Full Spectrum modifications that he does, and he had the following to say about this.  Taken from Gary's website:

 

 

1. Astro Imaging: A Full Spectrum modified camera can be used for astro imaging, without any external filters, if the telescope system being used is "reflector" based and contains no lenses. Mirror systems focus all wavelengths of light to the same point; the lenses in refractor telescopes do not. Even the highest quality APO refractors usually require the use of an IR block filter to block infrared wavelengths that do not focus at the same point as visible light. Adding an external UV/IR filter to the optical train, such as the Astronomik UV/IR clip-in filter or Astronomik Luminance (IR/UV) - 2" Mounted or the Baader 2" UV/IR filter among others, can be used for astro imaging with optical systems that do include lenses. Clip-in filters can be used with Canon camera lenses as long as they are not the EF-S type. Clip-in light pollution filters have become very popular with those imaging in areas of light pollution. An ultraviolet pass filter can be used with a Full Spectrum camera to image cloud features on Venus and Jupiter. Other specialized filters such as the Astronomik Planet IR Pro 807 Filter can be used for enhanced lunar and planetary imaging. Simple spectroscopy of bright stars can be done with a Full Spectrum DSLR camera.

 

So, I stand corrected.


Edited by GaryCurran, 05 September 2015 - 06:35 PM.


#5 tazer

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Posted 05 September 2015 - 06:55 PM

Gary,

 

A modified sensor (i.e., full spectrum mod) is sensitive to near-IR far beyond the Ha line and well into the UV range on the other end. Generally speaking, when imaging with a refractor with a full spectrum sensor, you'll get CA in the near-IR which is also known as IR bloat. Most refractive optics designed for visible wavelengths don't pass UV very well. A good UV/IR filter will cut out the UV and any IR beyond the Ha line.

 

Now a reflector will bring the entire range the sensor can detect to focus at the same point (at least that's my understanding) so the only consideration there is how much CA any reducer/flattener will introduce. My MN's corrector has a very small effect on the light cone so it's not nearly as bad as a refractor but probably not as well behaved as a "bare" reflector.

 

Interestingly, there are some folks that have experimented with using an IR pass filter on their guide cams. Since stars are strong in IR and IR is less affected by seeing, the thought is that you can get steadier guide stars this way.

 

Mark



#6 GaryCurran

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Posted 05 September 2015 - 07:17 PM

Again, Mark, I'm still learning, so this was a learning experience for me.  Most of the modifications I've been looking at use the Baader filters, which do not seem to be as wide as the Full Spectrum modifications.  I use two different telescopes for imaging, a small refractor and a 6" R.C. reflector.  So, it seems that if I wish to go with a Full Spectrum modification, I will need to purchase an IR band filter, while leaving the UV alone.  Or, a mild UV/IR filter to at least allow some of the Ha and OIII to pass.  But, by the time I buy the camera to modify, or purchase on of Gary Honis' pre-modified T3s, it's still going to be $700, and I can probably find a used CCD and filter wheel in that price range.

 

At this point in time, I am not ready to purchase anything anyway, so it's moot for now.  But, you have helped me learn something, and that is appreciated.




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