M16 with C9.25 and ST7 - not easy!

[Dennis]

Hi

Here is a “quick and dirty” image of M16, my first light with the ST7E and C9.25. Being experienced with using the ST7E with my Vixen 4” refractor at only 918mm focal length, I was more prepared for imaging at the longer focal length of 2350mm for the C9.25, which is much more demanding as I discovered.

The M16 image is a stack of 3 exposures of 3 mins each; auto guided using the ST7E although it suffers from the effects of imaging through thin, high cloud over Brisbane when I captured the individual exposures.

I also discovered that during auto guiding, I had a periodic oscillation in RA with the Takahashi EM200, which was slightly disappointing given the pedigree of Tak mounts. The RA corrections kept oscillating between ±1.5 units when I was expecting ± 0.5. I had to discard several images due to elongated stars. I am still investigating this oscillation to see if I can null it out.

Cheers

Dennis

[JohnG]

Hi Dennis

Nice shot, just a thought on the oscillations you mentioned, might the aggressiveness be set slightly to high for that focal length, might be over correcting slightly.

Cheers

JohnG

[Striker]

Is their anything you can't do Dennis.

Fantastic planetry images....wonderful bird terrestial images and now great DSO Images....your a machine....lol

Nice for just 3 exposures.

[Dennis]

Hi John

Thanks for the comments and suggestion re aggressiveness. You probably guessed (correctly) that I was using CCDSoft and I progressively reduced the "aggressiveness" value from the default of 10, down to 1, in steps of 1, and it only made a tiny difference so I'll have to look elsewhere.

I tried the usual trick of imbalancing the GEM to the East so the gears were always driving against the slightly out of balance load, and that didn't make any difference at all.

Weather permitting, tonight I'm going to try to reduce the guide rate in the Tak hand controller. I think the default is x1 and by flipping a switch from H to N, then holding button S1 and pressing button S4, the correction rate is reduced in steps of x0.1 from x1.0 to x0.1, for every button press…...

And yes, you really do have to make these adjustments by hardware button presses on the Tak mount!

Cheers

Dennis

[strongmanmike]

Well for a few short exposures and even under (an all too familair) duress, you have produced an image that would have taken a long time to photograph with film and been a gallery image in US S&T not that many years ago!

Keep at it Dennis as your scope/mount combination has the potential to produce some mighty fine images in due course. :thumbsup:

Mike

[JohnG]

You guessed that I guessed, ah now I am confused. LOL I found that when I go from the 820mm of the Tak at prime focus to using the extender at 1312mm I have to drop the Ag setting down to about 2 from my usual 3-4 on the ST-4, I very rarely go above 5. With the Gemini in Photo mode the drive corrections are down to about .05 Siderial.

JohnG

[asimov]

I wouldn't have a clue what you guys are talking about:lol:but thats a nice image Dennis! :thumbsup:

[Dennis]

Thanks Tony!

Hmm, now you mention it....yes....there is something I cannot do; take COLOUR images of DSO's. :( Must get myself a Hutech modified Canon like yours!

Cheers

Dennis