fwm891

These images were captured this week, others still to process but I have to say I can't remember the last time I've had 4 nights in a row - magic. Subs were all 300s in length for M16, 17 and ngc6888 (numbers in the file names) M20 was shot over 3 nights, mainly using 300s but there were a few at 120s and 2 at 600s in the mix. M20 was shot using a Baader UV/IR cut filter initially then I switched to an IDAS NBZ on the third night (last night). TS/GSO 8 inch RC with 0.67x reducer. ASi533 MC Pro OSC camera @ -12 C Processed in PI. Hope you like.

Hi Robert72, I've used Photoshop for all the drawings. I set scaling/resolution at 100 dpi which keeps the file sizes down. I tend to use layers a lot with these drawings and build things up by creating smaller drawings at the same resolution and transferring them to build up a full sized scope image. That way your working with smaller files making life easier all round. You can probably do the same with Gimp2 but I don't have any working knowledge of Gimp. I hope that helps

Managed a bit more metalwork today. I've made the pivot shafts with a modification. I've made the pivot end of the shaft 10mm rather than the 8mm originally planned. Also given some of the ali parts a light clean-up with #320 W&D paper (dry). The holes in the top of the box sections are for Allen key access. Other mod: I've inserted a snug fitting piece of timber inside each of the aluminium Box sections: a) provide extra support where the pivot shaft goes through the ali box section b) help to deaden any vibration within the framework.

Short answer No. The M42 thread (T thread) is used to attach the camera to a lens/telescope. You will need an adapter of some sort to take a 1.25" filter. I used an old cheap 1¼" barlow with the lens removed to do that. They have a M42 T thread at one end and a 1¼" filter thread at the other...

Great build - Mine is similar for a 10" dob. To be upgraded for a 16" build currently under way.

My initial thoughts on the axial adjustment of the spider to position the secondary under the focuser. The drawings are not engineering style nore are they photos so some might find them confusing but they work for me. The top left shows the end view into one of the 4 upper cage spacer tubes. The black fixed pressure plate is bonded to the inside of the tube (probably epoxy but may suppliment with m/c screws). The vane itself is held in the moving Vane anchor block with 4 m/c screws, there is a tension adjustment screw which passes vertically through the Vane anchor block to apply tension and radial positioning of the assembly. The travel locking screw passes through a shaped 'nut', tightening the travel lock applies pressure via the screw to the Vane block and outwards against the tube wall. This will allow circa ±20mm to the whole assembly inside the upper cage. Lower left is an end view of the secondary adjustment assembly giving tip/tilt adjustments via compression springs for collimation. View on the right is my attempt to show the same from a different angle. The pale slot outline against the fixed pressure plate is the slot the Travel lock moves in. TBC...

A RASA8 has a back focus of 25mm when measured from the M42 adapter or 28.7mm if measured from the reference surface (i.e. M42 adapter removed). You should download the 'White Paper' from the Celestron web site, it gives all the specs and drawings for the RASA scopes.

Next job on the list: The pivot bar, pivot shaft and triangles. I've already roughed the triangles to shape. I now need to form a domed pocket to their undersides and a flat bottomed counter bore to the upper surface to allow a 2.5mm retaining screw to passs through the triangles, the domed pivot and into the pivot bar. Not shown will be a cap head screw under the innermost support pad, this will act as a retainer for an 'O' ring linking the 6 plates and keeping them aligned. The pivot shaft will be turned from Hexagonal brass leaving a short hex section to act as a bolt head. The shaft will pass through the 1½" x ¾" x 16SWG support beam at 12mm OD, then down to 8mm OD to pass through the pivot bar. There will be a PTFE washer between the support beam and the pivot bar. Then a second PTFE washer and a cap washer and retaining machine screw ties the whole assembly together and allows side pressure to be adjusted so the pivot bar can rotate but not tilt sideways. That should keep me busy for a while...

I like that Peter. I like that a lot.

@Piero Thanks for the comments, I like your metalwork pieces you've posted, looks quite robust. The only 3mm dimmension posted is within the Newt-for-the-web screen grab and that is there purely because the program required a wall thickness for it to run. As this is a truss tube dob there is no solid wall so the 3mm is essentially meaningless. The truss poles are 16 swg or 1.6mm wall thickness and 22.25mm OD which I'm sure you will feel is under sized. The drawings I've posted have drawn for my reference and are evolving as I flesh out various details in different areas whilst keeping with the overall form of the scope. The upper cage being one area that is currently being fleshed out. I've always intended that the eyepiece will sit at at 45° angle to make life a little easier when viewing at lower angles, and also easier when viewing near the zenith by avoiding having to lean over the scope. They have been drawn that way to keep thing simple thus avoiding having to show a circle as an ellipse etc. The upper and lower truss pole seats/anchors are based around the use of register pins to avoid location errors and slippage not shown in any of the current drawings.. I've updated the Newt-for-the-web dimmensions sheet to reflect the 75mm secondary changes. I'll update the build images as things progress

I've spent the morning first printing off some of the drawings as A4 sections to build up a full scale drawing which I've taped to an odd board in the workshop. This is where I found suddenly everything is bigger than I thought !! Then to work on roughing out some of the parts. All will probably need some fine tuning but they're pretty close. I made a decision to increase the size of the gusset plates to allow more fixing at the main frame junctions. I used a bandsaw to rough cut pieces just over length, then used a disc sander to clean-up back to the lines. Bits get very hot when using a disc sander !!! Images show the parts cut so far layed out on the plan.

Heres an update on the drawing of the side view. A second focuser can be seen at the end this purely to show what's hidden behind the upper cage.

It will be 75mm more than fully illuminates any eyepieces I'm likely to use with it. If you look at the Newt for the web screen grab above (top right panel) you'll see with the secondary at 72 mm there's no vignetting. 72mm was the figure that just gave me those results. So with mine at 75mm I have a little cushion.

Tube anchors at the mirror box

@MarkRadice I'll make all the parts with the exception of the mirrors and focuser (Baader Diamond SteelTrack NT) I've been looking at the secondary mounting and come up with this based on a square aluminium box section, 4 thin spider vanes are clamped to the exterior of the box section using 3mm stainless cap head machine screws with washers and nuts. There's a fixed plate held inside the box section with an adjustable (tip & tilt) plate. A backing plate just smaller than the mirror is fixed to the tilt plate and the mirror fixed to that with adhesive pads. Not shown: the mirror will have a safety wire(s) to prevent accidental damage should the pads fail.

Well John Nichol emailed me this mornng to say the mirror is now finished and about to go off for coating with an estimate of 5 weeks for that. John has also given me a final focal length and blank thickness so I'll update drawings to suit the new data. I've also had an aluminium delivery ready to make the support framework. I'll order some 18mm marine grade Birch plywood for the mirror and rocker boxes. I already have some 12mm for the upper cage rings and mirror cover. Getting excited now...

Why not just place the whole OTA assembly on a piece of dowelling. Roll the whole unit back and forth and find the tipping/balance point from both directions and take an average. That will give you your balance distance.

Great project. I've been thinking along the same lines and I'm in the process of building a 16" f5 dob for visual use while imaging. I've ordered a mirror set from John Nichol and just taken delivery of some aluminium ready to start making the mirror support structure. Enjoy, and let us know how things fare. Francis

Almost too easy to be reliant on the gizmos, they get you close, probably good enough for visual. I've carefully centred the reticule in my CEM25P and then use a phone ap to give me the position of Polaris within the reticle. Gives me 3 min subs, longer guided.

For Bortle 9 skies I'd give yourself a pat on the back. Give some details of the image so people can comment and help you on your way.

I started using the Baader filter draw system but found a draw back with it in that you couldn't mount the filters so the correct face faced the sky. Whether that would have made a difference I don't know but Baader filters are usually intended to be used in a given orientation. I think there amy be a new version out now. I ended up making my own draw system to go with the ASi533 I use on the RASA 8. (Photo shows an ASi294 which I have now sold and replaced with the 533) both have the same back focus 6.5mm.

Not been in this section for a quite while but I now have a project in hand. I've ordered a mirror set from John Nichol (16" f5) 411 OD x 44(ish)mm thick with a focal length of 2032mm. Nothing to challenging in the design it's for visual use on an EQ platform, possibly some iamging but that's not the main intention. Mirror support cell will be an 18 point structure, the optical layout and support has been worked through with "Newt for the Web" and Plop respectively. Mirror box etc have been designed around these constraints. The drawing as it stands shows the layout (horizontally) across the top half with plan views of the mirror box and rocker box bellow. The upper cage is still a work in progress but show the intended angle of the output cone (45°). The focuser is a Baader Diamond SteelTrack NT (I have this already) this will be mounted on a plywood panel between the upper cage rings. I intend this to be a "floating" arrangement with enough movement to allow the final focal plane's position to be optimised. The secondary support will also be adjustable axially to enable it to sit directly under the focuser. Mirror support main frame will be made from aluminium (HE30) box section 1.5" x 1" with 16swg wall thickness (1.6mm) with inserts where box sections meet to allow parts to be bolted together with gusset plates for ridgidity. Brass bushing will be used for any pivot points. The whole cell will be adjustable for collimation from the front. The mirror box arrangement can be seen in drawing 2. The mirror edge support will be a pair of pivoted arms with pads. That's where I am at the moment. I'll order the aluminium so I can make a start on the mirror support construction. Marine grade Birch plywood will be used for all the 'timber' parts. I intend to use 8 poles in the usual triangles at each side. TBC

Fit your new focuser and tilt unit first. Keep the tilt unit as close to the focuser as you can. Then follow your intended route

Not used Gimp but I think it's like Photoshop without ($$$$$) sign. It should have a saturation adjustment. Try small adjustments and gradually build up colour, also levels, make levels adjustment in small increments but avoid using the sliders right at the blank and white adjustment ends as that will result in clipping these areas. Adjust saturation and levels switching between them . Once you've had more practice you'll get more confidence in bigger adjustments but I still prefer keeping changes small. With a Ctrl+Z after the check on differences...

Hi Grant - I've had a play with your image file. I've processed it in PI, theres a great deal of information in your image. There's a patch of blue in the lower right of the image I haven't tried to remove. Can I ask if you covered the camera eyepiece during exposures as that can cause awkward patches of light on images. My try...