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The "No EQ" DSO Challenge!


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You're doubling the focal length of the scope, and given the aperture is fixed your slowing the scope down, needing longer exposures. As you say, you're also making tracking more difficult making it more difficult to achieve even equal exposures let alone longer ones.

But, there are possibly occasions it's better. Planetary imaging, and maybe some bright planetary nebulae which are such small targets the longer focal lengths are worth it. I can get 15s exposures at 2350mm and over 90s at 400mm. I've not tried the 2350 with my 2.25x Barlow for anything other than Jupiter but I could probably get 4s. However, at f22.5 I don't know what I'd get in that time.

just finding my target with such a small fov makes me horrified! It was bad enough at f10 and f6.7 using a reducer!

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I'm still fairly new to imaging, but have had a good start with Planetary and Wide Field images. Obviously, like most of us, it's the Deep Sky stuff I'd like to glimpse, but time, location and more im

Assorted shots with a Nexstar 102SLT and a Canon 1000D. 30sec subs at ISO1600. Total exposures range from 5 mins (M20)  to ~1hr (M31). NigelM

this was taken a couple years ago on my AZGOTO mount with 130p...... about 50 x 5 sec subs, no calibration frames

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I'm in a bit of a quandary over this. Assuming that the target is small enough, should I use the native FL of 715mm, and crop the resultant image, or use my 0.79x reducer and gain a little bit of speed, but need to crop rather more?

I guess it rather depends on how much resolution on the target I'm going to achieve. Without the reducer and with my 16Mp APS sensor I shall get about 1.4"/pixel, whereas with the reducer it'll be about 1.7"/pixel. Both are below the recognised value of 2" for the seeing in this country, so I imagine on that basis it won't make any difference to the final image and I might as well go with the reducer to get as many photons on each pixel as I can. The deficiencies in mount tracking will be more evident in the image without the reducer, but as the image with the reducer will need to be cropped more, this will cancel out (assuming the target ends up the same size in both images). Is there any other factor at play here that I should consider?

Ian

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Thanks for sending me on a half hour googling mission trying to understand pixel resolution Ian!!! A lot of information to absorb there and it seems the topic draws very different points of view. 

From what I can see, the fact that your sampling rates, with and without the reducer, are well bellow 2" which is probably the very best seeing we can expect, the only other factors to consider are target framing and SNR - the more oversampled the image is, the higher the SNR. I'd go with the reducer, but hey, I may be completetly wrong!

At least, that's what I can extract from this article...https://starizona.com/acb/ccd/advtheorynyq.aspx

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I'd agree and go with the reducer on this one in order to 'go faster'. I can't imagine you'd beat seeing; at least not down to 1.4"/pixel. The other advantage is that the shorter focal length will be less sensitive to tracking errors and allow you to squeeze even more seconds per sub. Plus the wider field of view gives you more options to frame/crop. If the target is small within the image you could always produce a wide field image (sampled down to reduce it's scale) with an insert panel at normal resolution (or even slightly enlarged) to show the target.

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This little bit of research may explain why I struggled with noise so much in my attempt at M33. My Nikon D3200 & ST120 combo gives a resolution of 1.32"/pixel vs 2.04"/pixel with the ZS66. Perhaps I should go back to the little apo but the bahtinov mask and Baader semi-apo filter for the ST120 just landed on my desk...decisions, decisions.

On the subject of noise, have any of you guys actively experimented with and defined meaningful results from trying different ISO settings? I read this article http://dslr-astrophotography.com/iso-dslr-astrophotography/ and went on to get my sensor noise info at http://sensorgen.info which suggests I may be better off at 3200 with my camera as the noise gets less at higher ISO! The only problem then is that the saturation capacity is reduced, but given the 30s exposures maybe it will work out OK?

 

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1 hour ago, parallaxerr said:

 

On the subject of noise, have any of you guys actively experimented with and defined meaningful results from trying different ISO settings? I read this article http://dslr-astrophotography.com/iso-dslr-astrophotography/ and went on to get my sensor noise info at http://sensorgen.info which suggests I may be better off at 3200 with my camera as the noise gets less at higher ISO! The only problem then is that the saturation capacity is reduced, but given the 30s exposures maybe it will work out OK?

 

I have had a dabble with ISO settings a little.

The best setting for my cameras seem's 1600, but had a good result using 3200 and the LP filter.

6400 seemed to much noise, I stacked 3200 + 6400 in the flame & HH nebula, very difficult to process, I removed the 6400 from the stack and a much better image with easier processing.

800 ISO is good too. I have not tried with lower than 800 yet though.

Cheers

Nige.

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What camera are you using Nige? It'd be interesting to see if your results correspond to the noise charts on sensor gen.info.

EDIT: Scratch that, just turned signatures back on saw your camera listed! Looks like 1600 is the sweet spot for the 1200D and lower than 800 is a no go.

Edited by parallaxerr
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Hi guys,

I have a quick question hopefully one of you may be able to help with before I start imaging tonight...

I have just set the ST120 up ready to go. Focused with the new Bahtinov mask (turns out the focus was a good way off last time) and also fitted the new Baader Semi-Apo filter. I took a few test shots after alignment and I'm pleased to say that the sky-glow/LP is practically non-existent with the filter and CA, whilst still there, is significantly reduced :)

Based on my findings about sensor noise today, I took a few test shots at ISO 1600 and 3200 (supposedly 3200 has marginally lower read noise for my camera). At ISO 1600 the background sky is a nice deep blue but I noted the histogram spike is at about 1/4 from the left. At ISO 3200, the background sky is quite light grey/white but the histogram spike is nearly bang in the middle of the graph.

So the question is - which ISO setting? Does the histogram position tell a story and am I going to have problems processing with a lighter background sky if I set 3200?

TIA, Jon

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I aim to get the left of the histogram clear of zero; I.e. The darkest part of the raw image is >0. After that, I want to avoid over exposing stars so I reduce ISO until I get as much between those two limits. Sounds like ISO 1600 might be ok.

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OK sounds like 3200 is OTT.

Now, the reason I have dismissed 800 is because the read noise for my camera is higher at 800 than at 1600 (not by much). I thought the noise I was getting in my images was read noise but my darks are absolutely jet black so maybe the noise was something else?  

I'll try a few 30s subs to see how the histogram shapes up.

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Well what a horrid night so far...Cloud, lots of cloud. Technical issues, more cloud and target (M33) now above max alt. D'oh. HOWEVER, I think I have carried out some valuable testing. Thanks to @Filroden's post re: ISO settings, I have experimented with mine...

I took 30s subs on Capella (the only star that seem to be dodging the cloud) at ISO's from 100 up to 6400 and Hi1 (12800) and checked the histogram peaks. Only ISO100 showed clipping on the left side but from 200 up, there was no clipping. The lower the ISO, the darker and smoother the background sky with grain starting to show at ISO 800. As it happens, with my sensor, there is a dip in read noise level at ISO400, sooooooo ISO400 is where I'm currently at. At this level I also have 50% more dynamic range available and a higher saturation capacity, although I'm probably not reaching it with 30s subs.

I think at ISO1600, despite marginally lower read noise, the reduction in dynamic range raised the noise floor and it was all getting amplified by the ADC. Scope is now on M42 clicking away, surprising amount of detail in the subs and a very nice dark background sky. Watch this space......

 

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Yup. Was really clear at about 7.00-8.00. BBC showed clear skys all night. Opened observatory lid a few hours ago, and just went to check focus on moon/align and its cloudy... Thankfully in an observatory, I'd be in a huge sulk right now if I had to go and fetch my scope in after all the palava I used to have setting it up. Might take this as a chance to try the hibernation feature.

 

Saturday night might be the next chance. Will probably put my solar filters on seeing evening times are a write off at the moment!

 

 

Edit - Hibernation works! Why didn't I know about this before!?! For some reason I just assumed only fancy pants EQ mounts had it!

Edited by jimbo747
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11 minutes ago, jimbo747 said:

Hibernation works! Why didn't I know about this before!?! For some reason I just assumed only fancy pants EQ mounts had it!

Yup, I put mine straight into hibernate when the clouds came in. Woke it up again 2hrs later no problem :)

Got over 100 subs on M42, darks in progress now, then it's bed time. Good job I have a half day in work tomorrow!

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ILook forward to seeing your M42 Jon. 

Too many clouds here last night, again.  

All the recent talk of focusing and bahtinov masks I had to get one, it arrives today ?.hopefully focusing difficulties are in the past,  I had noticed with the LP filter focusing was a bit harder. 

Now all that is needed is a very large fan to blow the clouds away..

Nige.

 

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OK, I wasn't going to post this just yet but I have a question - in comparison to other M42's I've seen, there seems to be quite a lot less stars in mine. Probably a result of setting ISO to 400, but I really do like the dark background. What do you think? Stars vs noise?

M42.jpg

Note: this is far from fully processed. I threw this together at 2am so there's no flats or dark flats, just 101x30s light frames and x50 darks. The image has 4 noise hot spots towards the corners which I think is amp glow, I tamed it by over adjusting contrast and colour. Also it appears my computer monitor colour needs calibrating as this image looks far less vivid on the PC than on my other devices!

Also to note, this was taken with the Baader semi-apo filter. It made processing infinitely easier without the sky glow - subs were captured between 20°-30° Alt! After Autodev in ST, the image was washed out a lime green colour instead of red/brown but the wipe tool handled it much better, less noise probably helped. A little disappointed with the CA still.

Edited by parallaxerr
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You've almost resolved the trapezium at 30s. I'd have thought it might be overexposed! 

I personally prefer to see a non-black background as there will be very feint detail which will be lost if you clip the shadows. It does mean leaving noise visible but that can be reduced. 

Otherwise it's looking good. You've almost closed the loop of nebula and the running man is just visible. 

Darks and flats should deal with the corners. 

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18 minutes ago, Filroden said:

I personally prefer to see a non-black background as there will be very feint detail which will be lost if you clip the shadows. It does mean leaving noise visible but that can be reduced. 

I think you're right. There was very little noise in the subs to start with and Startools has tried to reduce it further, so this is a litle too dark. Looks like I'll be trying ISO800 next tiime despite all the theory and specification of my sensor suggesting it's not a good ISO to use, it goes to show that the final image is what counts!

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Jon, that's looking good,  be careful with processing M42, there's a lot of nebula emissions surrounding Orion which I mistook for background noise,  

I was given a lesson in StarTools processing on M42,  the difference between my image and Ivo's image was vast. So much more detail . 

If your interested I will post the processing data for you to ponder over ?  

Yesterday I had a play around with my M42 data,  I was amazed at how the slightest change in parameters changed the whole image for the worst.  Even just cropping a slightly different size . 

I tried binning 50% instead of Ivo's suggested 38% the gradients were impossible to remove.

Good luck 

Nige.

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52 minutes ago, parallaxerr said:

 

M42.jpg

I just took the liberty to quickly have a look at your image and enhance a little,  the detail is there, offset frames will help. This is a little work with photoshop express .

You can clearly see much more emissions although I have highlighted gradients.

I hope you don't mind. 

Nige.

PSX_20161111_091354.jpg

Edited by Nigel G
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