Adam J

Pritty sure that as a rule the imaging circle is larger when you use a flattener than a reducer flattener?

You would think every filter would be tested. Especially the fast stuff with tight tolerances.

huum I just refocus between filters, do 30mins per channel, focusing is only costing about 2-3mins on filter switch. You could say that you may not get 1.5 hours and not complete an image, but thats rubish who sets up for a gap of less than 1.5hours anyway and even if you did would you get a good image in that time? Adam

If I remeber correctly that is a FPL51 doublet if that is correct then i would not chose it for astro photography with that sized aperture.

Might make the stars look green if you drop the other two channels bit not sure what you would end up with if you then ran HLVG. Ok think it's worth a try.

Three things going on here. 1) You have too much back spacing from the flattner 2) You have sensor tilt. 3) You potentially have a slight colimation error, but you need to fix 1 and 2 and then reassess that. Test and adjust in that order. Adam

I have been using my ASKAR FMA180 quite allot recently in my current situation its a easy way to image and the image scale makes things easy. I note that when using it with my ASI1600mm pro I still get slightly larger stars in red and Blue in comparison to G, this is down to the speed of the scope and budget ED glass. Focusing between R G and B filters as I cycle them. I have been wondering if I would be better off focusing on blue when imaging with green. My reasoning being that this seems to lead to the star sizes being better matched between all three channels but at the expense of losing the max sharpness of the Green channel. The underlying cause of this is spherochromatism aberration as opposed to classic Chromatic aberration, hence the focus is nearly identical but the scope is just sharper in green in comparison to Red and Blue. Anyone ever deliberately de-focused the green channel like this? Adam

Can anyone post a dark frame fits file for the ASI485mc camera? 30seconds at gain 200 would be ideal thanks. Adam

The ASI120mm is fine for guiding no need to pay more.

Yes, but from the 20% example it looks possible to have a situation when every other ring is attenuated relative to unobstructed aperture even if the spacing between rings of differing brightness is the same. Suppose you could get an additional interaction with the pixel sampling of that effect occluding the underlying detail and only picking out the delta between the brighter and darker rings. The 130pds has a 36% obstruction though. I think that the reason that you still have the same fringe spacing is that more energy is placed into the rings created by the full aperture in comparison to the smaller amount contributed by the secondary shadow and as a result you will always see the primary rings and the secondary will simply modulate the primary rings as they move in and out of phase with them. If you hit the right ratio you can produce an effect similar to what is observed. As the size of the obstruction is increased the beat frequency will decrease and the attenuated rings will become darker / nulls deeper. Adam

I mean the shadow of the secondary cast prior to hitting the primary mirror, as opposed to the flat mirrored surface itself. It would be like cutting a circle out of the light incident on the primary of about 46mm diameter. I believe that you get interference between this secondary pattern and the pattern caused due to the primary mirror diameter that is the essence of why you get poorer contrast with a reflector as opposed to a refractor? Might be wrong though. Having said this I did operate this combination myself for a while (130PDS and ASI1600mm pro) and dont remember this effect. However I never imaged a bright star in narrow band and very good seeing conditions either. Adam

I took a look and can confirm your findings. Looking a little further you would expect the observed fringes to be produced by a aperture of between 31mm (3 pixels between fringes) and 46mm (4.5 pixels between fringes) the higher end is about the size of the 130PDS secondary mirror shadow and the bottom end is about the size of a 31mm filter. I suspect the secondary mirror on that basis. Adam

Am guessing that this is a very large aperture reflecting scope ? If so then it is actually normal and can indicate very good optics you will see this on bright stars in Hubble narrow band images. Find a high resolution image of the pillars of creation and you will see the effect, but now quite so obvious unless you can find a mono Ha only image. Adam

They do if you have dust on your filters or they centre a little differently when the wheel rotates to them.

Actually I will modify that statement I think the reason is the weight means it's harder for air to displace it as opposed to the size of the atom.

Every few years you have to replace the dessicant or it will freeze or dew. Takes about 30mins. The SX stuff is argon filled, as its dry and being a very large atom will not seep past the seals so easy. But it adds expense and replacing the dessicant is not such a hard thing to do.

For me the ASKAR is just a bit of fun for mobile imaging, don't expect to win any awards with it or anything but, it performs sufficiently for what I am using it for I think. It does have a tiny but of CA after using the L3 but is really is slight for me. Its easier to cope with when using a mono camera. But if you look in that thread @PadrePeace virgo galaxy group then he is not getting the same amount of CA as you on his ASI294mc pro. So I still suspect a issue with colour balance and or focus.

My personal opinion is that flat field APOs are more trouble than they are worth, you have more handles on a triplet + corrector, it allows to adjust when things are not quite rite, when scopes have an in built corrector then there is not much the user can do to fix them beyond sensor tilt. Sure they are great when they do work though. Another reason why the vixen would be my choice. I take it that this is a replacement for the ASKAR 180 then? Adam

Maybe a skymax 90? Put it on a AZGTI mount.

I think the Vixen for me if you want to pay the additional, just because I have a preference for the traditional focuser.

The solution to frost is to open the camera and replace the descant not wrap a heater around it. You can either recharge it or you can buy new tablets. But heaters are not going to help you. I described the process here. same for your camera. Adam

You don't say what camera it is. I still feel it's frost.

Possibly frost on the sensor.

This is not really a complete product more of a test shot really (no calibration used) I am going to get more panels in 2022 and take it up towards SADR + double the width. As it stands its 2 panels. 100 RGB per channel per panel at 20seconds 12 Ha per panel at 240seconds 6h total. ASKAR FMA180 ASI1600MM Pro Baader RGB + Astronomik L3, Astrodon HA 5nm. Not really done this kind of shot before so suggestions welcome, I could have pushed the Ha much further but it starts to look less natural and I was going for a natural look. Thanks for looking and commenting. Adam

Yes you need to be bang on focus, I use an auto focuser with mine so you are pretty much guaranteed accurate focus. But at F4.5 i would think that a focusing mask would be a minimum requirement. If you look at my star field above you will not see much bloat. Adam