bobro

I don't know if you are old enough to remember the days of separate exposure meters and interchangeable lenses, but once a light reading was taken, film sensitivity and exposure time was set in the meter, the f# was the result. It then didn't matter what lens was put on the camera - any lens set to the same f# delivered the same amount of light to the film plane (within reason) and hence the resulting exposure was correct. That's why I wanted to measure the 'true' f# on my scope setup - basically comparing it with a camera lens which doesn't have the same obstructions and light loss. Art enlightened me on the concept of T-stop, which is a more appropriate way of measuring what I was looking for. Using Excel, I previously did a basic calculation on my Newtonian scope, taking into account only the light loss due to the central obstruction - the result was about f5.6 (in non T-stop terms). My thinking about exposure and f numbers comes from many years of photography with cameras, so I tend to look at things that way. It does make me see a refractor scope as just a big camera lens and I would expect an f# measurement to turn out just as would a lens. A reflector scope doesn't seem quite as efficient - especially after my mods! I'll try and re-measure without the CC as rotatux suggested.

Thanks for looking into this Art. I hadn't come across the concept of a T-stop before - there's always something new to understand, making AP a great subject. No doubt you are aware that scopes such as the 130P-DS have the primary mirror moved a bit closer to the secondary mirror to help with DSLR focus. I've just read the 130P-DS has a slightly larger secondary mirror than the 130P to ensure light is not lost from the closer primary. With my scope the secondary is unchanged so there will presumably be some inefficiency there. I wonder what the T-stop of my scope is......

As previously mentioned, I wondered what was the actual f# of my scope, what with the obstruction due to the secondary mirror, scope shortening to achieve focus and the addition of a coma corrector. Here are the results of using the camera with a 135mm lens at f# from 5.6 to 16, comparing with the camera on the scope at a nominal 5. Same exposure time and ISO for all images.

A few things : Although the scope is nominally f5, it isn't really as the secondary mirror blocks some light (bought this way of course) 40mm was cut from the OTA to make it focus with a DSLR, likely resulting in some light loss at the secondary and also some loss up the focuser tube as the camera sensor is far up the tube The (DIY) coma corrector results in additional vignetting and some light loss

Replaced the scope by a camera lens (£18 from ebay). Note - the can is full of sand - contents weren't wasted!

Well if I can't find the target with the scope - put a wide angle lens on! 21 subs @ 120 secs with 135mm lens. Unfortunately didn't notice the camera display was on - some light from this registered on the sensor. It did show me how much brighter the image is with the lens at f4 when compared to my scope, so I think I'll do a comparative test to find out the real f# of the scope.

Art, are you aware that you can submit an image to nova.astrometry.net to identify where in the sky the image is? As I image from my back garden and have Internet access on my laptop, I take a quick snapshot and upload it as part of finding the target. You almost had the Pelican Nebula!

ISO setting may not do what you think (excuse me if you already understand this). The camera captures photons irrespective of the ISO setting. What the ISO setting does is enable amplification when the sensor is read. That is why a balance between the read noise and ISO setting is relevant - you want to keep read noise in the image as low as possible. So why not set the ISO to maximum? Well, to ensure bright parts of the image (stars) do not saturate and to leave room for stretching in software. So setting a high ISO value on a camera isn't necessarily a good thing - if I understand correctly (as always...). I understand you are going for high ISO as the exposures are short. That can help balance out read noise in faint detail, but won't help with the rest of the image.

Thanks Nige, I'm sure you are correct in that the southern part could provide more interesting imaging. A mosaic? That means I have to find it again - with accuracy of the area being imaged - aaarrrrghhh! I better get a goto upgrade!

The camera is set to ISO800 as I understand 400/800 helps to keep noise lower. It looks like longer exposure with my setup is still required.

Had another go at the NAN - without the UHC filter. A little better than my previous attempt, but I expected more nebulosity to show in the RAW image. 21 subs @ 360 secs.

Good to see the difference with longer exposure Nige - gives me motivation to have another go at the nebula with longer individual and total exposure.

So how did you achieve the colour change?

Believe it or not Neil, I was aiming for the Pelican but time was getting on so went for what I had as I couldn't get the final adjustment right! Yes, my captures don't seem to show the same number of stars as others achieve. It could be down to a number of things : 1) The OTA has been shortened by 40mm, possibly losing some light as the secondary is closer to the primary. Could also be light loss due to the focus tube being 1.25" and a long path up the tube due to the OTA being shorter and the camera relatively far out compared to scopes with shorter focusers. 2) The 1.25" Coma Corrector results in noticeable light loss and additional vignetting. 3) The UHC filter requiring longer exposure. You can see the difference in image brightness with and without the filter in the attached jpeg versions of raw images. Both are ISO800 240 secs and are of the NAN. As you say, guiding, together with the CC, are producing good round stars. What has also made a real difference is being able to very accurately polar align using SharpCap - DEC corrections are very infrequent when guiding. No issue with finding guide stars with the more sensitive camera which means the guide scope points in the same direction as the image scope - no field rotation (shouldn't be I know if PA is spot-on). I'm glad you posted your images of the NAN - lovely and sharp. Brings me on to something that puzzles me about filters and colour balance : If I try and do the usual lining up of RGB peaks after stacking to restore the colour balance following use of a UHC filter, will that not just result in degrading the image as some of the colour (especially green) will have been filtered out? Trying to achieve white stars would seem to be an issue. There must be a point I've missed about this type of filter..... Following your feedback, I think the next step will surely be longer exposures. I will try without the UHC filter first as it seems to be cutting out so much light. Bob

First attempt at (part of) the North American nebula. Didn't quite get focus. Tried a UHC filter - should this have helped with the image? Perhaps longer exposures would have been better as the subs were darker than normal due to the filter. Will try again to see if I can improve on it. 24 subs @ 240sec.

So just when I thought I was starting to understand something about imaging...... Well, to my untrained eye, the 10 sec subs have a tiny bit more detail and the 50 sec subs more noise. So that means the 10 sec subs win for me! As to why - no idea. Makes me wonder why I'm busy with guiding to obtain longer exposures. Perhaps it's a different story with exposures into minutes, though the short duration images (great composite) seem very good. Someone must be able to explain the result.......