Remote Flat Field Setup

My BSM system uses the Alnitak flip-flat, which works great for remote flats. Unfortunatly it's only around 7" diameter. You could place a mask around the periphery, but only part of the aperture would be illuminated, not sure if that would harm the flats, since it is so far from the focal point? They also make the Flat-man L for tubes up to 12". It might be possible to modify the flip-flat, to handle the larger Flat-man. Whether its extra weight and moment arm would cause a problem for the flip-flap motor, I don't know.

Mike

Thanks, Mike, but he Alnitak system will not work for me. I am looking for a free-standing system and wondering how big the screen need to be and at what distance to get a decent flat. Ic will be a challange, as I know from the poor near Ir response of my present EL panel. 

Ed

While I use twilight flats, I like the flat field arrangement used by the AAVSO APASS telescopes at DRO and CTIO. There use to be an image of the setup here: https://www.aavso.org/apass

Hi Ed, I think you could be interested in the following publication:

http://arxiv.org/abs/astro-ph/0510233v1

I have tested it but didn't find it well solving the problem of flat made at, or near, the pupil of the optics. Optically it's very different from a flat at the object distance (sky / twilight), the illumination distribution at image level is not set by the target uniformity but the light distribution following the rays angle. By the way the target uniformity is not so critical. Anyway such flats are never equivalent to the stars light distribution across the image like obtained using raster. This is mostly due to the (huge !) spurious light diffusion at undue angles that reaches the scope aperture. After many tentatives I have not found a good solution and have just decided to use the twilight, it's the best even if not perfect for photometry (but good for imaging ! ).

Clear Skies !

Roger

There are a couple of pictures of the APASS setup on page 36 of the June, 2011 issue of "Sky & Telescope."  The caption reads "(d)escribed in detail in the accompanying text, each survey installation uses a pair of 8-inch astrographs riding on a single mount.  A ring of LEDs mounted on each dew cap illuminates a white panel when making flat-field calibration exposures."

I took the following notes of Arne's discussion of flat-fielding during the 2014 AAVSO CCD School (I took the online version):

• No type of flat field is perfect or all-encompassing
• Beware of scattered light
• LEDs are problematic when used as a light source for flat fields because they don't cover the entire spectrum
• Electro-illuminescent panels have a similar problem toward the red end of the spectrum
• Incandescent bulbs emit at all wavelengths
• Projector screens
  • White paint is not white (TiO, poor blue response/red)
  • Extruded polystyrene foam board (insulation board) actually works pretty well if the backing is pulled off of the dull side, as does projection screen material (especially the glass bead variety)
  • Most professionals use a paint formulation developed at KPNO, but polystyrene foam works just as well.
• Installation
  • Screen needs to be far enough in front of the projector lamps to even out the illumination
  • Screen needs to be close enough to be out of focus
  • Reasonable guideline is to be ~0.5-1.0 telescope length distant
  • Usually make screen circular, large enough to cover entire primary mirror, with black border to reduce scattered light (Krylon flat black 'camouflage' paint)
  • Place lights equally=spaced around aperture of telescope
  • The colder you make a CCD, it changes the pixel response.  So taking flats at a different temperature than the science images makes a difference outsi8de the normal BVRI spectral region
  • Incandescent 12 volt lamps work effectively (don't work for U band). Arne uses bulbs from Edmunds that are used for model railroads. He uses 'banks' of lamps to change the intensity.
  • Arne feels that twilight flats can be superior to projection systems, but are harder to work with (clouds, etc.)
  • May need separate flats for each side of meridian due to mirror flop and associated effects.

I hope this helps.  Bear in mind that I'm just a newbie at this game, and may not have the necessary background to have understood Arne's comments correctly.

Mike

The concern regarding panel spectral output variations is not a big issue, IMHO. The BSM using Alnitak adjusts the exposure time to get the same mean ADU level between the filters. Yes, the panels are very poor in IR, but the exposure makes up for it. For example, the C filter uses 0.25 sec exposures, while the I filter uses 50 secs!!! They do provide a nice, reliable uniform illumination and can be done anytime when its dark, esp. when its raining ;)

Thanks for the paper, Roger, very interesting but probably not possible for an amateur to fully implement. But perhaps some of it is: (1) making the screen the same size as the aperture and (2) surrounding it with non-reflective black are great ideas.  I assume that the distance would be determined simply by looking through the scope. The choice of LED lights is also interesting; I wonder if one could simply substitute bulbs in an LED panel to broaden the spectrum. For example, a light box made with LED strips of different wavelength?

Ed