Problems with Capturing

[brenski]

Equipment:  72ED Pro, Zwo 183MC Pro Cooled, Star Adv 2i

Sharpcap Pro, 8.2s exposure time

I'm definitely doing something wrong here:

 

1. I think focus is out

2. I think my aim was out too

 

attached files are 18.3% and 100% crops of actual stacked tif file. Had the scope pointed at what I thought was Andromeda, but clearly not.

 

Any help appreciated

 

 

 

 

[vlaiv]

You will need to add IR/UV cut filter to that setup.

ED doublet scopes, while having decent color correction - still don't fully correct for chromatic aberration. In fact - almost no refractor does over extended range of wavelengths (maybe some super apos / quadruplet lens do).

For this reason you need to remove IR and UV part of spectrum (above 700nm and below 400nm) - that cause star bloating in your image - due to unfocused light in far ends of spectrum.

You'll even have trouble finding proper focus like this as stars will never be pin point.

https://www.firstlightoptics.com/uv-ir-filters/astro-essentials-uvir-cut-filter-125-2.html

(no need to spend much on decent IR/UV filter - these are pretty much standard)

As far as M31 in our frame - 8.5s is too short exposure to be able to tell if there is something there without very strong stretch (and even then, faint targets won't show up on such short exposure).

Fact that light is not properly focused only makes things worse.

First sort out stars with this filter to be able to focus properly and then try again to see if you are on target.

[brenski]

3 hours ago, vlaiv said:

You will need to add IR/UV cut filter to that setup.

As far as M31 in our frame - 8.5s is too short exposure to be able to tell if there is something there without very strong stretch (and even then, faint targets won't show up on such short exposure).

 

thanks for your advice, however your reply re: 8.5s is not strictly true., is it?

my DSLR with a 135mm lens attached managed to acquire the image below from a 10s exposure, so a Zwo 183MC Pro with a 72 ED should be able to capture something "recognisable" no?

(image is unprocessed)

[vlaiv]

7 hours ago, brenski said:

my DSLR with a 135mm lens attached managed to acquire the image below from a 10s exposure, so a Zwo 183MC Pro with a 72 ED should be able to capture something "recognisable" no?

I'm not sure.

DSLR has larger pixels and 72ED has about 3 times as long focal length, right? That must be 5-6 times higher resolution.

Light is much more spread over pixels.

I would expect image like above to be possible after stretching 8s sub - but not without processing.

[brenski]

1 hour ago, vlaiv said:

I'm not sure.

DSLR has larger pixels and 72ED has about 3 times as long focal length, right? That must be 5-6 times higher resolution.

Light is much more spread over pixels.

I would expect image like above to be possible after stretching 8s sub - but not without processing.

I do already have a Optolong UHC Ultra High Contrast Light Pollution Filter 2" - would that help? or is the IR/UV cut filter the only solution? Can both be used together?

 

[vlaiv]

10 minutes ago, brenski said:

I do already have a Optolong UHC Ultra High Contrast Light Pollution Filter 2" - would that help? or is the IR/UV cut filter the only solution? Can both be used together?

 

UHC is not very suitable filter for broad band targets like M31.

It is much more suited for Ha regions and emission type nebulae.

For above target - it will do more harm than good.

IR/UV cut is necessary if you want to remove star bloating and get good color. In general - it might not be necessary with UHC type filter - but that depends on UHC filter itself.

This is transmission curve for Optolong - UHC and we can't really tell (but it looks like it might need UV/IR with it as well) - because it seems to have transmission above 700nm (part of graph is missing, so we can't be 100% sure - but it does look like it is not falling back to zero).

You can stack filters - and resulting curve is multiplication of two curves - if either one is zero - result will also be zero (no transmission, as 0 * any number = 0).

[tomato]

M31 will fill your FOV with your scope and camera, so it won’t look anything like the DSLR/lens image. I don’t think you are on target, but definitely need to sort the stars out first.

 

[brenski]

@vlaiv @tomato thanks for that really useful information.

 

Can I ask a couple more questions:

1. Backfocus - I currently have an extension tube in place which adds (5mm 10mm 30mm) 45mm in total. Is that enough? Or do I need more?

2. For a Target like Andromeda that fills the 72ED FOV - what is the best way to manually locate the target correctly? Or is there a simple way to do this with the Star Adv 2i that I'm not aware of?

[vlaiv]

13 minutes ago, brenski said:

1. Backfocus - I currently have an extension tube in place which adds (5mm 10mm 30mm) 45mm in total. Is that enough? Or do I need more?

If you can reach focus - you don't need additional extension tubes.

In order to figure out if you can reach the focus - find bright star that you can easily see in short exposure. Iterate exposures and look at the disk formed by defocused star - moving focuser will make defocus disk either larger or smaller - go in direction when it's getting smaller. At some point it should reach "pinpoint" size and then it will start expanding again.

If you can make that happen - you have enough back focus, but if you reach the end of focuser travel as it the disk is getting smaller - but you can't reach that turning point - you need more back focus distance.

16 minutes ago, brenski said:

2. For a Target like Andromeda that fills the 72ED FOV - what is the best way to manually locate the target correctly? Or is there a simple way to do this with the Star Adv 2i that I'm not aware of?

Is there a way to mount small finder scope next your scope? Even simple red dot finder will do the trick.

First align it to main ota using an eyepiece and bright star. Then you'll be able to put target somewhere on the sensor.

After that - you need to do frame & focus exposures. Short exposures where you'll be able to recognize target (in stretched image, capture software usually has auto stretch function for this purpose) - and then you manually move your mount to center it.

[brenski]

thanks, will try that.

[tomato]

Yes, a good call to have a finder scope, Telrad  or red dot finder aligned with your main scope.  You can’t use Goto on your mount as it has a RA drive only, so a finder scope would be the way to go.

[brenski]

thanks guys, some really good pointers (pardon the pun).

 

for Andromeda what would be good exposure/gain settings for Sharpcap (the Star Adv 2i is unguided) 

[vlaiv]

I would try other software for DSO imaging instead of SharpCap.

SharpCap is my go to software for planetary and some quick work (and would be my go to for EAA if I wanted to do that now), but I prefer to use ASCOM drivers for long exposure imaging, and I'm used to other software with that.

Give Nina a try?

In any case, I'd use gain of 120 - I like working with unity gain. As far as exposure goes - as much as you can do without visible trailing. I don't know how precise is SA2, but there is a limit to exposure length that you ought to use. It depends on light pollution and you need to swamp the read noise with background signal.

I believe that sharp cap has capability to calculate this value?

https://www.sharpcap.co.uk/sharpcap/features/smart-histogram

If you don't know how to calculate it yourself - I think above calculator will give you at least rough idea. Go with that exposure length if your tracking can make it work (no star trailing).