Test shot, NGC2835. What's wrong with my stars?

[furgle]

I'm testing a new RC8 and when it comes to scopes with vanes, I'm used to seeing prominent diffraction spikes with small stars. It feels like I'm getting the opposite here.

Is it the focal length (1625mm), or due to the secondary central obstruction? Bad seeing?

The scope was collimated with a howie glatter laser, and a defocused star looks good.

Any suggestions? (not that I can do much about it with the current weather).

Higher quality image here (http://observatory.site/astrophotography/Spiral_Galaxy_NGC2835/Spiral_Galaxy_NGC2835_20180216.html )

[Atmos]

I suppose the first question is what the stars look like when using very short exposures.

[bojan]

Nothing much is wrong here...
Faint(er) stars are round (which is good), the brighter ones have some strange diffraction spikes, maybe the aperture is not clean .. perhaps the edges of the mirror are damaged ?

[furgle]

Unsure of this. Will try some short exposures if these clouds ever end.



Perhaps. The secondary looks ok to me, apart from a spider web which was removed. (attached)

[bojan]

No need for that, bright stars and short exposure is the same as faint ones with longer exposure.. if tracking was OK (and it was)

[bojan]

Spider web was a very possible culprit.

[Peter Ward]

Looks like spherical error to me. What do the intra and extra focal patterns look like?

[furgle]

Unsure as I was mainly interested in collimation at that point. As soon as it clears, I'll add that to the list of things to check.

[beren]

Are you using a reducer.......over sampling maybe ?

[furgle]

No reducer (yet), but I never got my head around over and under sampling. It's a 8300 chip binned 2x2, so using 10.8um pixels it should be around 1.4 arcsec/pixel

[beren]

This a handy ccd calculator http://astronomy.tools/calculators/ccd_suitability

I recently got the same OTA and have used both the HG laser and Tak collimation scope. Both have given different results (the stock focuser doesn't help), close though, but have gone with the Tak scope adjustments after some research here on the forum ( yet to do a star test for confirmation ).

Have you got an unprocessed image or one without the PI star shrink process done ? Btw very nice image though

[furgle]

I'll post a raw fits the next chance I get. Currently dealing with the trials and tribulations of my 4 year old not doing a damn thing I say. Kind of like my gear on a bad day...

[furgle]

Link to single unprocessed fits: https://drive.google.com/file/d/1rxKSoEDv3M9vapwOrpE_cbc5bcgRUVgO/view?usp=sharing

Link to calibrated, registered, std rejected stack (CCD Stack) fits: https://drive.google.com/open?id=1wDdAcfNQPJysY35tujTg2Jt-mKuT7n33

Link to all RAW data + calibration files (some of these files were not used in the stack, based on their aspect ratio, etc) 102MB: https://drive.google.com/open?id=1wHLpa_HqByD7TVGWZywIqgfkep LQVB9C

Stretched JPG also attatched below

[furgle]

Stars are starting to look a bit better now. I cleaned the secondary, adjusted the dew heater wiring, and used an iterative stretching method in PI.

Thanks for all the tips.

http://www.iceinspace.com.au/forum/showthread.php?t=165380

[alpal]

Hi Adam,
it doesn't look too bad.
Is the GSO RC8 design causing any weight from the camera to
be taken by the mirror?

cheers
Allan

[furgle]

It has a tilting plate to align the camera to the secondary. I don't think any weight from the camera is pulling on the mirror. Either way, since this photo, I recollimated using a shopping bag full of beans to simulate the weight of my imaging train (AO, ONAG, QSI683, SX Lodestar), approx 1kg (1.5kg - 0.5kg for the howie glatter laser).

If the tilting plate was not strong enough, or the mirror was loaded, I would expect the collimation to change as the altitude of the scope changed. It measured the same throughout the latest imaging session.

[alpal]

Hi Adam,
It was just a thought as I remembered a thread on the stargazers lounge
about people modifying the GSO RCs.
I thought it was an RC8 but I can only find an RC10 here:
https://stargazerslounge.com/topic/187671-gso-12-rc-dissambly/


I am away from my home computer so I may be able to drag up the article
I thought I read - on the weekend.

cheers
Allan

[alpal]

What a shame - all the images on that link from photobucket
are blocked -I thought it was the work computer but
it's photobucket itself.

There must be millions of articles on forums wrecked by photobucket.
Apparently they are demanding a ranson of $400 to unlock peoples photos.
:mad2:

[furgle]

Doh. I was hoping it was just my phone that didn't want to show the photos.

[beren]

:thumbsup: Latest image looks good Adam

Did some star testing on my RC8 after just receiving a tilt plate from Bintel.
Noted the new tilt plate didn't have any locking grub screws next to the three adjusting bolts. I used a simple collimation/chesire cap to align the focuser to the secondary marker using the tilt plate, then the Tak scope to align the secondary. Used the Tak scope to tweak the primary to get the baffle tube and secondary lined up { the primary lock down grub screw where not locked down on my scope}. Did five iterations of this and a visual star test {through an eyepiece, no diagonal} which looked spot on.

Encouraging to see the weight of your imaging train doesn't appear to interfere with collimation, been worried about that with the GSO RC design. Think Moonlite has a flange/collar that separates the focuser from the primary mirror housing, although I thinks it's only available for the 10' and up

[furgle]

My wife thought I was joking at first when I asked where the beans were.

I thought it was collimated until took the camera off and put the beans on. You can see the difference in this photo. Prior to this, it was collimated just with the laser, no on axis guider, camera weight, etc.

[alpal]

Hi Adam,
What a pity,
those photos would have been good to compare your mechanical setup
to see what was really happening.
We live and learn - don't ever use photobucket.

I believe that more expensive RC designs don't put any stress from the focuser
or camera on to the primary mirror.

I am surprised at the marvelous pictures from Paul Haese with his RC12
which is without any serious mechanical modifications,
but then again GSO may have made changes to their design over the years.
From the article I linked in this thread that guy called Doug, was having considerable trouble
but was smart enough to get it all going nicely with some mods.

cheers
Allan