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Arp 120 and a distant galaxy


Mike JW

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Hi, Rob's post "finally managed some spring galaxies" 30/04/19, prompted me to have another go at observing Arp 120, noting the settings Rob had used. Very pleased with the outcome.

Arp 120 -sometimes called The Eyes is NGC 4438 (the big galaxy) and NGC 4435. Arp classified them as ellipticals close to and perturbing spirals. Since Arp looked at this pair it seems that 4435 is not affecting its big neighbour as it a line of sight association. The most likely candidate is nearby M86. The mass of stars around NGC 4438 is quite thin and in fact I was able to pick up a distant galaxy at about 1.6 billion lyrs away shinning through this thin tidal cloud of stars - see line with number on attached image. I was also pleased to get hints of the wonky dust lane that zig-zags its way through the galaxy - see lines. On the southern end of the galaxy I picked up PGC 40913 (mag 17). In the southern part of the star cloud I picked up the long brighter section.

Details: C9.25@ f4.5, ultrastar mono. I tried subs at 1x1 but the best was using 2x2 binning and 6 x 25 sec subs. (20x 25 secs was no better).

HELP. I have often noted that during the stacking the image suddenly jumps and noise increases dramatically and that to get back to where the image was requires a few more stacks. This seems to happen after about 20 stacks - any thoughts.

 

997873483_Arp_120_NGC_4435.2X2_2019.4.30_22_10.24labels.png.33d20c82af50cd138800289846f83609.png

 

Mike

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This is great!  It prompted me to look again at one of my images of this area, taken back in March.  I was using an Ultrastar / C9.25 too, albeit at f2.3 with a Hyperstar, and 20 minutes of exposure.  I clearly see the features you’ve pointed out here.... 1.6 billion light years – that’s quite a long way!  

https://stargazerslounge.com/applications/core/interface/file/attachment.php?id=357330

I really must look more closely... I guess that's the difference between looking and observing.

Is your stacking issue not just a bad sub that would be best excluded from the stack?

Thanks again,

Tony

Edited by AKB
typo
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Hi Tony, Just looked at your link - wow. Must rush out and get a hyperstar? Wonderful shot and so full of detail. At heart I am a visual observer (without getting into all the discussions about what is EAA) and I am determined to study an image that I take as if I was still using my big Dob, read up about it, make notes, do labels etc. Otherwise I feel I am simply "seeing" something, ticking the box and moving on. Hence my post, but it takes a long time to read up etc about something I have imaged. Thanks for your positive comments. So much to see and learn about.

As to stacking issue. I remove the bad sub and carry on but the next sub to come in has all the extra noise...mystified.

Mike

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2 hours ago, Mike JW said:

Hi Tony, Just looked at your link - wow. Must rush out and get a hyperstar? Wonderful shot and so full of detail. At heart I am a visual observer (without getting into all the discussions about what is EAA) and I am determined to study an image that I take as if I was still using my big Dob, read up about it, make notes, do labels etc. Otherwise I feel I am simply "seeing" something, ticking the box and moving on. Hence my post, but it takes a long time to read up etc about something I have imaged. Thanks for your positive comments. So much to see and learn about.

As to stacking issue. I remove the bad sub and carry on but the next sub to come in has all the extra noise...mystified.

Mike

Maybe cloud? There is so much skyglow where I am I usually can't tell if it's actually cloudy or not! Woe is me.... Seeing your and others images tempts me to get a long focal length scope, maybe an RC6. But, there again, I already have too much stuff that I don't get the use out of!

Louise

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6 hours ago, Mike JW said:

Hi, Rob's post "finally managed some spring galaxies" 30/04/19, prompted me to have another go at observing Arp 120, noting the settings Rob had used. Very pleased with the outcome

Well done Mike, that's a cracking result, lots to see there. Thanks for the useful background info too, I didn't know they were just line of sight, I must have referenced an out of date source! That setup with 2x2 binning still produces really nice detail - I think in theory, by binning an Ultrastar you should get similar resolution to the Lodestar but much better sensitivity, your result looks better all round, I think the image scale is larger too . An Ultrastar is definitely on the list. What reducer do you use?

Regarding stacking, I get exactly the same issue - when I see the image jump, usually after 5 or 6 subs, I usually 'undo' the last stack which caused the jump and then just save what I have. Some nights are worse than others. I always thought it was due to tracking issues (periodic error or poor polar alignment) but it may be more complicated than that. I believe @Martin Meredith's Jocular software allows the best subs to be selected which could be very useful for dealing with issues like this.

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A great result and write-up. I've observed these a few times without realising they have an Arp designation. Interesting to hear that their interaction may have nothing to do with each other. 

Re the SLL issue, I vaguely recall Paul mentioning something about the stack being reset when a new keyframe is needed. It doesn't happen to me very much but does occasionally (perhaps something to do with the amount of field rotation in a given part of the sky causing too much movement for alignment to the original keyframe). If so, the solution is to set the max pixel displacement (under stacking) to a higher value. I've just checked and I seem to have it at 32.

Martin

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4 hours ago, Thalestris24 said:

Maybe cloud? There is so much skyglow where I am I usually can't tell if it's actually cloudy or not! Woe is me.... Seeing your and others images tempts me to get a long focal length scope, maybe an RC6. But, there again, I already have too much stuff that I don't get the use out of!

Louise

Hi Louise - yes I agree cloud (thin high stuff) can be an issue at times. As buying more equipment. I have always tried to use the equipment to its fullest extent before buying more .

 

3 hours ago, RobertI said:

Well done Mike, that's a cracking result, lots to see there. Thanks for the useful background info too, I didn't know they were just line of sight, I must have referenced an out of date source! That setup with 2x2 binning still produces really nice detail - I think in theory, by binning an Ultrastar you should get similar resolution to the Lodestar but much better sensitivity, your result looks better all round, I think the image scale is larger too . An Ultrastar is definitely on the list. What reducer do you use?

Regarding stacking, I get exactly the same issue - when I see the image jump, usually after 5 or 6 subs, I usually 'undo' the last stack which caused the jump and then just save what I have. Some nights are worse than others. I always thought it was due to tracking issues (periodic error or poor polar alignment) but it may be more complicated than that. I believe @Martin Meredith's Jocular software allows the best subs to be selected which could be very useful for dealing with issues like this.

Hi Rob. I use an Altair Astro 0.5 reducer. I too have had similar thoughts about tracking issues. Last night I was very closely aligned to the NP so error was low.

2 hours ago, Martin Meredith said:

A great result and write-up. I've observed these a few times without realising they have an Arp designation. Interesting to hear that their interaction may have nothing to do with each other. 

Re the SLL issue, I vaguely recall Paul mentioning something about the stack being reset when a new keyframe is needed. It doesn't happen to me very much but does occasionally (perhaps something to do with the amount of field rotation in a given part of the sky causing too much movement for alignment to the original keyframe). If so, the solution is to set the max pixel displacement (under stacking) to a higher value. I've just checked and I seem to have it at 32.

Martin

Thanks Martin. I will try changing the max pixel displacement from its very low current setting to something higher. I do not actually understand what is does. 

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I've always assumed it represents how much total movement (i.e. from the keyframe to the current frame) there is before a new keyframe is deemed to be needed. The reported  'mean displacement' value appears to increase as the stack progresses, and issues with new keyframes/noise appear when that value is greater than the max displacement threshold. I guess setting the max displacement high must run some risk of either taking too long or not aligning (otherwise why not set it high all the time), but I've not noticed too many problems setting it to a reasonably high value. Without knowing how the star registration works in detail it is hard to say more, but this has been my experience.

Martin 

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I have had problems with SLL when doing stacks but it sounds like it's different to what has been described here. Rather than jumping my problem has been the stacked picture suddenly becomes noisy all over. Hence I always make sure I save stacked images as I go along. However, since using Jocular and not stacking within SLL, I have not had any problems. (Now that's tempting fate isn't it?)

Bill

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Hi Bill, I too have the problem you described and save as I go along. I have had a look at Jocular but not being particularly computer savvy - it unnerved me a little but no doubt once I followed the guidance I would be OK. Mike

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