Art Gecko

Nice!! You can see the OIII it's just a little swamped in places by the Ha, I guess that's the difference of modding.. much brighter than mine though, it's good! Try it, it's not as faint as you'd think, I just had very poor luck with the clouds, they appeared as soon as the sky got nice and dark... half my subs looked so blue you'd think it was daylight and weren't really worth using! Just needed more dark time to collect the data..

Thanks Neil, I think it does have a lot of Ha in it too that I haven't been able to pull out... needs more time with my camera but it'd still be fainter than the blue... Would be good to see a comparison though!

This is my offering from last night... I chicken'd out of the crescent nebula and had a go at something a bit easier... the Western Veil Nebula, handy for me as it's mostly OIII which is right in the middle of the visible spectrum and great for unmodded imaging! So this is 20 mins in 60 sec subs at ISO 800 with darks and flats. Probably one of my favourites yet! Wish I could of got more data but the clouds rolled in after a while and stopped the fun.

What is the record?

I use PolarAlign on iPhone... can't say I've had any problem with it...

I think you may have cracked it there Bob, well spotted!!

I'm not saying it's a bad method, just suggesting that it may be the wrong method in this case.. It's the only obvious change to the setup that may have caused that odd looking flat... at least worth considering? Yeah, as Neil says, you will need to start the stack from scratch, as the flats are stacked first then the master flat subtracted from each individual sub in the stacking software

Hmmm there's nothing wrong with taking flats the next day... that's actually a pretty good method.. but there's definitely something not right with that flat... it should be slightly darker in the corners not all along the top!... Why are you changing the settings to AV, what does that do? I have a Nikon not a Canon but the only thing I alter when I take flats is the exposure length... Have you tried taking flats without changing the settings?

My first thought is, have you checked to see if your polar-scope is properly centered in the mount? If it's only slightly out it will impact your alignment quite a lot. To check it... centre it on a star (or distant object.. You can do this during in the day, I used the tip of a TV areal)... Then move your RA axis through 180 degrees... if the target object moves off centre... that's where the problem lies!... Use an allen key to adjust it so the target object moves half way back the the centre... then re-centre it and do the 180 flip again.... repeat this procedure until the target object stays exactly in the centre when you do the 180 flip... then when you polar align, it should be pretty bang on and you should be able to get a minute or more on individual subs (provided there are no vibrations, wind etc... the normal problems for everyone!!) HTH Art.

That's true actually Peter! And well done!... Maybe I'll give it a go if the clouds ever clear again. How come you're struggling with polar alignment though? and can you show us an example of your flats?

I think this one is a bit beyond me!... You guys are getting great results, but even so you're needing longer exposures than I can get and with modded cameras... If I try for it I have a feeling I will get nothing more than stars!

@mikey2000 That is your house... I've been secretly living in your attic for 3 months... I've searched the whole country and I can't find a darker garden to set up shop in... actually, I'm just kidding mate... if you find a hobo with a telescope in your attic it's NOT me! honest! Do post the pics of M53 though, if you've managed to get them, I appreciate a globular cluster shot as much as the next astronomer! interested to see your results.

I'm not sure if there's an actual scale of surface brightness.. I've never really looked... but shooting nebulae is definitely a level above galaxies in terms of getting everything just right.... filters will be your best friend since your camera is modded, make the most of the narrow band access! RE summer targets.. I'm totally with you on that... this time last year I was pretty much just an 'observer' and the summer months were basically a wash out, not bothering to stay up late enough to actually see anything... since getting into imaging, I've learned so much more about the things in the night sky I never thought I'd get to see!... and I realised the summer is the best time to see some of the most beautiful treasures of the greatest galaxy of them all..... our galaxy.... it passes over our head every night

Nice capture Bob, you're doing well in this warm weather with short nights!! Magnitude is not really the best way of assessing whether 2 objects will image similarly as magnitude and surface brightness are very different things... magnitude is a measurement of the total light from the object as if it were compacted into a star like point... whereas surface brightness is exactly what it says on the tin.

Agreed... From what I've read, that does seem to be the case.

I agree, I quoted the T-stop explanation to demonstrate that Bob's f# hadn't changed.... the modifications he's made will have only altered the transmission of the light. The f# is still focal length/apeture and is governed by the primary mirror. Why there is such a big difference between his f5 scope and his f5.6 lens..... I have no idea!!?

What you've found there Bob is a T-stop. The focal ratio hasn't changed, but the transmission of light has... Here's an extract from wikipedia that explains it better than me... T-stop A T-stop (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (transmittance). A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N. A particular lens' T-stop, T, is given by dividing the f-number by the square root of the transmittance of that lens: T = f / (transmission square root) For example, an f/2.0 lens with transmittance of 75% has a T-stop of 2.3: T = 2.0/ (0.75 square root) = 2.309... Since real lenses have transmittances of less than 100%, a lens's T-stop is always greater than its f-number.[7] Lens transmittances of 60%–90% are typical,[8] so T-stops are sometimes used instead of f-numbers to more accurately determine exposure, particularly when using external light meters.[9] T-stops are often used in cinematography, where many images are seen in rapid succession and even small changes in exposure will be noticeable. Cinema camera lenses are typically calibrated in T-stops instead of f-numbers. In still photography, without the need for rigorous consistency of all lenses and cameras used, slight differences in exposure are less important, however, T-stops are still used in some kinds of special-purpose lenses such as Smooth Trans Focus lenses by Minolta and Sony. Had to ditch a couple of diagrams as they were coming out black with black background, so I wrote in the equations. Art

Ah, I see, so the mirror is still f5, but you want to measure the amount of light loss from the modifications.. Fair enough, that makes good sense!

That is an impressive display of DIY skills!!! It's not quite what I meant though... You said you need to test to find the f# of your scope, but how has it changed from when you bought it?

Looking at your picture, I think I can see where I went wrong with my star hopping from Deneb... and NAN does look a lot brighter than pelican! Interested in how you've altered the focal ratio of your scope Bob? @mikey2000 those are two fantastic pictures!

No I was not aware of that! Thanks Bob!..... Hmmm I'm not sure it'd help me in quite the same way though, as I shoot untethered as well, so I can't really do that until I've detached my camera at the end of the session..... Unless I buy the dongle to send the pictures to my phone, that might work. I got completely the wrong nebula then haha... Thanks for identifying it though!

Well, my attempt last night was just an epic fail! I've never shot Nebulae before (except the obligatory M42 on my iPhone) so I spent about 2 hours imaging... only used about 40mins of subs... and processed to find I'd been off target all along!!... So you might find me behind Bob in the queue for a Goto! I did get a tiny bit of nebulosity... but not enough to identify any particular part of the nebula to see how far out I was.... Bare in mind I'm using a stock camera (unmodded) so I'm only gathering 20-30% of the Ha wavelength... so it's quite dim even though I've stretched it beyond a ridiculous amount! Maybe I should stick to Galaxies...

Thanks Mike, yeah, I have read that page before, it is a very informative write up! Art

I wasn't totally unfamiliar with the ISO settings, but I do admit I haven't been using a DSLR long andthat was well explained! Nice one!... I don't normally push my ISO up that high either... well I do for test shots, but then lower the ISO and find a nice balance between that and the exposure length..... ..... to get a base line for my background, as Neil has explained.... It is my personal choice to do this I guess, but I feel its a good idea to have a base value for the background sky.... As I've said before and will no doubt say again, I may be wrong in what I'm doing but it's a learning curve and I'm still very much at the bottom of it!! So I'm quite happy for all the pointers, thanks guys!