New SynScan, new 55mm lens, and Sadr et al

[PeterC65]

Last night was my first session of the season. I’d bought a new (to me, but actually an antique) Pentax Asahi Super-Takumar 55mm Lens that I wanted to try. I like widefield and this would give me super widefield, great for observing the Milky Way I thought, and being an antique it had manual focus and aperture controls which seemed better for astronomy. So I set up the 72mm refractor with its x0.6 reducer alongside the Pentax 55mm, both with a Uranus-C camera.

The second thing I wanted to try was the new release of SynScan Pro (v2.5.2) which includes mount auto alignment using plate solving. Widefield would make the plate solving easier to do. I’d tried the setup in daylight and could see a live camera image, and in the dark the whole auto alignment process worked well.

You have to select a number of alignment points to which the mount slews, capturing a frame, plate solving it, then creating an alignment point to use for its pointing model or for local pointing accuracy enhancement. I didn’t know how the mount would slew so I hit the go button then rushed outside to check that the mount wasn’t doing anything silly. At least for three alignment points, they lie along the diagonal of a rectangle 120° wide and 30° high, one at each end and another in the centre, with the starting position of the mount at the bottom left corner. Next time I will make sure I start from the bottom left corner of the area I plan to observe.

The main part of the Milky Way is never very high above the horizon for me so I started with that, and some of the objects within it, just as it was getting dark. Anything below 30° of the horizon seemed very noisy, perhaps due to the dust we have in the atmosphere at the moment.

The Pentax 55mm was easy to focus although I couldn’t get an image with the Bahtinov mask so just focussed on a bright star. The aperture ranges from F2.0 to F22.0 but at F2.0 the stars were very bloated and fringed with purple. Disappointing as I was hoping take advantage of the big aperture. At F22.0 everything sharpened up though.

I’d fitted a dew heater to the refractor but not to the Pentax and it needed one. An hour in and I began to see a central disc of noise which I confirmed was dew on the lens. I belatedly added a dew heater which eventually drove off the dew but leaving some residue and hence some central noise. I did briefly get some good images with the Pentax and those that were noisy showed some promise.

Here is the best of them, M16, with a tree to the right …

M16

M16 looked a lot better with the 72mm refractor but I really need to point the Explorer 200 at it sometime and see if I can observe the Pillar’s of Creation.

M24 was unfortunately too large to fit in the field of view of the refactor, but here is part of it …

M24

M11 looked better but would have benefitted from more magnification …

M11

With the Pentax 55mm dewed up and the lower altitudes murky I slewed to the Veil Nebula and the Sadr region. First time around there was very whisp cloud which appeared as noise in the image. I considered packing up since it was midnight, but persevered and was rewarded with some of the best views I’ve had of this part of the sky. That may have been due to the Optolong L-eNhance which replaced my original Astronomik UHC filter (which is meant for visual and so passes IR). The jury has been out on the L-eNhance but last night it made a big difference.

I could see lovely wispy detail in both parts of the Veil Nebula …

NGC6992

NGC6960

I’ve always been disappointed with the Sadr region and only once manged to make out the shape of NGC7000, but last night I could see masses of Hα emission nebulosity around Sadr …

SAO049528 Sadr

including some nice wispy bits around the star SAO049603 which doesn’t seem to have any special designation of its own …

SAO049603

and I could make out the shapes of the North American Nebula (NGC7000) …

NGC7000

and the Pelican Nebula (IC5070) …

IC5070

These were all observed with the L-eNhance and looked really bright with the lights off inside at night.

It was getting very late for me by now (2am) so I finished off with M31 / M32 / M110 which was very clear …

M31

and M45 which was a bit too close to the Moon.

Then a final snapshot of Mars and Jupiter together in the same field of view with the Pentax 55mm. Just bright dots, but the aperture blades caused a nice six point diffraction pattern …

Jupiter and Mars

 

[bosun21]

Very nice Peter. I particularly like the Veil and Sadr. The L Enhance working nicely. What sort of exposures were you capturing with?

[PeterC65]

4 minutes ago, bosun21 said:

What sort of exposures were you capturing with?

I was using 8s exposures which has become my usual setting for EAA. NGC6960 is 74 frames, NGC6992 is 111, the Sadr region is 59.

 

[happy-kat]

Looks to have been a very successful night. On camera lens I use a Y mask, take an image then zoom  in on the image to check the Y mask result.

[PeterC65]

17 minutes ago, happy-kat said:

On camera lens I use a Y mask

Do you happen to know where I might get one of these? I will try the Bahtinov mask again with the lens stopped down but I'm not holding out a lot of hope for that.

[happy-kat]

I make what use from cardboard the first version I did just used rubber band to hold the Y mask on as I gave each bit a long leg to fold over the lens. 2 examples

 

[Ratlet]

I've got the 135mm and 200mm version of the lenses.  Basically they focus the blue and the green to the same spot but not the red.

If you use a mask you will end up in the same position, or possibly worse with red bloat which is far more obnoxious looking and if imaging far harder to process out.  I think with blue/green defocused photometric calibration will often see it as the star and process it out itself.  At least this is my experience.  Red just lingers like a bad smell.

The trick is to get it in focus then back it off slightly.  Your basically trying to make the blue/green and red equally defocused.  It doesn't take much and won't over bloat the stars.  The good thing with this is that you can open up the lens to faster speeds.  I found that I only need to drop down by one stop or maybe 2.

Finding the sweet spot is a pain though.  Part of the reason I'm building an autofocusser is so that I can hit this spot reliably.  I just need to figure out how much offset is required.  I can the run an autofocusser and (hopefully) tell Nina to use a filter offset to advance to the correct point to get rid of the halos.

I've attached a couple of images, one showing what I considered to be good focused and another... Well.  I think each was maybe 30 minutes of capture with the 135mm from last year.

[Ratlet]

But they are great lenses imho.  You would need to spend a LOT more to get much better performance.

[Elp]

F22? Really?

Yours maybe different to mine, I've got the 50mm F1.4, and it works around F4, better at F5.6 (most taks I've used work well around F5.6). The edges are however astigmatic/coma'd slightly with the 50mm, no issues as 50mm is very wide (also difficult to use if surrounded by street lighting) and can support a crop.

The right takumars have that red/green focus points as advised above and usually you have to do the infinity focus mod so you can focus past the red bloat and into the green then focus to where the red halo around stars just disappears before the green appears. They are relatively well corrected overall for 50 year old lenses. Note most of the best ones have thorium lens coatings (they'll be yellow by now) and I believe was common to manufacture at the time for the faster lenses, thorium of which is radioactive so don't go physically touching the glass too often, likely won't do much harm if handled and stored appropriately.

Finding good modern lenses is also extremely difficult, most don't even correct as well as the takumars.

[PeterC65]

Looking again at the aperture scale on the Pentax lens, it runs from F2 to F16 and I was using it two clicks from the limit, so F9. I started off at F2 and the images looked awful, just masses of star bloat, so I just adjusted the aperture to (close to) the opposite end and got much better results. I will try using it at F5.6 next time and follow the focusing suggestions mentioned above.

The lens in mine is indeed yellow.

 

[Elp]

Backspacing also plays a part when using astro cameras with lenses. Some of mine I have to pack the camera backward 0.7-1mm though I've never done this with the takumars.

[PeterC65]

I spent some time researching the correct back focus to use with M42 lenses. From what I discovered my lens needs 45.46mm of back focus to the camera sensor to just achieve focus at infinity. I've actually set it to slightly less than this (44.3mm) so that I can focus past infinity. The back focus has been set using a Baader M42x1 to T2 Adapter, Baader T2 to 1¼” Ultrashort Clamp, and a couple of Baader 1.4mm Spacer Rings which gives me 44.3mm when used with my camera and its nosepiece.

Does that sound sensible?

 

Edited Tuesday at 17:08 by PeterC65

[Elp]

Is that the full range of BF? Sometimes they don't state the 10/11mm thickness a typical t ring takes up, or the inside of the camera creating a 55mm BF total.

I utilise the ZWO eos adaptors (there's two types, I've got both):

1. I use a m42 to eos adaptor ring on the back of the lens,

2. Lens then connects to the ZWO eos adaptor,

3. Then on the back of the ZWO adaptor I normally use the 11mm female to female m42 ring,

4. Then raw zwo camera, the more common type with the upstanding "nose" that the black 11mm ring threads onto.

This gets me near enough normally, or I pack the nose of the camera with spacers to move the camera back.

The same item under point 1, connected to an Eos to Sony e mount adaptor works the same on my Sony body camera.

I'd have to measure it now whether the distance I've got is near 44mm or 55mm.

Edited Tuesday at 16:58 by Elp

[PeterC65]

I got the back focus distance from this Wikipedia page which says it is the distance from the M42 flange to the camera sensor. I was expecting it to be 55mm but perhaps this is only for more modern lenses. When I set it to 45.46mm, infinity focus in daylight was right at the end of the focus travel. Reducing the back focus a little allowed me to focus just past this point, again, in daylight pointing at a ridge a few miles away.

I have some modern Canon EOS lenses and had considered the adaptor you are using, screwing the camera directly in to the adaptor. But I normally use the camera with its 1.25" nosepiece fitted, having configures all of my astro kit to accommodate this, and wanted to do the same with the Pentax.

[Elp]

The near 44mm would be right, just measured my SY135 and it's set roughly 38mm + front of camera to sensor 6.5mm = 44.5. My takumars would sit roughly 1-2mm forward due to the m42 to eos adaptor ring. I always measure from the physical stops.

[PeterC65]

@Elp do you use a UV / IR  cut filter with your 50mm camera lens? I don't currently have one.

If you do use one, which one do you have?

 

[Elp]

45 minutes ago, PeterC65 said:

@Elp do you use a UV / IR  cut filter with your 50mm camera lens? I don't currently have one.

If you do use one, which one do you have?

 

If using with an astro camera that has a generic AR window then yes, any luminence filter will do as they're the same as UV/IR cut (I've got Optolong and Astronomik), mine are 2 inch, off hand I think the front of the takumars are typically 49mm so I use a 49-48mm step down ring out front and screw them on there, or if I'm using the filter drawer adaptor they go in there.

[PeterC65]

6 hours ago, Elp said:

If using with an astro camera that has a generic AR window then yes, any luminence filter will do as they're the same as UV/IR cut (I've got Optolong and Astronomik), mine are 2 inch, off hand I think the front of the takumars are typically 49mm so I use a 49-48mm step down ring out front and screw them on there, or if I'm using the filter drawer adaptor they go in there.

I've ordered a Hoya 49mm UV/IR cut filter from Amazon. Maybe it will also help a little with the star bloat.