Two parts...Vignetting by smaller adapter and Flats

[Lenny]

Question #1-
I want to mount my 383+ EFW2 filter wheel with 1.25" filters on my NexStar11GPS using a T-thread adapter. Will it vignette the filters? It is an adapter I'll have made by Precise Parts.

Question #2-
What is the optimum range for flat frames for the 383 L+ Mono? I did some images with 10 minute subs and applying flats with a range of 26,000, 33,000, and 52,000 I still see some radial uneven background...especially in the middle.

[Lex]

Hi,

On the C11 you will always encounter vignetting with the "big" chip of the 383, if you mount the camera as close as possible to the EFW2 the 1.25" filters won't vignette the chip. The filters themselves will not be vignetted at all. If you will use a Focal Reducer thing will change, with the f6.3 I had slightly heavy vignetting that can be easily cropped. The center image is nice and tight!

For the flats the normal ADU value is between 25000 and 35000 depending on the source. What kind of Flats do you take, ELP, domes or twilight? Also always think to collect many flats and calculate their median to retain a homogene result.

Best

Lex

[Lenny]

The camera and EFW2 are mated in the normal way attached together the way they came. My concern was making an adapter that fits for scope side of the filter wheel and will attach it to either the f/6.3 reducer/corrector, the larger optical back (3.3"), or an standard T-thread adapter. The latter is my choice. Whatever it is it's got to be custom made since the threads on the camera nosepieces don't seem to attach to anything I've found.

I had read somewhere that the nominal range was around 24,000 but it almost looks like that's overdoing it some. I took some tonight at 15,000 and 25,000 and I'll see how they turn out. I use a translucent plastic cover over a 14" square foil lined box with an incandescent bulb inside on a dimmer. The box sits on the floor.The light through the box is reflected off a flat white foamboard panel set at 45 deg. to the scope. There's another piece of translucent plastic over the scope so the light gets very well diffused. Sounds more complicated than it is.

[bwa]

If you're talking about adapting the female M54 thread on the EFW2 to female M42(T) thread, I know of at least three different commercial adapters that work.  ATIK, Baader and QHY each make one.

bwa

[Lenny]

Can you supply part numbers or web sites where I can find them? I've seen nothing on the Atik web site about any adapters.I'd appreciate it. Thanks-