Refractor dew control and flat fields

[smr]

Hi,

I've recently acquired my lovely HEQ5 Pro mount so I just need to buy a refractor now. I'm pretty much set on the Skywatcher ED80 Pro but I'm wondering how to control dew with it, what would you recommend?

Something else I've been wondering... at the moment I have a Star Adventurer mount and shoot with a 55-250mm lens mainly. In Lightroom (or any other raw converter) I can apply lens profile correction for this lens which flattens the field curvature. 

Why is it that you can't get a lens profile for refractors? I assume you can't as that's why you have to take flats with refractors but I don't know why you can't.

 

 

[Skipper Billy]

I am sure someone far more knowledgeable than me will chime in but my view is.....

You cant go wrong with a SW ED80 - fine scope and great value for money and a match made in heaven for a HEQ5. Many of us cut or teeth on excatly that setup - inc. me !

For dew control you need either a camping mat home made dew shield or of budget permits one of these https://www.firstlightoptics.com/dew-prevention/astrozap-dew-heater-tapes.html and one of these https://www.firstlightoptics.com/astronomy-cables-leads-accessories/hitecastro-four-channel-four-port-dew-controller.html which will also allow you to add a further dew band for your guide scope when you get one.

Taking flats isn't to correct distortion like you would with a lens profile with a camera lens - flats are taken to remove the effects of dust motes in the system and vignetting. 

There are some examples of home made dew shields and the better dew strap system on an ED80 mounted on a HEQ5 here - scroll down towards the bottom of the page.... https://sites.google.com/view/astro-imaging/equipment 

Hope that helps!

[vlaiv]

Yep, flat field is one thing, and flattening the field is other

I know that this sounds silly, but here is what each mean:

Flat field correction is removing uneven illumination of the sensor that can arise from couple of factors - vignetting and dust being main. So it is "flattening" of illumination, and has nothing to do with curved field - that you can correct with appropriate software or in case of telescope using field flattener. Sometimes software that comes with your camera for day time photography has this option as "correct vignetting", and can be incorporated into "lens correction", but will not deal with dirt/dust on sensor or filters - just uneven illumination due to internal lens obstructions on large sensors.

Curved field simply means that focal plane of telescope or lens is not perfectly flat, while sensor surface is. You can think of focus plane being very small patch of sphere (or some other curved surface). When it is small compared to radius of sphere it is almost flat, but for larger sensors, corners can be in zone where focal plane is no longer on surface of sensor but some very small distance away.

Effect is such that stars in the middle of the sensor are in perfect focus, while those in corners are slightly out of focus.

In principle you can correct for this aberration in software, and that is something lens models do in specialized software that comes with your camera. Problem with telescope images is that for algorithm to work well you need very high signal to noise ratio - since process tends to amplify noise while trying to correct the signal. This is ok for day time photography when there is plenty of light and signal is so strong that all the noise sources are negligible hence SNR is very high.

In astrophotography light is scarce, and SNR is often not as high as we would like it to be. While some aberrations can be corrected with specialized algorithms (see example below), it is often not feasible to do it, and "hardware" solution is preferred - using dedicated field flattener. Also, software would need to know exact "model" of scope to attempt field curvature correction (or let you adjust various technical parameters to try best combination).

Long time ago, I was struggling with high periodic error of my mount, so my images had quite a bit of elongation of stars due to this:

(image stretched to emphasize star shapes)

So I tried algorithmic approach to solve the issue in this particular image, and after much trial and error (to adjust parameters of algorithm) I ended up with following:

So stars are indeed looking rounder, but it was at expense of SNR, and I would not call this a good image (well, I would not call it good even before star correction ).