CraigT82

A big newt like this will require some kind of thermal management in order to perform well. I have a pretty decent 140mm Noctua fan taken off my Fullerscope, which will work nicely. I made a baffle from 6mm plywood to close up the rear of the scope, which will allow the fan to work most efficiently. The fan is working to suck air out of the tube, and has a good strong airflow which should hopefully be enough to completely suck away any warm air rising off the primary. The baffle is attached to the cell using velcro and the fan is also attached to the baffle using velcro. The velcro provides some mechanical isolation and using a mechanic's stethoscope against the cell and OTA I can't detect any vibration on either with the fan running.

I don't get on well with the standard collimation arrangement of these cells: three adjustment screws and three locking screws. The adjustment screws have coils springs but these are pretty feeble and so I have fitted much stronger springs to enable me to get rid of the locking screws altogether. I bought a length of 15mm OD compression spring with 2mm wire and cut this into the required lengths. I finished the ends of each of the three springa by heating and manipulating the ends and then grinding a flat. These springs are very strong with a 130N/mm compression strength.

I marked out new locations for the edge support studs and drilled/tapped new holes, so that each stud now contacts the mirror right on the CoG pencil line I made earlier. Cruxis suggests that three points at 120 degrees is perfectly sufficient for this mirror, however Cruxis is assuming that the mirror is in an Alt az mounted scope, whereas this being an EQ mounted scope I'm not sure that three points is good enough (edit: seems OOUK think 3 point edge support is OK for EQ mounted newts, as seen on their mirror cells, whereas Aurora in the US make cells for EQ Newts with 6 point edge rollers for a 12" mirror- who to believe?!). There's not much I can do about that as the rim of the cell between the existing three studs is not high enough to drill and tap holes for another three studs and have them at the right height, although I could simply glue some nylon studs in the right place but they wold not be adjustable and great care would need to be taken to ensure all 6 points contact the mirror uniformly. For now I will resort to turning the OTA in the rings so that the mirror is resting on two studs as equally as possible during an imaging session, and will make a slip ring to facilitate this. This isn't too bad a situation to be in for now as the majority of my hi-res imaging I'm only going to be looking at a relatively small patch of sky about 10 degrees east and west of the southern meridian where my targets will be at their highest. I also modified the mirror clips as there is no need for them to be so large and clunky!

Having a good look over the primary mirror cell I found a couple of issues that needed attention. The cell is a 9 point floating design with 3 nylon studs for edge support. It was immediately obvious that the triangles were solid and most definitely not 'floating'. This was fixed by loosening the centre screws and removing the small silver coloured pins (one for each triangle) which were supposed to prevent rotation of the triangles but pretty much prevented them from moving at all. Three strips of tape were added linking the triangles to prevent then from rotating too much. The edge support comprises three m6 grub screws with nylon tips. This is actually quite handy as the clearance between the edge support studs and the mirror can be fine tuned. In order to check the studs were contacting the CoG of the mirror I removed the mirror and marked on CoG lines (calculated in Crux). Putting the mirror back in the cell it was clear that the studs were too low, missing the CoG by about 6mm.

Having just taken ownership of an old blue tube skywatcher 300mm f/5 newt I'm starting this thread to document the modifications and upgrades I'm making to it. This scope is going to be put to use for lunar and planetary imaging pretty much exclusively. The primary mirror has a rather odd graze in the coating which is easily visible and was disclosed by the seller so I knew that it was there before I bought it. I could see from pics that the defect was likely to be in the shadow of the secondary and so I wasn't too fussed. What is surprising is that on removing the mirror from the scope it seems like whoever had made the scrape had tried to remove it with something, most likely a scourer! Luckily all the scratches are within 30mm of the mirrors centre and so all are withing the shadow of the 70mm secondary.

These are great! I don't think you've got much to worry about, maybe seeing could have been better and in which case the mono cam and red or IRpass filter would help.

Ah ok, hadn't considered that you can easily remove the secondary to fit hyperstar system, In that case the warranty must be ok. I wouldn't have thought the defects in the glass would have any effect of the mirrors performance ( @Peter Drew?) but its probably worth checking with Celestron to be sure. They may just ask to send the secondary back and not the whole scope which would be much easier that sending the whole lot back

Theoretically you should be OK with this... https://www.firstlightoptics.com/rainbow-astro-mounts/rainbowrobotics-rst-300-mount.html

Well I reckon you're going to have to live with it seeing as you have almost certainly voided the warranty. Were the views OK when you used the scope or were there any issues? What prompted you to remove the secondary?

For a phone pic this is a real belter. Loads of recognisable surface features on display. Classic blue top and red bottom is atmospheric dispersion. Little soft due to it being a single image (?)but if lucky imaging could be used with that set up it could be much sharper. Very impressive for 4"

Good for you. You must be very proud of your stuffed furniture.

Haven't managed to get any new lunar images for a while so I've been looking through older images trying to pick up on lunar features I've previously overlooked. I found this image of Mare Insularum captured in January (Fullerscope, 290m and blue filter), and I don't know how I missed then the first time around, but there are a good number of lunar domes visible due to the favourable illumination.... I count about a dozen. For reference Hortensius crater with it's associated domes, is towards the lower centre-right. I've reprocessed the image and pushed the contrast and exposure a more than I would normally to bring out the Domes as much as possible.

Wow what a fantastic result for your first go!

Yeah hopefully!!

Hopefully soon, have missed out on the recent clear run due to being away from home but now I'm back the weather has turned... typical! It's usable now but I needed to fettle the mirror cell a little as the floating triangles were locked solid and definitely not floating, ok now though. Got an Astrosytems secondary holder and spider on order as I don't think the standard SW affair is really suitable for the 70mm secondary. What rings did you go for?

It is equally fundamentally unscientific to say that refractors don't require collimation (not that you said that, but many do).

What a fantastic scope, we'll done! What mod did you do to the astrosystems secondary holder (I've just ordered one). Also what made you decide on a glatter sling over a whiffle tree arrangement?

Very nice Neil👍

The input box asks for ambient temperature in centigrade. US users would have to do their own maths!

The thing is that the longer the wavelength the worse your scope's resolving power is. Radius of airy disc = 1.22λ/D. So at 850nm your scope is only resolving fine details twice as large as if you were using a blue filter (at 425nm ish). This is why people say it softens the detail... it doesn't so much soften the detail as not capture it at all!

This isn't as easy as you think. Even if the mirrors are fixed there is always flex/movement in the OTA itself, especially after the odd drop or knock. A truly fixed collimation reflector would probably weigh an awful lot considering the extra strength of the OTA necessary. Besides, refractors are supposed to not require collimation yet I've owned three fracs and the first one was out of collimation in the lens cell and not adjustable. The second had misaligned objective and focuser which wasn't adjustable either but was bodged with shims to line up. The third was perfect.

Maybe you could use the simple Skywatcher DC motor, and for visual just use the battery operated hand paddle. For computer control you can get the Hitecastro DC focus controller.

I also went for the contrast booster for my 102mm f/11 achro when I had it, that frac didn't give much CA anyway but I wanted to eliminate it completely (being a reflector man I cant stand CA!). The contrast booster does indeed dim the image quite a bit as you can see from the test shots in that pdf, but if you're just going to use it on the moon that's probably the one I'd go for personally. Just for info here are a couple of phone shots I made with the 102mm f/11 I had, first is unfiltered and second is with the contrast booster.

The A62ss a 62mm f/8.4 achro which on the face of it might not show much CA, but it is actually a 4 element design with a shorter FL doublet up front and another doublet at the rear acting like a barlow lens, so this means that the CA is likely to be quite bad due to the short FL doublet up front producing the CA and the barlowing pair at the back simply magnifying it. Might be worth going with one of the stronger filters if you find CA objectionable? Here is a pdf with some test shots through the Baader filters, might be useful if you haven't already seen it. testreview_baader_anti_fringing_filters_astronomy_technology_today.pdf

Sorry not sure on where you can get the mirror info from, but Astrosystems.biz (in the USA) make spiders and secondary holders if you didn't fancy making them yourself and cant find an OE replacement. https://www.astrosystems.biz/sechold.htm