New reflector problems

[thegableguy]

I just took delivery of a new GSO 8" f/5 from Andrews Comms and had a shot at Helix last night. For some reason it was far, far darker than it should have been, ie several stops. Trying to work out why.

I've collimated it this morning and it was significantly off (and there are smudges on the primary - WTH Andrews???), but it's perfect now. I've just compared it to my ED80 with the same camera & settings, and have been dismayed by the results. Using the same crop-sensor DSLR at the same exposure setting, the same target is actually slightly BRIGHTER through the ED80 than through the Newt. Surely this shouldn't be..?

Worth noting:
- the Newt is using the GSO coma corrector which adds about 1.1X to the focal length
- the ED80 uses a 0.85 flattener / reducer

Even so, the focal ratios are now f/6.375 for the ED80 and f/5.5 for the Newt. Any image through the vastly bigger Newt should require a shorter exposure, surely...? My much older Skywatcher 200mm f/5 certainly did. I'M CONFUSED.

Any help appreciated!

[RickS]

You'll lose some light due to the shadow of the secondary and reflectivity of the mirrors vs the transmission efficiency of lenses but I'd still expect the newt to be slightly faster.

Cheers,
Rick.

[bojan]

Vignetting? F5 will cause some due to mirror & mirror box.

too late... LOL :-)

[glend]

You can't really blame Andrews, those scopes are shipped to, and held in, a bonded warehouse. Andrews gets the box out of the warehouse when they get an order. Andrews never opens the box, i have bought several GSO scopes from them and they were all factory sealed boxes. If there was factory incompetence, rare for GSO these days, but you can talk to Luke at Andrews and he would likely replace it.

That said, most newts will need collimation checking after being road shipped, never mind being container shipped from Taiwan. You should always check it before you use it. Make sure your collimation laser (if used) is properly collimated itself, some cheap ones may not point straight. Get a laser you can align or adjust. They are easy to check with a V-block aimed at a wall target, rotate and watch for spot movement, etc.
Make sure the secondary is properly centred under the focuser, if not it is possible you could still point it towards the focuser tube but being misaligned you would lose alot of light. Be aware of the slight secondary offset of the f5, but more likely it could be mirror rotation. When you look into the focuser tube with a Chreshire is the secondary a perfect circle?

[thegableguy]

Vignette is pretty heavy with a full-frame DSLR on the Newt, but a crop sensor isn't too bad. That and the light blocked by the secondary might add up to a little, but it still surprises me the Newt isn't significantly brighter.

I use a cheshire eyepiece for collimation, no lasers involved. The secondary was WAY off - about 5-6mm too far down the tube.

Anyway, I've just re-assembled my old Skywatcher equivalent and it's exactly the same exposure as the GSO - in fact the GSO is ever so slightly brighter. It's weird!! Maybe there was some fogging on one of the surfaces last night, perhaps the camera-end glass of the CC. All the subs I took were extremely dim despite really cranking the ISO; same results with two different DSLRs. I'll try again tonight, weather permitting.

I just spoke to Luke at Andrews; if I can get to Sydney next week he's agreed to swap it for another one. You're right, not his / their fault at all. The last GSO telescope I bought from them was perfect and nicely collimated; this seems to be a rare miss.

[thegableguy]

FIGURED IT OUT.

The 30mm spacer between the camera and the GSO coma corrector.

I tried removing it and there's about a quarter of a stop difference.

So combining the light blocked by the secondary, the vignetting and the 30mm spacer, I can see how that would all add up to about a half a stop.

Conclusion:
I should've spent the extra for the Baader MPCC, which doesn't require as much spacing (and would have saved a lot of effort getting the damn thing into the Skywatcher's focuser), instead of going with the cheaper GSO. Sigh. Lesson learned.

[glend]

The conditions on the central coast over the past few nights have been very dewy, and i actually put my newt away after the first clear night because the secondary fogged over (no dew heater on it), been using the Mak-newt since as it is easy to heat the corrector. I suspect you have a foggy secondary, hard to tell on solid tube scopes unless you pull the camera and look, might be able to tell by the reflection from the primary if you look in the front. Do you have a good dew shield for this new scope? A dew shield will buy you some time, will not prevent it just delay it.

[thegableguy]

No I don't have a heater or a dew shield. Are they essentially an extension of the main tube that sticks out a little way, like my ED80 has? I could probably make one easily enough if you reckon it's worth it...? Is there a particular ideal length for a 200mm?

My initial thought last night was dew; my red dot finder was littlerally dripping within half an hour, but in my (very limited) experience dew makes it just plain not work. It wasn't great, but I could still see stuff. Perhaps when conditions are right - or wrong! - it's possible to get a view that's worse but not entirely ruined..?

[glend]

There is actually a formula for dew shield length, i think it is 1.5x the tube dia, but that could be wrong. I have made them out of thin yoga mats cut to suit and duct taped on the seam, works fine, use black foam, you can order them online very cheap. If you have an old hikers matt that will work as well. You can buy proper Farpoint ones from Bintel $$$.

Secondary heaters are more complicated, check them out at Kendrick Astro online to learn which one might suit you best.
You can usually still get some light through when fogged up but it does look like looking through fog - sort of like your image.
Hair dryers can work but i always consider them abit dangerous when your holding an AC powered device on wet ground, in an observatory no risk but you have to keep checking to see if you need to do it again.