Narrowband with an Achromatic Telescope

[avandonk]

I just purchased a Skywatcher 150mm F5 Achro. Here is a pic with the modded 100ED as a guidescope. The other picture is a single JPG straight from the 5DH. Exposure details 8 min at 1600ISO with Hutech focal reducer (0.85) and Astronomik HA filter. Focus could be better. The intention is to use it for narrowband data and just simply establish focus position for any filter and recombine the data with no chromatic aberration!

The Hutech FR is flattening the field very well. Will post a combined image when I get the data as it is a race against predicted rain.

I reckon this will work.

Bert

[leon]

Knowing you Bert, I'm sure it will work.

Leon

[tornado33]

Great idia Bert, who needs to worry about colour fringing when each colour will be a separately focussed narrowband image :)
Nice deep Helix there for a single frame. I cant wait to see it after multiple frames and in the other colours
Scott

[h0ughy]

wow this looks promising, fantastic Bert! unprocessed there is heaps of data in that shot!!!

[Lee]

I've often wondered about using achro's and narrowband myself - will be keen to see how it goes! Looks good so far....

[robin]

Great rats nest of wires Bert! Very pro-looking.Looking forward to seeing your images mate

[ballaratdragons]

:eyepop:

You mean you've had it all of one day and you haven't hacked it up yet :lol:

Bert says "Give me time" :rofl:

[avandonk]

Here is a quick combined image. No flats.
HA 8x8 min at 1600ISO
O3 6x8 min at 1600ISO

HA red to Red
O3 green to Green
O3 blue to Blue

Large image 1.1MB
http://members.optusnet.com.au/~fmlee/hel_nb_acr_01_6.jpg

As you can see the field is nice and flat even to the corners. I am going to have to install a larger aperture focuser as there is some vignetting. The central part of the image sees the whole of the 150mm achromatic lens.
The actual focal length of the scope with the Hutech FR is now 670mm and the f ratio is now f/4.46. This matches quite well to the modded 100ED (FL =686mm )for focal length.
The real aim is to mix colour images from the 100ED with narrow band from the 150mm F5 now 150mm F4.5. This should get most of the really faint nebular data.

Now where is my hacksaw?

Bert

[h0ughy]

a few dust moates, but man - thats a great result so far.

[janoskiss]

Fantastic! Achromat for narrowband makes perfect sense. 6" should give you nice resolution too.

[Geoff45]

Certainly something worth trying. One thing occurs to me though. The focus points are slightly different for each colour, which means a slightly different focal length, which means the image scale will be slightly different for each colour. I wonder if this is a big enough effect to affect the final image?
Geoff

[avandonk]

You are correct Geoff. I have a means of knowing the accurate focus position for any wavelength ie filter and then it is simply a matter of using RegiStar to get perfectly aligned images and using ImagesPlus to recombine them.
See the Helix image
Bert

[Geoff45]

Of course--it's just a matter of scaling, so lots of software packages can do it. I should have thought of that!
Geoff

[avandonk]

No only RegiStar gives you this near perfect registration!

Bert

[avandonk]

You are so right but first the achro has to have 'good' correction ie lack of spherical aberration.

I did a careful check and this finally committed me to trying it out.

http://aberrator.astronomy.net/scopetest/html/refractor_150_3.html

I am not the old fool you all take me for and I am sure it could be worse.
Just joking!

Somebody has to try to be original in thought and action.

Below you will see an image of the helical focuser I have fitted to the 150mm F4.5 and there is almost now no vignetting. Only because I could not find my hacksaw! I had to use the araldite. Behind the helical focuser is the filter box and after that the Hutech FR/FF.

Bert

[Dr Nick]

Looks good! ;)

[avandonk]

Here are a few clearer pictures of the modified focuser. Note the scale on the helical focuser makes it easy to change to the correct focus for each filter very quickly.

Bert

[tornado33]

Ahh yes having a scale on the focuser makes life very easy. Even the numbers on the 300mm lens make focussing easy for me, taking test shots, checking focus then moving focuser a mm or so 1 way, if worse twice as far the other way, doesnt take long till I have sharp focus.
Scott