Whilst waiting for Sirius to climb high enough a few nights ago, I filled in time by grabbing a few shots of M42 using the Canon 40D.
I used the Takahashi Mewlon 180 with Tak X0.8 Reducer/Flattener (efl of approx 1700mm) and ran into a whole lot of coma. The Mewlons are of the Dall-Kirkham design which does have significant coma, but I thought that the x0.8 Reducer/Flattener would produce better results? Maybe I didn’t have the optimum spacing in between the x0.8 R/F and Canon 40D?
This is my first attempt at using Layer Masks in CS3 to blend 3 separate images to cater for the extremely high dynamic range of this object. Some noise has crept in and more practice will improve things, but overall, it was an enjoyable experience.
Welcome to the real world. It is sad is it not that all is not perfect.
Bert
Hi Bert
Although somewhat ironic, all is indeed perfect! The DK design has inherent coma and displays this, quite perfectly.
By using a DIY Canon DSLR to Tak adapter (cobbled together from bits and pieces), I may have flaunted the design spec for correct sensor spacing from the Reducer/Flattener, so once again, the results are indeed perfect, in accordance with my mis-use of the overall system!
Truly, there is no thing such as a free lunch in this game! One real challenge with Tak gear is gaining an understanding of, and then acquiring all the necessary adapters to make sure you are using the gear as the designer(s) intended.
Oh well, back to the drawing board and time to do some measurements.
i am also the owner of a Mewlon180 with the Tak reducer. Maybe you should have to check the metal back distance to be respected : 56mm.
Clear skies.
Pierre.
Hi Pierre
Thanks for the details on the correct metal back distance of 56mm.
I have the Moonlite focuser fitted on my M180 and although I have a Tak adapter that couples my SBIG ST7 to the Reducer/Flattener, I do not have sufficient in-focus travel to focus when using the Canon 40D. I estimate I am approx 2 or 3mm short?
So, I cobbled together a few threaded adapters and have used this – I will go and measure the spacing to see how far off from 56mm I am.
The coma didn’t show itself on the (smaller) 765x510 pixel ST7, but I was expecting some to appear with the 3888x2592 chip of the Canon 40D.
Doesn't look all that severe though Dennis. I have the same reducer/flattener, but always understood it's prime function in life was to sort the under 800mm focal length refractor problems. Here the scope is way over this and also not even made in the same country, not pushing it a bit are you, LOL.
Gary
Good work with such a long focal length Dennis....spacing reducers/flatteners is a pain and It seems that you can never get It right till you stick you hand in your pocket and fork out for the right parts . (Tak)
Doesn't look all that severe though Dennis. I have the same reducer/flattener, but always understood it's prime function in life was to sort the under 800mm focal length refractor problems. Here the scope is way over this and also not even made in the same country, not pushing it a bit are you, LOL.
Gary
Hi Gary
I have the ridgy didge Tak x0.8 Reducer/Flattener, which reduces the M180 from F12 (2160mm) to F9.6 (1728mm), which I reckon can do a better job than my set up and technique has produced.
Pierre put me onto the required spacing of 56mm which is something I need to check. Also, when I looked at my 60 sec and 30 sec exposures, I didn’t see any (as much?) evidence of the flaring, so I’m going to investigate further to check for flexure in my side by side set up. I have a lot of overhang even though the plate and fittings are very substantial and stiff.
Quote:
Originally Posted by atalas
Good work with such a long focal length Dennis....spacing reducers/flatteners is a pain and It seems that you can never get It right till you stick you hand in your pocket and fork out for the right parts . (Tak)
Hi Louie
Yep = you’re dead right; you need to be faithful to the Tak design and adapters – something that I’m learning the hard (and expensive!) way.
Seeing must have been nice for you mate... you've split the trap cleanly into 5, only one more to go! I tried this a few weeks ago with the C11 @ F/6.3 (1756mm) I managed to split a,b,c & d however e and f still eluded me.
It has nice detail present. More subs would clean up the noise a bit, however, considering this image was basically a time filler, its turned out nicely.
Ps... When did we have clear skies??? I must have been sleeping or something silly like that..
I managed to find a few holes in the clouds on the night I split Sirius A & B. Things started off okay but then the cloud bands rolled in occasionally, which killed long exposure imaging so I switched to the DMK31 to grab 40-60 sec AVI’s between the sets.
I did manage to get the Trapezium A, B, C, D, E & F using the M180 pumped up at F48 with the TeleVue x4 PowerMate, here is a 700 frame stack showing the 6 of them. IIRC, the exposure was around ½ sec, hence the soft stellar discs.
The jet stream map seemed to promise more than the night produced, but there were some local winds that seemed to predominate.
Oh well, it was a good night for an equipment shake down – had lots of those recently! LOL!
Yes – that set up should do this nicely. I’ve found that when using the DMK to image multiple stars that have a wide brightness range, especially through Barlow’s, I have often achieved my best results by playing around with the Gamma, somewhere between 16 to 18 (range 10-22) at the time of capture.
This setting (and do experiment) seems to attenuate the overwhelming glare of the brighter component whilst “teasing” out the fainter component.
I’ve found that by just increasing the Gain (linear), the bright component can overwhelm the fainter component(s) in its bloated glare. Using Gamma seems to produce a non-linear stretch that seems to help bring the fainter star out of the background.
I have the Moonlite focuser fitted on my M180 and although I have a Tak adapter that couples my SBIG ST7 to the Reducer/Flattener, I do not have sufficient in-focus travel to focus when using the Canon 40D. I estimate I am approx 2 or 3mm short?
So, I cobbled together a few threaded adapters and have used this – I will go and measure the spacing to see how far off from 56mm I am.
The coma didn’t show itself on the (smaller) 765x510 pixel ST7, but I was expecting some to appear with the 3888x2592 chip of the Canon 40D.
Cheers
Dennis
Hi Dennis,
The 56mm metal back distance is "naturely" obtained with the addition of the T2 ring and the dslr whatever the brand is (Nikon, Canon and so on).
Now, 2 or 3 mm short or long might be the tolerance... To be checked
For example, according to the french tak dealer, the Epsilon 160 metal back distance is 56 mm with +/-2 mm.
Although somewhat ironic, all is indeed perfect! The DK design has inherent coma and displays this, quite perfectly.
I tried this a few weeks ago with the C11 @ F/6.3 (1756mm) I managed to split a,b,c & d however e and f still eluded me.
