ASI585MC – 9 April 2024

[Fir Chlis]

It had been two weeks since the last clear night and a promising forecast suggested a good night ahead. I’d spent a lot of time looking at my collimation since the last session and had bought a StellaLyra collimator (having previously used a collimation cap). After a minor tweak to the secondary mirror’s tilt, everything was lined up. I captured a flat, with dark correction, with the bottom of the OTA protected by a black shower cap that I’d recently bought to reduce light leakage around the primary.

There was a brisk Westerly, so I set up on the lee side of the house, which gave me a good view of the Eastern sky. This is an unusual location for me, as the clear nights tend to come with a Northerly or Easterly wind.

A long wait then until darkness came and I started SharpCap polar alignment. I tried a new procedure this time, starting the scope at 3h RA left of home, rather than the home position. This was when the night’s frustrations began. A glance up showed a line of black cloud coming in fast on the wind, with telltale streaks of rain. What do I do? It looked like it might just pass me by, so I quickly park the scope, switch off the mains power, grab a big bin liner and cover everything up. The wind was rising, so I stood out there holding the bin liner down, feeling the odd drop of rain on my head. Fortunately, the sky was clear behind the cloud and the wind soon moved it away. Switch everything on again, and polar align went the quickest I’ve seen it, so the offset start position is definitely worth trying again.

Goto first target, then run SharpCap’s Brain – as usual it came up with gain 300 exposure 4s, so I quickly captured a dark, but looked again at the histogram and decided to go for 8s. Time was pressing, so I used a previous dark at a similar camera temperature.

I have a big list of targets, but I’d built an observing list in Stellarium from the Cloudy Nights April EAA Challenge, so decided to go for these.

The first was M53, and I’m delighted with the clarity and detail on this one, although only 96s total exposure.

This was when the frustrations of the night continued, with a lump of black cloud blocking the view and Sharpcap continually dropping frames that weren’t bright enough, so I moved on. The rest of the night was spent chasing holes in the cloud – I’d goto a target, then the plate solve would fail because it was cloud blocked.

After two more targets, M87 (5min) and M65 (3min), I gave up. There’s evidence of hot pixels on all the images, despite using dark correction. I’m wondering if these pixels have popped since I took the dark frame (it was from a few weeks ago).

 

Geoff

 

 

Edited April 10 by Fir Chlis

[PeterC65]

It may have been a frustrating session but you got some very nice images.

 

[Fir Chlis]

28 minutes ago, PeterC65 said:

It may have been a frustrating session but you got some very nice images.

 

Thanks - and I think that the coma from previous images has gone/reduced since the collimation improvements, though I could do with a few more images to confirm this. 

[PeterC65]

54 minutes ago, Fir Chlis said:

Thanks - and I think that the coma from previous images has gone/reduced since the collimation improvements, though I could do with a few more images to confirm this. 

I can't see any coma in the images above.

 

[Richard N]

Nice images. I always get rather blobby stars. Not sure why? Yours look nice and sharp.

[Fir Chlis]

2 hours ago, Richard N said:

Nice images. I always get rather blobby stars. Not sure why? Yours look nice and sharp.

Hi Richard

I can tell you what I do - but my apologies if I'm telling you what you already know. I'm also not sure what gear you have. I've got the 200PDS Newtonian Reflector on an HEQ5Pro mount, with the ASI585MC.

Mount leveling - I don't worry too much about this, as long at the tiny spirit level on the mount looks roughly level.

Collimation - I've spent a LOT of time on collimation and since I started with EAA at the end of January I've made gradual tweaks - but when you do collimate, be gentle, don't make big adjustments and try and understand what you're doing before making too many alterations - it's a slow gradual process. After last night I think I've got about as close as I'm going to get with my gear. I have a collimation cap that I'd used when I was just doing visual and managed to get fairly good results with that, but I've recently bought a StellaLyra Premium Cheshire from FLO, which I think has given me a bit more accuracy. @PeterC65 pointed me to this article on collimating Newtonians: Collimation Checks – Astro Diary (catshill.com). This is also good: Collimation - Newtonian Telescope - Reflector - Astro Baby's Guide to (astro-baby.com). I've also used the method described in this thread: Collimation of Newtonian Telescopes using SharpCap - SharpCap Forums - but you need an all sky lens. I've tweaked the focuser alignment to make sure it is perpendicular to the length of the tube, but I've tilted it slightly in the circumferential direction to get the secondary mirror centred in that direction. I've then done three or four iterations of collimating the secondary then the primary.

Polar Align - I use SharpCap's built in polar align tool and can always get the polar align error down to less than 1 minute of error even with the fairly small field of view that I get with the 585MC (more often than not it's Good, sometimes it's Excellent) - I've got used to tweaking the alt and az bolts by very small amounts. I'm not sure what mount you have, but I think that an equatorial like mine will give a better result than an alt/az or dob. If you aren't using an EQ mount, then I think that shorter exposures (assuming you're aligning frames when live stacking with SharpCap) are better - maybe no more than 4s or 8s.

Focus - I use a Bahtinov mask with SharpCap's focus tool - I got the mask for about £12 from someone on eBay who 3D prints them and, again, I always take time to get the best focus I can before locking the focuser with the thumb screw - and this usually lasts me for the night. I have a dual speed Crayford focuser that came with the scope, so can make very small tweaks and as it comes into the best focus I usually overshoot slightly then bring it back.

Wind direction - I'm fortunate that I can almost always put the scope on the downwind side of the house, so that tends to dictate which bit of the sky I'm going to look at on a particular night.

Frame Filtering - I normally use SharpCap's brightness frame filtering set to auto, which will eliminate poorer quality frames.

Darks and Flats - I always do these, a flat when I'm setting up the scope in daylight, and a dark at the start of the session. I usually end up using the same gain and exposure all night, so generally just need one set of darks. Although these won't affect sharpness, I found that the darks really made a difference to the background noise but I'm still undecided about flats.

Experience - when I look back to my first attempts in January, and what I got last night, I can see that I've improved over time. Here's M52 from 29 January, followed by M53 from 9 April.

 

Hope this is helpful.

 

Geoff

Edited April 10 by Fir Chlis