Is there a cheaper TRF-2008?

[MikeWilson]

0-400 Mike? The TRF2008 is for 400-600 range, I assume thats what you mean

Not sure on alternatives, I used a skywatcher one with some success, certainly performs well for its cost of about £60.

The TRF-2008 is fine but for the extremely short focal lengths, there's nothing else. The WO IV is for 500mm-1000mm and the TRF2008 from 400mm but there's nothing else that would work at 400mm - or is there?

All the best,

Mike

[NickH]

I want one for the TMB105 F6.2, and people have repeatedly said the TRF is the ONLY one to consider for that scope... be happy to know if someone has had perfect results with the lower cost WO ones on that scope?

[Beyond_Vision]

The TRF is not perfect for the TMB as it is 650mm in focal length. Also if you have invested in a triplet do you want to compromise the CA with a reducer that is not fully apochromatic.

Regards

Kevin

[NickH]

Problem with the TMB (unlike the F8/100) is that there is some small degree of coma at the edge..speaking to a few people who have the same scope, they all rated the TRF...so this is interesting Kevin as I know you're a TMB fan too

[MikeWilson]

My TRF-2008 arrived today and is fitted to my Orion ED80T CF (also arrived today), filter wheel and camera. I'm keen to see how well it all performs tonight or tomorrow!

[shaunster]

Be an interesting first light Mike as I am considering a new scope now and this one is on my list

[Deneb]

Just make sure you get the spacing right Mike !

[MikeWilson]

I calculated the spacing pretty tightly:

The Televue TRF-2008 requires 56mm +/- from the back of the reducer/flattener glass to the CCD.

The QHY8L has 20mm of back focus

The SX USB filter wheel has 29mm of back focus

The filters add 1mm of back focus

= 50mm of back focus without any adapters.

So I added a single 6mm t thread and there should be exactly 56mm of back focus from the reducer/flattener to the CCD.

However today the QHY8L window heater arrived from Bern @ Modern Astronomy and I thought this would wrap around the neck of the camera. But oh no - it's effectively a heated t thread spacer(!). Fortunately it's about 6mm but maybe +/- 1.5mm either way so hopefully it won't cause any spacing problems. I think I'm about right replacing a 6mm t-spacer with the 6mm 'ish' heater.

Anyway tonight I was completely tormented by the gods of astro since my power extension cable fused (burnt out). So tomorrow I will go get another one.

All the best,

Mike

[dmahon]

I thought your ideal spacing would be 8mm, not 6mm.

56-20-29+1=8

[Ben Ritchie]

The Televue TRF-2008 requires 56mm +/- 2mm from the back of the reducer/flattener glass to the CCD.

In my experience, the recommended spacing does not necessarily give you best performance. I had a TMB 80/480 + TRF-2008 (...and was a complete dumbass for selling it, one of the best 'scopes I ever owned...) and got significantly better corner performance with something like 64mm than 56mm, I remember from a thread on CN at the time that I wasn't alone either.

So ideally you really need to be prepared to do a bit of experimentation with a stack of small spacers and a copy of CCDInspector to find what works best.

[NickH]

Got one £150 immaculate condition - chuffed!